The Fishes of Great Britain and Ireland. By Dr. Francis Day, F.L.S., &c. (London: Williams and Norgate, 1880.)

THIS work is to be issued in nine parts, of which the first, containing sixty-four pages of text and twenty-seven 'plates, is now published. Waiting until the completion of the work for a more extended notice, we may for the present mention that in it the author purposes to give a natural history of the fishes known to inhabit the seas and fresh waters of the British Isles, with remarks on their economic uses and on the various modes of their capture, and that an introduction to the study of fishes in general is promised.

The synonymic lists of the species are given in great detail; the descriptive diagnoses treat of internal peculiarities as well as of external form; a good many interesting details appear under the headings of Habits, Means of Capture, Baits, Uses. The plates are from drawings by the author, and are very excellent.

A Manual of the Infusoria. By W. Saville Kent, F.L.S. (London: David Bogue, 1880.)

THIS Sometime promised work has now advanced so far in its publication as the third part; when completed it will merit a somewhat lengthened notice, as the most important work on the subject which has issued from the British press. It is intended to include a description of all known flagellate, ciliate, and tentaculiferous Protozoa, British and foreign, and an account of the organisation and affinities of the Sponges. Each part (roy. 8vo in size) contains over 140 pages and eight plates. The general get-up of the work is magnificent, rather too much so for the poor student, already weighed down by the burden of the parts of Stein's " Infusionsthiere," but very pleasant for the book fancier, and forming an imposing shrine wherein to inclose the records of these early-life forms.

The first five chapters (pp. 1-194) are introductory, treating of the general history of the group: on the subkingdom Protozoa, on the nature and organisation of the Infusoria, on spontaneous generation, on the nature and affinities of the sponges. The sixth chapter treats of the systems of classifications of the Infusoria, adopted by various authorities, from the time of O. F. Müller to the present date. The seventh chapter commences the systematic description of the Flagellata. The third part, just published, carries the work as far as the 432nd page and to the twenty-fourth plate.

...

A Complete Course of Problems in Practical Plane Geometry. with an Introduction to Elementary Solid Geometry. A New, Revised, and Enlarged Edition. By J. W. Palliser. (London: Simpkin, Marshall, and Co., 1881.)

THIS is a cheap manual, the cost of which can be easily met by any artisan desirous of studying the subject, while at the same time its contents enable it to fully satisfy the wants of all examinees in first, second, and third grade and similar papers of the Science and Art Department Examinations. The figures are very clearly drawn, well showing given, constructional and required lines; the form of the page enables four propositions to be fully treated of with the accompanying figures in four spaces on each page. In the constructions we do not look for novelty, but we have conciseness and great clearness generally prevailing. Here and there elegance of expression is sacrificed to brevity ("for all the Government examinations, the requirements of which this is a textbook, the same rules will apply, with exception of Nos. I and 6"). We have detected only three points which call for our notice in Prop. 12 it strikes us as being simpler to use the same radius throughout, thus doing away with the necessity of taking two cases, as Mr. Palliser does; in Prop. 37, note, it is necessary to add how the point is

obtained; in Prop. 212 the letter E is made to do double duty in the proof. We can confidently recommend the book.

Bericht über die Thätigkeit der Botanischen Section der Schlesischen Gesellschaft im Jahr 1877. Erstattet von Prof. Dr. Ferdinand Cohn.

MOST of the papers in this part are in abstract; a few however are given at some length, and are of considerable interest. The additions to the phanerogamous Flora of Silesia and the record of new localities for rare plants Occupy a considerable part of the pamphlet. Perhaps the most interesting paper is that on the Date-palm and Palm-forest at Elche in Spain, by General von Schweinitz. The palms there grow to a height of from seventy-five to eighty feet. The plants grow for about 100 years, then become stationary, and next decay. Each tree bears from the fifth year two to five bunches of fruit, each with from 500 to 600 dates, the weight of dates yielded by one tree being sometimes three centners. Many of the papers in this part are contributed by Goeppert and Cohn, and deal with all departments of botany. Dr. Thalheim describes a series of models of diatoms made in paraffin and glycerine soap, which exhibited the structure of all the chief groups of this order of plants.

