The English Mechanic AND and more precise (distinguishing between groups and isolated spots) series of observations than that of Schwabe, in order to determine, if possible, the exact average length of period, searched all the accessible records of the observations of solar spots since their first discovery. In doing WORLD OF SCIENCE AND ART. this, he was led to the conclusion that by assuming it to be 11.1 years, all the observations would satisfactorily be accounted for, or as much so as could be expected when their imperfect nature was considered. I have already mentioned that Dr. Lamont, who himself had detected a period of ten years in the variation of the magnetic FRIDAY, NOVEMBER 11, 1870. ON THE PERIODICITY OF THE SOLAR declination, would not allow that the length of the SPOTS. BY W. T. LYNN, B.A., F.R.A.S. (Of the Royal Observatory, Greenwich.) solar spot period, so far as it could be deduced from all the observations, exceeded 10-4 years. The view which Professor Loomis comes to, from a careful examination of the records, is that the T number, and frequency of the solar spots exceed ten years. been suggested, but rejected, HE question of the length of the period, period is even a little less than this, and does not An influence of the planet has become extremely interesting from the hope because his period of 11.9 years is longer than the which it inspires of obtaining some clue to the cause of this variability, which may again, at longest of the periods in question. Professor some future time, help us to a better understand-Loomis, however, points out that the interval being of the nature of the spots themselves, and tween two successive heliocentric conjunctions of even of other remarkable celestial phenomena. Jupiter and Saturn amounts to 19-9 years; that, It is well known that the fact of a periodicity therefore, a heliocentric conjunction or opposition amounting in length to somewhat about ten years of those planets occurs every 9-9 years; and that was first discovered by Herr Schwabe, of Dessau, "if we suppose that the action of these planets and that the Royal Astronomical Society of upon the sun has some analogy to that of the London afterwards marked their sense of the moon upon the earth in raising a tide, then we value of the persevering industry and energy by shall have a cause whose period correspouds which the discovery was accomplished by award- quite accurately with the mean period of the ing him their gold medal in the year 1857. The maxima of the solar spots." fact acquired great additional interest when it was found by the labours of General Sabine and Professor Lamont that a period of similar length appeared to exist in the magnetic disturbances and in the variation of the magnetic declination, the times of maxima coinciding with those of the frequency of the solar spots. More recently it has been shown that a great abundance of aurora is frequently seen in years when the solar spots are most frequent; and that an aurora is always accompanied by an unusual amount of magnetic distaruance is too well known to need dwelling upon. I suppose no one can have watched the streamers of an aurora, who has also been an observer of comets of any remarkable size, without the idea being suggested to his mind that some sort of analogy exists between them, and that some common cause may act upon both classes of phenomena. Bessel long ago dwelt upon the high probability that we must seek in magnetic or polar action for the cause of many of the appearances exhibited by the tails of comets; and his elaborate explanation in this way of these appearances remains, though it may require some modifications, by far the most likely to be the true one of any that have yet been suggested. Thus a great number of facts seem unanimously to point to a mutual magnetic action between some at least of the heavenly bodies as being a very hopeful subject in future astronomical progress. And astronomy, which has recently called in the aid of chemistry (and, we may add, in doing so has richly repaid the debt), seems likely to avail herself also of the assistance of other phy sical sciences. Before we can arrive at a satisfactory theory of the periodicity of the solar spots, we must come to a positive conclusion as to its real length. This has been a subject of some dispute, owing partly to its irregularity, and partly to the great uncertainty of the earlier observations before a systematic record was kept by Schwabe and others. There seems to be no doubt that it is usually about ten years; but Dr. Wolf, of Zurich, has maintained that it amounts to 11-1 years; whilst Dr. Lamont, of Munich, has contended that it does not exceed 10.4 years. An interesting paper on this subject, by Dr. Loomis, Professor of Natural Philosophy in Yale College, has recently been published in the American Journal of Science and Art, and I have taken the opportunity of here giving an account of the investigation contained in it, which has been performed with the scrupulous care and scientific discrimination which distinguish Professor Loomis. Schwabe began his long series of observations (which he still continues) in the year 1826; the maxima which have occurred since that time took place in 1830, 1837, 1848, and 1860; the minima in 1833, 1843, 1856, and 1867. From these dates the most probable length of period would seem to be about 10 years. But the whole interval is scarcely sufficient to establish with more accuracy than this the exact length. Hence Professor Wolf, who began in 1849 an equally persistent We seem indeed here to be on hopeful ground, but much requires to be done before a theory can evident variability of the length of the solar spot be considered to be positively established. The period seems to require the admission of some change in the magnetic condition of the sun; its relative condition with regard to the planets appears to undergo some change, as the maximum of from conjunction or opposition of the planets. spots does not always occur at the same interval The last heliocentric conjunction of Jupiter and Saturn took place in 1861; and the next heliocentric opposition will occur in 1871. Every one knows that the solar spots are at the present time rapidly increasing in number and frequency; whether a maximum will occur this year or next fessor Loomis thinks he can trace a smaller we cannot, of course, at present decide. Properiod of less amount dependent on the heliocentric positions of Venus and the Earth. There seems to prevail a national prejudice against the metric system, but its intricacies amount only to fancy; it requires no excellent memory, the reductions are performed easily, for there are no questions to solve as in our inconvenient English method, that of bringing grains to tons and barleycorns or lines to miles. Its perspicuity makes its easy to teach; the stages running by series of tens from the unit, require no effort of memory as in our method, where nearly every stage of every system is irregular. The only difficulty I have found in teaching is that of impressing the relative bulks of the metric weights, as regards our pounds, &c., upon the student's mind. The relative bulk of the litre and the subdivisions of the litre may be easily demonstrated by flasks marked in the neck. But and iron for the low sum of two shillings, it was before Messrs. Griffin & Sons, of Garrick-street, introduced their set of ten metric weights in brass not in every student's power to compare the kilogramme with our pound. FOUNDATION OF THE METRIC SYSTEM.