LETTERS TO THE EDITOR

[The Editor does not hold himself responsible for opinions expressed by his correspondents. Neither can he undertake to return, or to correspond with the writers of, rejected manuscripts. No notice is taken of anonymous communications. The Editor urgently requests correspondents to keep their letters as short as possible. The pressure on his space is so great that it is impossible otherwise to ensure the appearance even of communications containing interesting and novel facts.]

Dr. Carnelley's Hot Ice

THE remarkable observation made by Dr. Carnelley that ice in a vacuum is very permanent, even though surrounded by and apparently in contact with very hot bodies, has caused him to suppose and maintain that the ice itself is at a high temperature ; calorimetric determinations. This proposition has naturally met a supposition which has been apparently confirmed by preliminary with a good deal of scepticism, and certainly requires ample and cautious verification; but I venture to think that there is nothing in it contradictory to our present knowledge of the properties of matter, though if verified (as, for the reasons to be stated, I fully believe it will be) it constitutes an important addition to that knowledge.

The notions which have occurred to me have made the essential

part of the phenomenon so much clearer to myself that I fancy they will not be uninteresting to your readers.

By the term " vapour-tension at a given temperature I mean, as I believe is usual, the pressure at which a liquid and a vapour can exist permanently together at that temperature, or the maximum pressure which the vapour is able to exert at that temperature, or the vapour pressure under which a liquid ceases to the term "boiling-point " I mean the temperature of a liquid evaporate, or the total pressure at which it begins to boil. By under a total pressure equal to its vapour-tension.

Now in order that a solid may sublime or pass directly into the vaporous condition without melting, it must be either at a temperature below the melting-point, so that no liquid attempts to form, or else at such a temperature that any liquid formed shall instantly evaporate; which it would certainly do if it were above the boiling-point, that is if the total pressure on it were less than the vapour tension.

A solid, under either of these circumstances, gives off vapour from its free surface at a rate depending on, but not necessarily proportional to, the supply of heat; for there is no definite subliming point for a solid, any more than there is a definite evaporating point for a liquid, so that the temperature of the solid need not remain constant. When a liquid is evaporating, the more you heat it the faster it evaporates, but not at a compensating rate, and the temperature rises as well: if this be true for a liquid, much more will it be true for a solid, whose

evaporation is always more encumbered, partly, no doubt, because its evaporating surface is a fixture. The only limit to the rise of temperature of a liquid is its boiling, but if this be prevented it may get superheated; and, funless the solid boil (i.e. disintegrate internally) it can become superheated to any extent. The possibility of this internal disintegration we will examine directly, but at present we will consider it practically nil.

Let us grant then that a subliming solid always rises in tempe. rature if heated at a sufficient rate, and Dr. Carnelley's proposition follows.

We have seen that no liquid can exist at temperatures below its freezing- or above its boiling-point, so that if we wish to prevent the possibility of its existence, we need only make these two points coincide. This can always be done by diminishing the pressure, for the boiling-point of all substances is greatly affected by changes of pressure, while the freezing- oint is only slightly altered, and even, in the case of ice, in the opposite direction.

Start then with the solid below its melting-point, and reduce the pressure on it till the boiling point coincides with, or passes below the melting-point. There is now no region where liquid can exist, and the solid must therefore sublime; but, by our supposition, a subliming solid if heated will get hot, hence the solid may now assume any temperature you please; and the hotter it gets the more pressure may be brought to bear upon it without causing it to melt, i.e. the pressure may be allowed to increase to anything short of the vapour tension at the new temperature. If heated sufficiently, then the whole atmospheric pressure may be let in, and no melting will occur. All that is necessary is that heat shall be supplied at a sufficient rate to compensate for the rapid evaporation (which however will not be so rapid as in the vacuum), and to prevent its temperature falling to the boiling-point; for if it reached this, part (or all) would quickly liquefy, and the whole fall to (or towards) the melting-point.

Thus we have the remarkable proposition that if, by the process of lowering the boiling-point to coincide with or pass below the melting-point, we manage to get ice across the gap which ordinarily separates the e two points, it may be heated to 120° or to any other temperature; and that when at 120° it will be permanent, and will not melt even under the whole pressure of the atmosphere. To prevent its melting you must keep on heating it if allowed to cool to 100°, five-eighths of it will be instantly crushed to water, and the whole will be at o° (assuming, what is not likely to be correct, that the specific heat of hot ice is ). There is still the question of the possibility of internal melting

:

or sublimation to be considered.