-Before the year 1789 France was possessed of such a variety of weights and measures that the people naturally sought redress by applying to the National Assembly. The Assembly saw the necessity of dealing directly with such an important question; they knew that the provinces were inundated with many varieties of livres (pounds)—land measures varied and with cloth measure, it varied according to the fabric, the aune (ell) was intended to measure. These different weights and measures offered were in many cases used for fraudulent purposes. many impediments to the course of trade, and In 1790 M. Talleyrand was authorized to report upon the subject of weights and measures, in consequence of the total abolition of local weights and measures feudally constituted. The folreport:-"The innumerable varieties of our lowing is a translation of an extract from his weights and measures, with their odd denominations, tend necessarily to a confusion of ideas and the embarrasment of trade; but the variety which was regarded as a source of error and infidelity, is still less than the difference of things nominally the same. Such a more common than one might at first suppose, medley, which is an enticement to fraud, is much seeing that the mind has been used, under cer tain names, to attach an idea of a fixed measure, such as a foot, an ell, &c., there exists a multitude of very apparent differences. Nothing is reserved for the National Assembly." can justify such an abuse as this; its annihilation M. de Talleyrand was quite correct in his remarks, possess is a national disgrace, and I think we can such a variety of weights and measures as we scarcely say that in this respect we are much more advanced than the French were prior to the year 1790. frequency of the auroral displays. It is neces- Talleyrand's report being issued England was met with no response. The question was then put written to by the French Assembly, who of course before the Academy of Sciences, when they recommended the ten-millionth part of an arc of the meridian as the unit of length, which they called a metre and divided into 10, 100, and 1,000 1000 traits 1 metre. 100 doigts 10 palmes Metre = = This law was to have come into force on July 1st, 1794, but the standards were not ready, and the system was postponed until April, 1795. In 1795 a law was passed firmly establishing the system, but the subdivisions and multiples of the various units were altered. The law states that: "The METRE is the measure equal to the tenmillionth of an arc of the meridian, comprised between the north pole and the equator." "LITRE, the measure of capacity for dry and liquid matters, the quantity contained in a cube whose side is one tenth of a metre." "GRAMME.-The absolute weight of a volume of pure water equal to a cube whose side is one centimetre) one-hundredth part of a metre at the temperature of melting ice." The law also decreed that the one-hundredth part of a metre should be called a centimetre and the tenth part a decimetre, the same with the thas : = Centiare 1-100th are Deciare 1-10th litre and gramme. Ten metres were called a ten metres side." I is divided and multiplied decametre, one hundred a hectometre, and one thousand a kilometre. The multiples of the litre and gramme were similarly denominated, the prefixes being taken from the Greek, while the decimal divisions were Latin. From 1800 to 1812 permission was given to use the original names for the weights and measures as were first proposed by the Academy of Sciences. This was repealed in 1812, and the old French weights and measures substituted to a great measure. Two systems were thus allowable, the metric system and the old French, which was not decimal in any point. The two systems continued to be used till the year 1837, when the government of Louis Philippe thoroughly established the metric system throughout France from the 1st of January, 1840. The law runs thus : Commencing from January 1st, 1840, all weights and measures, others than those established by the laws of the 18 Germinal,* An III., and 19 Frimaire,† An VIII., constituting the metric decimal system, will be prohibited under penalty enforced by Art. 479 of the penal code.' When the metric system was first proposed, a natural standard was selected, this being found in the meridian passing through Paris, the tenmillionth of the quadrant from Dunkirk to Barcelona being styled a metre. It has since been found that the metre is not exactly the specified length, and the Paris standards authorities have been petitioned to correct the error, but their replies have always been similar to the translation given below from M. Regnault to Herr Brix : "As for the stability of our metric system, I will respond very plainly. The French Government has never thought, I say more, cannot for a moment think to modify it, though it may be the metric system now adopted. The scientific reasons which are put forth for a change of this kind are absolutely puerile and fall after causing a laugh from people of good sense. It has been said that the metre is not exactly the tenmillionth part of the quadrant of the meridian which passes through Paris-two meridians may not be identical, and I defy the most accurate observer to measure twice, the same arc of the meridian and arrive at the same result. The metre type, that which no one can either contest or change, is the standard metre placed in the Archives Imperiales. I will say as much for the kilogramme. The true kilogramme is the standard platinum kilogramme belonging also to the Archives. No other is admissible with us." The metric system is not confined to France alone. Belgium and Holland have adopted it and made its use compulsory, with the North German Confederation, Italy, Spain, Portugal, Mexico, Chili, and the Southern Republics of South America. The United States of America, with Great Britain and Ireland, have so far adopted the system that contracts in its terms are rendered legal; while other countries have adopted mixed systems, which render calculations nearly as difficult as before. Greece introduced the system in 1836, but they relapsed to the old Turkish, which they now continue to The French unit of length is the METRE, and the decimal parts and multiples are as follows:Millimetre = 1-1000th of a metre M.m. use. 10 Metres = 1 decametre 100 1,000 10,000 hectometre = = kilometre myriametre c.m. d.m. m. decam. hectom. kilom. myriam. The metre is the measure used for purposes similar to the English yard, its English equivalent is 39-37079in., 3-28089ft., or 1-09363 yards. One yard is 0.91438 metre. By employing one or more of the above equivalents metres may be converted into English measures or these latter as easily converted into metres. When long distances are to be measured kilometres are used: a kilometre is 0-62138 mile, and a mile would be equal to 1.60931 kilometres. The law of the 18th Germinal, An III., decreed that the unit of measure for land should be called an ARE, and be "equal to a square of The 7th month of the first French Republican calendar; it answers to the 7th of April. Germinal commenced March 21st, and ended April 19th. The 3rd month of the Republican Calendar, commencing Nov. 21st, and ending Dec. 20th. The 10th Corresponds to Dec. 9th. ARE = = unit = 10 ares 1 decare = 1 hectare 100,, As the are is a square of 10 metres side it contains 100 square metres, the centiare then is one square metre, and the deciare 10. The centiare 10-76429 square feet. The are = 119-60332 square yards. The hectare, which is generally used for large surfaces, is 2-47114 acres. 1 square 9-28996 square decimetres, 1 square yard foot = = 0-83609 centiare, 1 acre = 0.40467 hectare. The French measure of capacity is the LITRE, which is subdivided as the metre, and is that quantity of water which is contained in a cubical vessel of one d.m. side. SIEGE having been decided on, the first operation is to surround the fortress, so as to cut off all communication; this is generally done suddenly, to allow no time for preparation. The beseiging army forms a double chain around the fortress with the same object that was intended by the lines of circumvallation and contravallation; the inner line to keep the garrison within the place and the outer line to keep off all assistance. This is called the investment, the necessity for which at once becomes evident. It was owing to the imperfect investment of Sebastopol that the siege was more prolonged than any other of nodern times; ground was broken on the 10th October, 1854, and it was not until the 10th of September, 1855, that the Russians retired, the siege having lasted just eleven months; this was entirely owing to the fact that the Russians were able to pour in a continual supply of men and materials. It is during the investment that a reconnaissance is made to determine which side is most favourable for an attack, and now also all hands are hard at work preparing the materials. Some idea of the immense quantity required may be formed from the following actually used at a siege in France. It will be as well first to explain a few of the terms used. A fascine is a long faggot of brushwood, used in supporting earthworks; it is 13ft. long and 9in. thick, and weighs 140lb. A gabion is a wicker basket, cylindrical in form and open at both ends, used for supporting the interior slopes of field parapets; it is 3ft. high and 2ft. in diameter. Entrenching tools consist of spades, shovels, pickaxes, &c. The number used of each was as follows:80,000 gabions, 300,000 fascines, 30,000 entrenching tools, and 200,000 sandbags, besides the guns, mortars and their appurtenances. All this enormous quantity must be had in readiness. The first parallel. When the garrison has retired, the most suitable ground for the trench is selected; the parks of artillery and engineers are formed, so as to have everything at hand. The position of the first parallel being determined, it is traced a little before dusk (on the first night) by the The gramme equals 15-43234 grains. The kilo-engineer officers and their assistants, who are provided with balls of white tape, with which they gramme is 2-20462 avoirdupois pounds, or mark out the trace of the trench to be made; the 32-15073 troy ounces. One grain is equal to working parties are placed in position, as soon as 0-06479 gramme. A pound 0.45359 kiloit is dusk, lying down at 6ft. apart, until the One hundredweight 50-80237 kilosignal is given to commence, then each man begins by digging a hole 3ft. deep in the centre of his position and then extends right and left until the whole trench is completed. In easy soil a man can remove about a cubic yard in an hour, so that by daybreak a trench 3ft. deep is made, and the earth thrown out before it to a height of The metric system having been laid before the 3ft., a complete cover throughout the whole length student, with its history, he will be able to see of the trench is made, sufficient to enable the the simplicity which characterizes it. There are working parties to continue during the whole of certainly some long calculations to make when the next day widening the trench and increasing metric are to be converted into English equiva- the thickness of the earth in front of it. This is lents; but this is not the fault of the system, it to be completed the first night and following day, is the result of the adoption of those clumsy the next two nights are devoted to the construcweights and measures with which the English tion of the enfilading batteries and the zigzags in population have been so long satisfied. Were front and rear. these old weights and measures suppressed, and the French system substituted, its simplicity would be a recommendation for its general adoption, and the uniformity it would introduce would be more than sufficient to compensate for the difficulties which it is stated would occur in practice. Having seen how the cognate sciences of light and heat have influenced chemical philosophy, theoretical point of view, experiments still contheir coadjuvancy having been most valuable in a tinue to be made, for there are many things yet to be discovered; things that are looked upon as simple and correctly proved, may, by the genius of some modern experimentalist, be regarded as erroneous theories. That theory is probably correct which explains everything to which it this is the haven of rest looked forward to by relates, without a single exception; and although every truthful observer, who can say that the science of chemistry is much nearer that point than it was in Dalton's time? CORDOVA EXHIBITION.-This exhibition will open March 1, 1871; but the trials of agricultural machinery will take place on the 15th of December, 1870. Intending exhibitors should apply at once to Messrs. Johnson & Sons, Castle-street, Holborn. The first parallel and zigzag communications being completed the approaches towards the place are made. Formerly a trench similar to the first parallel was continued along the zigzags towards the place, but in future the sap will become necessary, and under favourable circumstances the flying sap may still be used. The flying sap is executed in the following manner :-The working parties assemble at dusk, each man taking with him two gabions, a shovel, and a pickaxe; sentries and supported by battalions as near as they are preceeded by a covering party having possible; the gabions are placed in front of the men, and as soon as they are ready they begin to fill them with earth so that in ten minutes they have a musket-proof cover which they continue to strengthen by excavating the earth and throwing and width of trench can be finished by each man. it over the gabion; in this manner a good cover When this cannot be done a regular sap must be adopted. The process of sapping will by the subject of our next paper, as the latter part of a siege must be done by that means. The necessary strength of a besieging army varies with the number of the garrison, but one example will suffice-say for a garrison of 5,000 The number of men required for guarding the trenches must be at least of the garrison men. FIC.7 Guards of trenches in three reliefs.. Working parties in four reliefs General duties of the camp, estimated at 1-10th of the whole army, in four reliefs .. 12' FORTIFICATIONS. FIC.8 5,000 x 3 = 2,000 × 4 = 11,250 12,830 18 at night is easily taken in those places where it occurs as it rests on the flowers in full view. sometimes A particularly local butterfly is that pretty species the Marbled White (Arge Galathea), and as it very seldom flies far beyond the meadow where it was bred, there is some encouragement for the collector to look for the caterpillar, which feeds upon various grasses. It is but small when it hybernates, remaining apparently without eating all through the winter, though some species, also in three reliefs, that being the greatest number the Scarce Vapourer are hatched about the end of grass-feeders, nibble the blades occasionally if they are likely ever to have to oppose. July, from eggs deposited in a cluster (sometimes the weather is mild. When feeding, at the least as many as 400); and at first emergence, they annoyance or alarm the caterpillar of the keep pretty closely together, separating from each Marbled White falls from the plant in a curved other by degrees as they increase in size, but they posture, lying without motion for some time. The 8,000 grow very slowly, and soon prepare to hybernate. head is rough, sometimes green, As is the case with other species, they do not brown; the hue of the body also varies simialways locate themselves for the winter on or near larly, but always a little darker than the head; their food-plant, which renders a search for them down the middle of the back is a dark stripe, on more difficult. According to my observations, they each side of this is a narrow stripe, pale red, and do not form any protection for themselves, usually just below the spiracles (which are deep black) resting near the ground on a twig near the centre there is a