Now I suppose that if a solid is perfectly homogeneous, a change of state in its interior would with great difficulty occur, and the solid might readily be superheated. But an excess of pressure at any point, such as would be produced by a bubble of air, would readily determine a melting-centre. In Prof. Tyn dall's ice-flower experiment the nuclei are probably minute bubbles of air, and the ice walls of the cavities so produced are subject to the pressure of this air in addition to that of the vapour; and accordingly melting sets in and spreads. But Dr. Carnelley's ice is formed in vacuo, so that no air-bubbles are pos sible, and the only nuclei that can properly exist are little bubbles of enclosed vapour; and these, I imagine, can scarcely be absent. Let us inquire then what can happen in the case of one of these bubbles when the temperature of the ice is raised either by radiation or conduction. Initially, while the temperature is constant, the vapour is saturated; but no liquid is formed because this temperature is below the melting-point. When heat is applied, the ice, being less diathermanous than the vapour, will get heated first, and so long as the temperature keeps rising it will always be a little hotter than the vapour, which consequently is not quite saturated, and the pre-sure it exerts is less than the "vapour-tension" (ie. the temperature is above the boiling point), and no water can be formed. The cavity will of course enlarge by sublimation, but very slowly, much more slowly in fact than outside, if a vacuum is there artificially maintained.

But if cooling be permitted the ice will cool the fastest; and the vapour at once becomes over-saturated and condenses. The temperature is now below the boiling point, and liquefaction instantly sets in and rapidly spreads, the ice consuming its own heat in the process.

Internal disintegration therefore will not occur while the temperature is rising, but it will set in at a great pace if it be allowed

to become stationary or to fall, unless there be an utter absence of nuclei. If the temperature rises very high the pressure of the internal vapour will of course be great, and ultimately might even be able to burst the ice, but this would scarcely occur under several atmospheres.

It would be interesting if Dr. Carnelley would kindly try the following experiments :

1. Heat ice in vacuo with a pressure gauge, and, still heating it, stop the passage to the condenser so that the pressure is allowed to accumulate, and note the pressure and temperature when collapse occurs.

2. Heat ice up to any temperature, and, still maintaining a good vacuum, remove the supply of heat, and see if the ice does not collapse.

3. Heat the ice up to 120°, and, still heating it, let in the atmosphere gently (but make the air come in through hot pipes, or it will melt the ice), and see if the ice does not last rather longer than it would have done in the vacuum, because the evaporation will be more obstructed. But if the second experiment succeed, the temperature must never be allowed to fall much or to remain stationary long.

Finally, it is important to point out explicitly that the Carnelley experiment has no bearing on the change of the melting-point of ice with pressure. Our knowledge on this point remains as it was, viz. that the value of de about zero centigrade is '0071; that is to dp say, the melting-point rises and falls about 0071° centigrade per atmosphere of pressure decrease or increase.

Note. With reference to the above second experiment and the reasoning which suggested it, it is important to remark that I have all along assumed that the vapour-tension of ice at any temperature is precisely the same as that of water at the same temperature. But Prof. Foster considers it possible that the vapour-tension of ice may be less than that of water, and would hence explain the permanence of vapour inside an ice-cavity without attending to whether the temperature were rising or falling, provided it were not falling too fast. This would be a most important fact to discover and verify; but I think the Carnelley experiment in its present form does not inform us concerning its truth or falsity.

Another thing it may be interesting to note is the rate of variation of boiling-point with pressure at different temperatures, which can be calculated on thermodynamic principles (after Prof. James Thomson) from empirical data for the latent hent of steam, and for the density of saturated steam at any tempe

I HAVE a great respect for Dr. Watts's spectroscopic work, nevertheless the experiments he has described in NATURE, vol. xxiii. p. 197, appear to me singularly inconclusive for the purpose for which he has adduced them. How could any one expect to get a tube of gas free from hydrocarbons when the joints were of india rubber and melted paraffin? I have long since found it necessary to forego rubber joints if I would exclude hydrogen. Salet has shown that the hydrocarbons from the blowpipe-flame used in sealing in wires, &c., and the last traces of dust, can only be removed from tubes by burning them out in a current of oxygen. But more than this, I have found