whitish stripe. The body is stout, of a bush. In some cases, they have been found rather fusiform, and at the anal extremity exactually on the earth, and observers have re-hibits two points, in which all the stripes meet; ported also instances where these caterpillars every part is studded with minute warts and had sheltered themselves by weaving a web short hairs. No preparation for becoming a around a leaf or branch, but this is probably ex- chrysalis appears to be made by this caterpillar; ceptional. Remaining without food until the spring, descending from its food-plant, it settles low the caterpillars of the Scarce Vapourer are quite down amongst the herbage, and turns to a short ready in April to commence an attack upon the sal- brown chrysalis, partly transparent, from which low or hawthorn buds, taking afterwards to the oak the perfect insect soon emerges. or hazel, which they prefer. The ground colour is a beautiful orange with four rows of black spots coalescing so as to form stripes; from the fifth to the eighth segment, we find a brown upright tuft of hair arising from each; the second segment has two long pencils of hairs which point fowards; on the last segment there are three tufts of black hairs, directed backwards. A considerable portion of the hairs are used by the caterpillar in forming its cocoon, and the chrysalis is also hairy. Total, independent of sick and casualties.. 32,080 Fig. 7 is a section of the first parallel; Fig. 8 a section through the second parallel and zigzag of approach, when executed by flying sap. J. E. OLDFIELD. (To be continued.) CHAPTERS ON CURIOUS CATERPILLARS. HE entomologist has many opportunities for in looking for one thing you find another," so often does he go out intent upon discovering some insect, the locality of which he thinks he knows, and while he fails to get this, lights upon other species he did not dream of finding. Also, while searching for insects in one particular stage, specimens turn up in other stages; when hunting for imagos especially we come across caterpillars, and if seeking caterpillars we very frequently discover chrysalides. Amongst other choice chrysalides taken this month, by what we call a fortunate accident, the insect-hunter between united leaves of birch, beech, or oak, finds the cocoon of the Lobster Moth, the caterpillar of which we described last month. The chrysalis is without any notable singularity. The cabinet of the entomologist is not likely to derive much advantage from his researches amongst the caterpillars in November, for though there are a few which may be taken now nearly full-fed, and do not require much care or attention to bring them through, the bulk of the hybernating individuals, if brought in-doors, or otherwise placed in confinement, are difficult to rear. But still, those who desire to gain a thorough insight into caterpillar history will devote themselves, as opportunity offers, to the work of seeking for colonies, or isolated individuals, even at a season when such a pursuit is less attractive than in the summer season. And there are certain species, it must be remembered, which may be detected in winter, when the trees are bare and herbage is scant, with more facility than when vegetable life is at its height. Ingenious, too, as are the modes of concealment adopted by some hybernating caterpillars, they ought not to baffle the determined collector entirely, though he may require several successive seasons to pass by ere he can say that he has succeeded in fully elucidating the life-history of some one or other of these. Amongst these hybernators is one to whose peculiarities I have paid much attention, having at various times reared a large number of the caterpillars. This produces the moth known as the Scarce Vapourer (Orgyia gonostigma), and the species is highly interesting because it is so closely allied to the very common Vapourer Moth, which we see dashing wildly about in London suburbs, and in the vicinity of other towns during the summer. The other species is confined to a very few localities, one of which is in Surrey, near the metropolis; and as the female moth is destitute of wings, and cannot journey from place to place, it is not likely to become more common. The caterpillar, by the movements of which alone could it be distributed, is not very migratory; and the circumstance that it lives through the winter, exposes it to various dangers which the common species escapes, since through the cold months it is in the egg state. The caterpillars of A very local moth is that called by collectors the Reed Leopard (Macrogaster arundinis), and it is one of those species which beguile the insect-hunter into the uninviting fenny districts of Huntingdonshire and Cambridgeshire. Like the swallow-tail butterfly, each year tends to diminish the numbers occurring, through the reclaiming of the fens. The caterpillar lives from summer until the following spring, and probably feeds through the winter unless in severe weather. It is of a dirty white colour, with a horny head, and a plate of similar texture on the next segment, very much resembling a maggot in appearance. The egg is deposited by the mother moth on the stem of the common reed, only one being laid on each plant. The young caterpillar at once eats its way to the interior of the stem, and generally works its way upward towards the top of the plant, though it has the power of moving up and down. The chrysalis is long, and has rows of minute hooks, by means of which it is able to change its position if needful, giving birth to the moth in June. The history of the Black-veined White Butterfly (Aporia crategi) is very interesting. Classed, as it is, with the common Whites, and bearing a resemblance to them in some particulars, it is much scarcer, and differs greatly in its habits. The caterpillars, directly they are hatched, construct a sort of tent, under which they feed, but do not attain any considerable size during the autumn. A similar, though thicker, tent serves them for an abode through the winter season, when the prying eye of the entomologist, scanning the boughs of the hawthorn or the pear, may perhaps discover it, and carry off the colony as a prize. Certain districts in Wales and contiguous English counties yield this species most abundantly in some seasons, and it occurs in Kent near Faversham, and also, it is said, in Devonshire. The caterpillar of the Black-veined White, when full grown, has the head of a smoky black, covered with hairs of two different lengths, the shorter being black, the longer white. The body is a rather deeper black, and it has two rustcoloured stripes, which a moderate magnifying power resolves at once into a number of minute spots; in the centre of each of these there is a black dot, from which springs a rust-coloured hair. Underneath, the surface of the body is grey, sprinkled with black dots, and dotted with whitish hairs. When full-fed, this caterpillar spreads a silken web over the twig on which it has been feeding, and, fixing itself thereto, turns to a chrysalis. Sometimes, however, like its relatives the Whites, it will crawl from the tree or shrub to some wall or paling. The butterfly, which appears in July, flies swiftly by day, but In places where the common broom grows plentifully, we are almost sure to find in November, and through the winter, the young caterpillars of the Grass Emerald (Pseudoterpna cytisaria). Hatched during July, they grow but little in the autumn, and then fix themselves upon the stems of the plant, and with the head raised from the surface they remain unmoved in the coldest weather, re-awakening to life about the end of April. When getting near their full size they rest on the twigs in a singular position, with the head bent under and the legs crowded together and brought close to the mouth. The whole body is covered with small points, as if shagreened, the head being deeply notched on the crown, while from the segment behind it rise two blunt protuberances, which point over the head; on the last segment are two points of a pinkish hue. The general colour of both head and body is dull green, with pink on the crown of the head and the protuberances behind it; a very narrow brownish stripe runs down the back, and along each side is a white stripe, edged with red, which is interrupted on the fourth segment; the spiracles show distinctly, being paler than the ground colour. When these caterpillars have ceased to feed they draw two or three leaves loosely together, and under this shelter become chrysalides about the end of June. Another hybernating geometer caterpillar, which bears some resemblance to the preceding, is that of the Common Marbled Carpet (Cidaria russata); unlike it, however, it feeds occasionally during the winter in mild weather, being then found upon or near the wild strawberry, in the summer it has also been detected on birch and sallow. This caterpillar rests usually with the body extended, but, if touched, at once raises its head and bends it under, in the "volute form." The head is of a rather duller colour than the body, the eyes, which are black, showing conspicuously; the body pale yellow-green, with a stripe of dark green down the back. In some specimens there is a beautiful purplish stripe along the sides. All over the body are minute white warts, each giving off a hair; at the anal extremity are two protuberances, usually rose colour; the legs and claspers are of a dull red colour. The individuals of this species which have lived through the winter turn to chrysalides in May, and there is a second brood of the caterpillars in June and July. The curious caterpillar of the Scolloped Bar (Scodiora Belgiaria) occurs on heaths, and is rather uncommon. It may be looked for at this time on patches of the common ling, near the roots of which it remains in a state of hybernation, rousing itself to eat in April. When alarmed it rolls into a ring, and will remain thus coiled up for an hour. In colour it is brown, with indistinct greyish markings, and a short white stripe near the first pair of claspers. On each segment there are two warts on the back, and there is a conical short horn above the anus, behind which are two longer and slender horns, which are usually pressed closely together. The female caterpillars are perceptibly less in size than the males, a rather unusual circumstance. The cocoon is spun upon the ground; it is of slight texture. On various heaths and commons, though less fully justify the term "star-drift," which I have abundant now than formerly, we find the cater- applied to the stellar proper motions. pillar of the Clouded Buff (Euthemonia_russula), Remembering that the stars which are visible to which, though hatched from the egg in July, does us lie at very different distances, we see that if a not become adult till the following May. Its favour- real star-group exists in space, with real dimensions ite food-plants are the mouse-ear hawkweed and which cause it to appear to cover a widely-extended the common dandelion. At this time it is about region of the heavens, we must expect to find that other stars in the same region do not belong to the one-third grown, and is dull brown in colour, with group. Not only so, but it must be looked upon as hairs of a reddish brown, and a faintly indicated highly probable that the stars seen in any given stripe down the back. When the weather is direction may belong to three, four, or more starfavourable, it appears to feed on various low groups, if the existence of star-groups is a real plants, as does also the caterpillar of its hand- fact. Therefore we might expect the existence of some relative, the Wood Tiger (Chelonia planta-star-groups to be more or less marked by the effects genis), so named from its partiality to plantain. which would follow from their apparent intermixClearings in woods yield this species, especially ture. If, for instance, a general concurrence of in the south, and the caterpillar may be detected proper motions in a definite direction is to be held indicative of the fact that the stars so moving in the winter season by the persevering insect- form a single system, then we might expect in hunter. It is of a greyish black tint, dotted over general to fail in detecting such systematic drift, with numerous warts, and covered with long on account of the perplexities introduced by some hairs, which increase in length towards the anal other drift belonging to a set of stars apparently extremity, being black in colour at the head and mixed up with the former. And if three or more tail and reddish brown on the middle seg. star-drifts were mixed together in this way, the ments. In its habits it appears very sluggish, problem would become still more perplexing. as is also the moth, which is rarely seen on the Thus, all that was to be hoped for (as it seemed), wing. was that here and there some sufficiently well marked cases of star-drift might not be masked by the effect of other motions. The result, however, was more satisfactory than I had anticipated. STAR-DRIFT.* BY RICHARD A. PROCTOR, B.A., F.R.A.S. (Concluded from page 127.) 3 PEAKING of this result nearly a year after the Astronomer Royal's labours had been published, Mr. Main, then president of the Royal Astronomical Sciety, remarked that "the inevitable logical inference deducible from Mr. Airy's researches is that the whole question of solar motion in space, so far, at least, as accounting for the proper motions of the starsi concerned, appears to remain in doubt and abeyance." It must be understood, however, that the doubts hers expressed by no means rest upon the fact of the sun's motion. We are forced to believe that some of the assumptions upon which we had been proceeding with confidence, are more than questionable. It may be, for example, that our ideas of the stellar distances require to be molified. Or again, it may be that the stars are not moving independently, but according to some system which tends to falsify all the general assumptions we had formed on the hypothesis that they are independent suns like our own. And, again, there may be laws of motion which associate tue stellar movements in a special manner, without any actual association of the individual orbs. I was led some time ago, by considerations wholly distinct from those which I have been dealing with above, to form the opinion that if the proper motions of the stars were mapped, there would be rendered apparent signs of association between stars much farther apart on the heavens than the members of the widest double or multiple star-systems. In the Student for March, 1868, I pointed to the fact that associated proper motions would afford significant evidence in favour of the theory that the signs of stellar aggregation in certain parts of the heavens are not accidental. I therefore proceeded to map down the proper motions which the Astronomer Royal had used in his calculations. It need hardly be said that a map of proper motions speaks much more intelligibly and clearly respecting the meaning of those motions than the most carefully constructed table could do. Astronomical tables are necessarily arranged in such a way that stars which lie near each other in the heavens are often far apart in the catalogues. It is therefore impossible to form any clear conception of the general character of the motions prevailing in any given region of the heavens. When a chart had been so constructed as to exhibit the stellar motions to the eye, this difficulty was at once removed. In some regions large groups of stars are seen to be drifting bodily in a definite direction. The most remarkable instance of this sort occurs in the stars which form the constellations Gemini and