that even with joints all made by fusion of the glass it was well nigh impossible to get rid entirely of hydrogen. Mr. Crookes has, I believe, found that the last traces of moisture adhering to glass can only be expelled by heating to the softening point of the glass. This tallies with my own experience. In a series of experiments on the ultra-violet water spectrum I had occasion to photograph the spectra of sparks in sundry gases wet and dry, and found that in gases which had been passed through a tube full of phosphoric anhydride the water-spectrum still appeared strongly. Even when the gas had been passed very slowly through two tubes each half a meter long filled with calcium chloride, and then through a similar tube full of phosphoric anhydride, and the part of the tube where the wires were sealed had been heated strongly for a long time, while the current of gas was passing, traces of the water spectrum still often appeared. But Dr. Watts did not see the hydrogen lines in his tube. My difficulty has always been to avoid seeing them when the pressure of the gas was sufficiently reduced and a large condenser used with the induction coil. True: tubes of gas may not always show them even when hydrogen is known to be present. The spark takes a selected course of its own, and does not always light up all that is in the tube. Carbonic oxide does not generally show oxygen lines, and in tubes exhausted by a Sprengel pump the lines of mercury do not usually appear until the pumping has been carried far. A real test would be to see whether when the spark gives the line-spectrum of carbon the hydrogen lines do not also appear. The experiment with naphthaline Prof. Dewar and I have repeated and discussed else. where, so I will say no more on it than this, that purity in regard to chemicals is a relative rather than an absolute quality, and that it is only from a long series of experiments chosen with a view to eliminate the effects of accidents of all kinds that any safe induction in this kind of spectroscopy can be reached. Cambridge, January 4 G. D. LIVEING

[To save time we submitted Prof. Liveing's letter to Mr. Watts, who sends the following reply.-ED.]

I SEE no reason why india-rubber stoppers may not be used in the construction of an apparatus to be filled with a gas at atmospheric pressure, or nearly so. The case would be altogether different if we were concerned with the construction of a vacuum tube, and I take it that most of these statements of the difficulty of getting rid of the last traces of moisture and of hydrocarbons adhering to the glass refer to cases where the pressure is to be only a few millimetres. But when a current of cyanogen at atmospheric pressure, made from dried mercuric cyanide, is passed through a U-tube filled with phosphoric anhydride, the gas is surely dry to all intents and purposes (I do not say that the glass would not give off traces of moisture, &c., if the pressure were to be reduced to an extreme point); at least there can be so little hydrogen present in the tube that to ascribe the spectrum given by the tube to the hydrogen present in it is to adopt an extreme hypothesis, which must be supported by cogent experimental evidence before it can be accepted.

But if the defect of the experiment be in the use of indiarubber there can be no great difficulty in constructing the apparatus entirely of glass, and if we are to give up the view that the groups (5165 to 5082) and y (5635 to 5478) are due to carbon, it must be shown that they are not present in the spectrum of the spark in cyanogen at atmospheric pressure when sufficient precautions are taken to obtain the gas pure. I have never examined the spectrum of the spark in cyanogen without seeing them, and have every confidence that Prof. Liveing will still find them there after he has taken all the precautions he may think necessary.

But admitting for the sake of argument the justice of Prof. Liveing's contention that the cyanogen in my experiment con. tained a trace of hydrogen and that the naphthalin contained a trace of nitrogen, then this seems to be the theory offered for our acceptance-that the spark in nitrocarbon gas containing a trace of hydrogen gives the lines of hydrocarbon, and that the spark in hydrocarbon gas containing a trace of nitrogen gives the lines of nitrocarbon. Does Prof. Liveing hold both of these hypotheses W. M. WATTS

to be reasonable?

Geological Climates

THE letter of Prof. Haughton in last week's NATURE SO bristles with figures and calculations that some of your readers

may feel a little puzzled and may be unable to detect the fallacies that lurk among them. The question is far too large a one to be fully discussed in your columns. I shall therefore confine myself to pointing out the erroneous assumptions and false inferences which vitiate all the learned Professor's calculations, having done which my own theory will remain, so far, intact.

The whole argument against me is based upon an "ideal icecap," extending from the Pole to lat. 60°. A considerable but unknown thickness is given to this imaginary field of ice, and it is then calculated that the three great ocean streams, even if admitted to the Arctic area in the manner I suggest, would not get rid of this mass of ice. There are however several important misconceptions and illogical deductions underlying the whole argument, and when these are exposed the results, however accurately worked out, become completely valueless.