Cancer. All these, amounting in number to seventy or eighty, are drifting towards the neigh bouring part of the Milky Way, with the exception of three stars, which seem to belong to another system. Another remarkable instance is to be found in the stars of the constellation Taurus. This is the instance of concurrent proper motions on which Madler founded his theory that Alcyone is the central sun of our galaxy. The drift in this part of the heavens is in a manner opposed to that in Cancer and Gemini. The two systems are, in fact, divided by the Milky Way, and each seems drifting towards that zone. More commonly, however, two or three forms of star-drift are seen intermixed in the same region of the heavens. And here it might seem difficult to pronounce whether in reality there are any associated movements, since it would clearly not be impossible that a mere chance distribution of motions might stimulate a tendency towards two or three definite directions. There is, how ever, a circumstance which at once serves to establish the true significance of the observed relations. If the motions in one direction have, besides, a general agreement in respect of magnitude, we can clearly assume with a much greater degree of probability that they indicate a real drift, or rather, that the stars they belong to form a real system. Nay, the mere fact that a number of stars in a given region have a very minute proper motion, while all the rest have large motions, would show that the former form a system perfectly distinct from the latter. One instance will serve to show the power of this new mode of discrimi nation. Of the seven bright stars in the Great Bear, five are travelling in a common direction with uniform velocity. The other two are travelling in another direction, and also with a common velocity. We cannot doubt that the first five, at any rate, form a system, drifting along bodily. For let us sum up the evidence. First, we have the comparatively weak evidence derived from the general equality of the five stars, a peculiarity which has in all ages led observant men to entertain the impression that these stars are in some way associated. Next, we have the fact that the five stars are travelling in the same apparent direction, and the significance of this point it is easy to estimate, because the antecedent probability that, taking the direction of one The plan I a lopted was to attach to each star star of the five as a standard of reference, the other a little arrow, whose direction and length indicated four would be found to be travelling in the same directhe character and magnitude of the star's proper tion, is demonstrably minute. Lastly, we have the motion. In order that these arrows might not be evidence derived from the equality of the motions of too small, I had to give them the length correspond- the five stars, and here again the antecedent proing to the star's motion during a very long interval bability of the coincidence is so minute as to force of time. For convenience, I made one degree on upon us the opinion that the actual coincidence is the map correspond to a stellar motion of one-tenth not accidental. The combination of three lines of of a second; the result being that the motions evidence leads to a feeling of absolute certainty that actually represented were those which would take the five stars are associated into a single scheme or place in the course of 36,000 years. Even so mag-system. nified, most of the motion-arrows were inconveni- Let us pause for a moment to contemplate the ently minute. significance of this result. One of the stars of the set The maps included at first only the 1,167 stars of five is the middle star in the tail, which I have dealt with by the Astronomer Royal, but I sub- already referred to as having a companion visible to sequently added upwards of 400 stars, whose proper the naked eye. Now, it had long since been proved motions had been calculated by Mr. Stone, of the that the bright star and its small companion are Greenwich Observatory, from observations as trust-really connected. The evidence had been no other than worthy (having, in fact, been made by the same that community of proper motion which I have observers) as those which Mr. Main had used in been dealing with in the case of the five stars. preparing the catalogue of 1, 167 stars. Mizar and Alcor, then, were looked on as a wide The maps, which I have before me as I write, double, and astronomers had contemplated with interest and amazement the wondrous cycle corresponding to the motions of stars separated in reality by an enormous interval. For the nearest of the From the Stulent for October, 1870. stars in our northern skies is more than 720,000 times farther from us than we are from the Sun. Mizar is presumably much farther away. Now, whatever distance separates Mizar from us, cannot be more than about 240 times as great as that which separates Alcor from Mizar; so that Alcor must be at least 3,000 times farther from its primary orb than we are from the sun. then, must be the period required for the revolution of the two stars around their common centre of gravity! How enormous, But this is not all. Mizar has a close companion, as well as its distant companion, Alcor. This close companion has the same proper motion as Alcor and Mizar, and belongs, therefore, to the same scheme as the other four stars. A sort of dignity is thus given to the star-system we are considering, by the triplicity of one of its members. At present, however, we are dealing with another considerationthe magnitude, namely, of the cyclic period appertaining to the scheme. Now, the close companion of Mizar, though undoubtedly it is in reality travelling round that star, moves yet so slowly that no sign of a change of place has as yet been detected by astronomers. Therefore the period of revolu tion, even for this comparatively close pair, must be very large, and Alcor must have a very much longer period; so that we may accept without surprise Baron Mädler's estimate that Alcor occupies no less than 7,659 years in travelling around Mizar. But what sort of periods can we assign to the cyclic revolutions of the five stars, when the comparatively close companions of one of the set occupy periods of revolution so enormous? Again, how can we resolve the questions which at once suggest themselves respecting the relations which prevail in such a system? That the whole system revolves around its centre of gravity is of course certain. But there are numberless ways in which the revolution may take place, depending on the relations between the weight and magnitude of the different orbs forming the system. Any two of the five may really form a pair, any three may form a triplet. We cannot tell where the centre of gravity of the scheme may be. We have no knowledge of the true relative positions of the five orbs. We cannot guess what the real direction of their orbital motions may be. We are, in fact, altogether in doubt on every subject connected with the system, except the main fact that the whole system has a drift carrying it bodily forwards at the rate of many millions of miles per annum. It is in this relation that the appearance of such systems as these in the heavens seems to me so interesting-I may almost say, so imposing a phenomenon. The life of man is a period too short to tell us anything even of the subordinate motions of such a scheme,-the motion of Mizar's companion about its primary, or of Alcor about both; but the duration of the human race, nay, of the solar system itself, may be out-lasted by a single revolution of the great star-system placed out yonder in the celestial depths. From the far-off times of the Chaldæan shepherds the great Septentrion star-system has looked down with seemingly unchanging aspect on the rise and fall of many nations and races of men. When the human race