We first have it stated that if heat and cold were uniformly distributed over the Polar regions the whole would be permanently frozen over, and an ice-cap be formed of great but varying thickness, diminishing from the Pole to about lat. 60°. But even this preliminary statement is open to serious doubt; for ice cannot be formed without an adequate supply of water, and over a large part of the Polar area no more snow falls than is annually melted by the sun and by warm southerly winds blowing over the heated land-surfaces of Asia and America. Admitting however that any such ice-cap could be formed, it would certainly not form in one year but by the accumulations of a long series of years; and any estimate of the total heat required to melt it has no bearing whatever on the annual amount that would be sufficient, since this depends solely on the average thickness of the ice annually formed, of which Prof. Haughton says nothing whatever.

The amount of rainfall in the Arctic regions (mostly in the form of snow) is certainly very small. It is estimated by Dr. Rink to be only twelve inches in Greenland, and this is probably far above the average. All that falls on the inland plains of Asia, Europe, and America is however melted or evaporated by the action of the sun and air far from the influence of the Gulf Stream. The thickness of ice formed annually over the whole area of the Arctic Ocean I have no means of estimating. In open water in very high latitudes it may be considerable, but perennial ice-fields can only increase very slowly. I should therefore very much doubt if the thickness of ice now formed annually over the whole Arctic area averages nearly so much as five feet; and Prof. Haughton himself calculates that our own Gulf Stream is now capable of melting this quantity.

The first assumption, therefore-that the amount of heat required to be introduced into the Arctic regions in order to raise their mean temperature above the freezing-point is "accurately measured" by the amount required to melt an "ice-cap " covering the whole area to a thickness of several hundred feet-is grossly erroneous; and it is so because it takes the hypothetical accumulated effects of many years Arctic cold under altogether impossible conditions, and then estimates the amount of heat required to melt this whole accumulation in one year!

But we find a second and equally important error, in the assumption (involved in all Prof. Haughton's arguments and figures) that all the ice of the alleged "ideal ice-cap" must be melted by that portion of the Gulf Stream which actually enters the Polar area, where its temperature is taken to be 35° F. or only 3° above the melting point of ice. A large quantity of the Arctic ice, however, even now floats southward to beyond lat. 50° in both the Atlantic and Pacific, and is melted by the warmer Now, as it is an essential part of my theory that much of Northern water and atmosphere and the hotter sun of these lower latitudes. Asia and North America were under water at those early periods when warm climates prevailed in the Arctic regions, it is clear that whatever Arctic ice was then formed would have a freer passage southwards, and as the south-flowing return currents would then have been more powerful and more extensive than at present, a much larger proportion of the ice would have been melted by the heat of temperate instead of by that of Arctic seas.

Prof. Haughton admits that the Kuro Siwo and the Mozambique currents together, if they entered the Polar seas, would be equal to the melting of a layer of ice more than thirteen feet thick over the whole area down to lat. 70°. But if our own Gulf Stream is sufficient to get rid of the whole of the ice that now forms annually-as Prof. Haughton's figures show that it would probably be, and as it would be still more certainly were Greenland depressed, thus ceasing to be the great Arctic refrigerator and ice-accumulator-then the heat of the other two currents would be employed in raising the temperature of the Arctic seas above

the freezing-point; and if we take the area of the water as about equal to that of the land, we shall have heat enough to raise the whole Arctic ocean to a depth of full 180 feet more than 20° F., or to a mean temperature of 52° F., and as this would imply a still higher surface temperature it is considerably more than I require.

Unless therefore Prof. Haughton can prove that the amount of ice now forming annually in the Polar regions is very much more than an average of five feet thick over the whole area, his own figures demonstrate my case for me, since they prove that the rearrangement of land and sea which I have suggested would produce a permanent mild climate within the Arctic circle and proportionally raise the mean temperature of all north-temperate lands.

Briefly to summarise my present argument :-Prof. Haughton's fundamental error consists in assuming that the true way of estimating the amount of heat required in order to raise the temperature of the Polar area a certain number of degrees is, first, to suppose an accumulation of ice indefinitely greater than actually exists, and then to demand heat enough to melt this accumulation annually. The utmost possible accumulations of ice in the Arctic area, during an indefinite number of years, and under the most adverse physical conditions imaginable, are to be all melted in one year; and the heat required to do this is said to be the "accurate measure of that required to raise the temperature of the same area about 20°, at a time when there were no such great accumulations of ice and when all the physical conditions adverse to its accumulation and favourable to its dispersal were immensely more powerful than at present !