has perished from this globe, when the earth has become what the moon now is, a scene of utter barrenness and desolation, the star-system will doubtless have exhibited many changes. But only when millions of æons have passed, and the earth is nearing the scene of its final absorption beneath the solar oceans, will the stately motions of the star-system have begun to work out the full series of cyclic changes appertaining to a scheme so extensive and so complicated. But the star-drift in Ursa Major is only one instance out of many. Looking more closely than we have yet done into the sidereal scheme of which our sun is a member, we see it breaking up into subordinate star systems of greater or less extent. Our sun himself may not be a solitary star as has been commonly supposed. From among the orbs which deck our skies, there may be some which are our sun's companions on his path through space, though countless ages perhaps must pass before the signs of such companionship will be rendered discernible. On every side we see drifting starschemes, and comparatively few stars are to be recognized as voyaging in solitary state through space. From the complexity of such systems as we see in Gemini and Cancer, to schemes such as the one in Ursa Major, and thence to solitary stars such as Arcturus and Sirius appear to be, w recognize a number of gradations, and it yet remains to be determined to what class of these schemes our sun belongs. Verily much remains to be learned respecting our galaxy. Since the days of Sir W. Herschel, or rather since the younger Herschel conpleted the noble series of labours commenced by his father, a sort of rest has fallen upon astronomy, so far as the science deals with the relations of the great sidereal system. But there is room for much new labour in this wide field of research. The Herschels dealt with generalities. They discussed the galaxy as a whole, and it was no part of their work to examine into the details of the stellar scheme. The work they took in hand to do they accomplished with marvellous success, insomuch that they have left little for others to accomplish in the same direction. But it would be a mistake to renew the Herschelian mode of inquiry-to continue to neglect details and consider only the grander features of the galaxy. The work of survey has been completed. In examining, part by part, the field which has been plotted out for us, we must adopt new principles for our guidance. To deal now with generalities alone, as the Herschels did, would be to destroy those scarcely recognizable indications which can alone guide us to new knowledge. We must in future examine the sidereal scheme detail by detail, feature by feature. The work will not be light, and many workers will be wanted. But the result will be worth the toil. Not in our day, perhaps not for many generations, may the fruits of such labours be reaped. But gradually astronomy will gather in her harvest, and when it is garnered, the rich reward of many years of toil will be found in a clear knowledge of the relations presented by the wondrous galaxy to which our sun belongs. THE MAGIC LANTERN EDUCATIONALLY camera up to three (Concluded from page 151.) THE THE Microscopist may, to a limited extent, display the minute structure of natural or artificial objects themselves on an enlarged scale by means of the gas microscope; in other cases, by raeans of photograpes of microscopic preparations, first enlarged in the inches in diameter, then again enlarged by the lantern to any desired extent, after the method so successfully carried out by Dr. Middox and others. The Geologist may illustrate his lectures by scenes taken direct from nature, of the mine and the quarry, the natural cleavage of slaty rocks, or the stratification of aqueous deposits, the disintegrating action of the atmosphere on granite and feldspathic rocks; the stupendous erosive action of water, as at Niagara Falls; the volcanic cone and crater as in Piazzi Smyth's views of Teneriffe; the eruptive Geysers of Iceland, mineral veins of eruptive rocks, or metallic deposits; the characteristics of glaciers and glacier action. That paper" Reynard the Fox." Again, how Hogarth and of Arts* on the present subject. attracted the attention of a young officer in the morality may be combined, in the counterparts of Russian artillery, an amateur photographer, who the celebrated engravings of the "Good and Idle has since become not only a distinguished professor, Apprentices," and how the great stories of Sacred but Inspector of the Military Colleges of Russia. History may be illustrated simultaneously with With the courage of youth, he determined to dissertations on the bold and vigorous designs of introduce the method I advocated into the old Schnorr. We may also show how the singing-master system of instruction, where he had the power to may avail himself of our method, so as to place the words and music of a hymn, or other appropriate use it, in his own lectures on General History. a choir, when such subjects as This gentleman commissioned me to procure for song, before him accurate counterparts, both as to drawing and Schnorr's Bible Pictures are being exhibited in the colouring, of such subjects as would thoroughly dark. By this method the works of our great masters illustrate," The Manners and Customs of Nations from the Earliest Historical Periods to the Present ancient and modern may be placed before a class, Day," to which end the works known as the best on a scale sufficiently large for their merits or authorities on Assyrian, Persian, Egyptian, Grecian, peculiarities of style to be pointed out in detail Roman, and medieval history were ransacked to Nothing can be more beautiful than the reproducsupply the necessary material, the officers of the tion of sculpture, when projected on the screen, British Museum courteously rendering me every faci- growing-after the eye has dwelt upon it for a few seconds into stereoscopic rotundity. lity for the artists I put upon the work. My notion of 1862 thus became un fait accompli in 1869, and I think it may be regarded as a step in the right direction, for though the lantern has been employed at institutions and in schools, it has never yet been used in colleges, and he would be a bold man who would venture to suggest to the Dons of Oxford or Cambridge the introduction of a magic lantern for illustrating the University courses of History. But why not? "Because it has not been done before" is not a sufficient answer in these days of rapid progress and wide reform; for by a well selected and carefully executed series of pictures from authentic data, we may take the student back into time, and make him familiar with the features of those who have been celebrated for good or evil, in politics, war, literature, science, and art; the aspect of the people of the various nations, their costumes, the buildings they erected, the chambers in which they lived, prayed, or died, the vases with which they decked them, the gods they worshipped, carved in ivory and gold and of prodigious size, their manners and customs, how they lived at peace and in war, their arms and armour and modes of attack upon an enemy, and in what grandeur they carried their illustrious dead to the grave. And in placing such records of man's history on earth, in the form of existing scenes, we appeal to in a manner the most impressive, verbal, or printed descriptions could never convey to the mind, and so in these days, when a wider range of knowledge is expected from our educated classes, we may hope to establish a more rapid and impressive system of instruction, and of a kind not so likely to pass out of the student's head after he has left school or college. the eye, or In all artistic reproductions the negatives should be taken from the originals, so that the characteristic touch of the master may be ensured. Descending to the artistic requirements of every What can be more delightful than to day life. bring back reminiscences of travel, taken from our own points of view, with the miniature cameras at last coming into vogue (cameras that are no longer a burden to the tourist), and on one's return from a summer's trip, placing before family and friends enlarged transcripts, depicted by Old Sol, of the scenes that have given us health and pleasure? By the time the method of illustrating instruction in literature, science, and art herein advocated can come fully into play at our educational establishments, it is hoped that a systematic series of photographs of scarce or unique specimens and art treasures scattered through European museums, may, by the co-operation of their curators, have been secured; for every bee in the great hive of science should sympathize with the efforts of his fellow-worker and give a hand to attain a desired result. ever But photographic magic lantern views, when not in use, may be made available for educa tional purposes; for I would suggest that, if they represent natural history subjects, instead of stowing them away in boxes, they should be placed in the open cases of museums, &c., beside allied objects, care being taken that they are fixed at such an angle that light should be reflected through them by aid of a piece of white paper placed behind the transparency, or by mountground glass, they might serve as appropriate ing the views in long frames, backed with fine borders to the windows of a scientific institution. The advantages I claim, then, for magic lantern views, when educational value is aimed at, are delineations truthful to nature and abounding in detail; cheapness, compactness, as compared with not in use for their legitimate purpose. paper diagrains, and their utility in museums when The Paleontologist may show the extinct forms of animal and vegetable life that mark the boundary lines of great epochs in the world's early history; the weird skeletons of mighty animals, frogs as big as rhinoceri, reptiles larger than whales, birds as long-necked as giraffes, stags of gigantic size, and In Geography we may show by maps the political elephants as great as Behemoth, and the probable aspect of such creatures in their living state, as boundaries of our globe, or the natural divisions restored from their fragmentary remains (not by climate and other physical causes have marked upon fanciful or haphazard guesswork, but by sound its face. Portraits of the types of men who inhabit inductive reasoning, founded on anatomical know-its various regions. The vertical range of plants ledge) by Cuvier, Owen, and Waterhouse Hawkins: and animals, from the greatest mountain heights In conclusion, I would say, that every exploring or even entire resuscitated landscapes, including to the lowest measured depths of the ocean, their horizontal distribution over the face of the plants and animals, as Unger, of Vienna, has so artistically reproduced in his "Ideal Views of the earth, and their limits in latitude and longitude. expedition should be accompanied by its official The physical phenomena that characterize its photographer-that every national museum, obserPrimitive World."t as monsoons, hurricanes, vatory, and hospital should have its appointed phoseveral regions, such waterspouts, mirages, snowstorms, glaciers, icefields, tographic operator, and then the hoped-for time avalanches and landslips, thunderstorms, volcanoes, may come when we can, in systematic manner, place geysers, whirlpools, mountain torrents, caverns, the records of scientific travel, the transcripts of coral reefs, stalactitic formations, basaltic islands. nature's treasures, and the history of the progress The theatres, so that we may carry our student round The physiognomy of mountain peaks, and other of fell disease upon the screens of our lecture physical features of the earth's surface. costumes of various countries. The buildings that the world, back into time, and into the depths of characterize different nations, from the snow hut of space. the Esquimaux to the European palace, or those of people who have passed from the face of the world, whether it be the pyramid of the Egyptian, the ON THE VIBRATIONS WHICH GIVE RISE palace of the Assyrian, or the temple of the Aztec. The general aspect and characteristics of the The Astronomer has, for long years past, availed himself of the lantern for illustrating his discourses on those vast and illimitable realms of space beyond our own globe, studded with other planets, star-groups, comets, and meteors. By mechanical agency and transparent orreries he has shown the orbits and relative motions of the planet, but recently photography has come to his aid, and sun and moon depict their own portraits, of which Warren De La Rue has secured such an interesting series. Again, the spectra of the heavenly bodies, those mysterious protuberances surrounding the sun's edge, extending to a vast height into its phoface, or the various stages of our luminary's eclipses, tosphere; the dark spots that travel over the sun's may be truthfully rendered by photography and the lantern. SCHOOL TEACHING.-As yet I have only spoken of the application of photography and the lantern to scientific demonstrations; but what can be more valuable for educational purposes than to place before the student or schoolboy accurate as well as striking pictures of the subjects of their studies, whether in science, geography, or history, on such a scale as to become impressive, while appealing to the eye, and so serve as an artificial memory, giving the next best thing to students having seen the objects themselves? I feel convinced that schoolmasters would save time and teach better were they to make history and geography subjects for evening lectures, illustrated by the lantern. In Russia the thin edge of the wedge has been introduced into the colleges (not schools), and the historical courses have been thus illustrated; geography, in the widest sense of the word, is to be treated in the same manner a scheme I have had the pleasure and honour of assisting to carry out. IN HISTORY.-After the close of the International Exhibition of 1862, I read a paper before the Society By SAMUEL HIGHLEY, F.G.S., &c. + "Ideal Views of the Primitive World in its Geological and Paleontological Phases." By Dr. F. UNGER. Edited by SAMUEL HIGHLEY, F.G.S. Quarto; seventeen illustrations. great cities of the world, their engineering feats, TO MUSICAL SOUNDS.* BEFORE entering upon to consideration of Theory of Sound which it is my purpose to bring When the Sound cannot pass through empty space, as is proved by the old experiment of striking a bell under the receiver of an air pump. of an observer placed close to the air is exhausted, the sound is quite inaudible to receiver. It follows that sound must consist of some movement which, travelling through the intermediate air (or other interposing medium), strikes upon the auditory nerves. But the naturalist, historian, and geographer * See Society of Arts Journal, Jan. 16th. 1863, "On the application of Photography to the Magic Lantern, educationally considered." Of what nature is this movement? Experience furnishes us with two distinct and well-marked types of motion, or rather with two different ways in which movement may be propagated to a distance. In the one we have a simultaneous transport of the movement and the body moved. This is the case with any vehicle or projectile. In the other type the movement is not that of a single body carried from place to place, but of a succession of particles, which in turn take up the impulse from the particles behind, and pass it on to those before, none of the particles being themselves more than slightly disturbed from their places. * Read before the Royal Dublin Society by Professor PURSER. |