When this fundamental error is corrected, it will be seen that Prof. Haughton's calculations are not only quite compatible with my views, but actually lend them a strong support.

ALFRED R. WALLACE

By the courtesy of Mr. Ingram I am enabled to say that the tree at Belvoir supposed to be Araucaria Cunninghami is in reality, as surmised by Capt. King, Cunninghamia sinensis. The Cunninghamia is a native of Southern China, whence it has been introduced into Japan. In this country it was originally grown under glass, but, as the instance at Belvoir illustrates, such protection is not absolutely requisite. The tree is however somewhat tender, and so far as I know has never produced its cones in this country in the open air.

As to the Bamboos bardy in this country, it may be well to warn those who are not familiar with the plants not to expect to see the gigantic and rapidly-growing grasses that go under this name in the tropics. Rarely indeed do they attain in this country the dimensions even of the Arundo donax, so familiar to travellers in Italy. As accuracy of nomenclature is proved in this and the foregoing instance to be a matter of much moment, it may be well to say on the authority of the late General Munro that the Himalayan plant commonly grown in gardens as Arundinaria falcata is more correctly called Thamnocalamus Falconeri, that the Bambusa gracilis of gardens is the true Arundinaria falcata of the Himalayas, and that the Japanese Bambusa metaké is Arundinaria japonica. General Munro's monograph of this group is to be found in the twenty-sixth volume of the Transac tions of the Linnean Society, part 1, 1868, while his remarks on the cultivated species may be found in recent volumes of the Gardeners' Chronicle, particularly in vol. vi. 1876, p. 773.

The simultaneous flowering of Thamnocalamus Falconeri a few years ago in all parts of Europe created much attention, and was indeed a remarkable illustration of hereditary tendency manifested under very varied climatal conditions. The flowering of this grass was by no means looked on with unmixed gratification, as it entailed as a consequence the death or protracted enfeeblement of the plant.

A visit to Kew or to any of our larger nurseries will suffice to show that there are other Bamboos (that is, grasses belonging to the group Bambuseæ, if not true Bambusas) which are hardy enough to withstand even such rigorous winters as those of 1878-9 and 1879-80. MAXWELL T. MASTERS

Climate of Vancouver Island THE letters on this subject which have appeared in NATURE (vol. xxiii. pp. 147, 169), have reminded me of a "Prize Essay on Vancouver Island. By Charles Forbes, Esq., M.D., M.R.C.S.Eng., Surgeon Royal Navy," which was published by the Colonial Government in 1862. It consists of sixty-one

Dimorphic Leaves of Conifers

IT is now generally believed that some of the varying forms assumed by individual plants or animals in the course of their de velopment are as it were the reflex of an ancestral state of things. From this point of view the different forms of leaves assumed by particular importance. The Retinosporas now so common in some Araucarias, as well as by many other conifers, become of our gardens and on our balconies represent an immature stage of some Thuya, the proof of which statement is occasionally furnished by the plants which suddenly assume the foliage characteristic of that genus. In various species of juniper, notably in the Chinese juniper, two forms of leaf representing the juvenile and the adult condition occur together on the same branch.

Assuming that the juvenile, or "larval" forms, as they have been called, do really represent previous conditions in the history of the species, it might be expected that some of the fossil coniferæ would be characterised by the possession of this larval foliage to the exclusion of any other. But if I mistake not both forms of foliage have been met with in fossil as in recent conifers, and the pedigree of these plants is by so much the more pushed back.

The resemblance in the form and arrangement of the adult leaves in some Thuyas and allied plants to the disposition of the leaves in Selaginella should not be overlooked in this connection nor the close resemblance between the foliage of some species of Lycopodium proper and the "larval" leaves of many conifers as above referred to.

MAXWELL T. MASTERS

Dust and Fogs

THE meteorological conclusions of Mr. Aitken's important paper, published in NATURE, vol. xxiii. p. 195, will, if adopted without further examination, even temporarily, exercise an unfortunate influence upon the present attempts to rid the atmosphere of our large towns of their ever-recurring fogs, glooms, and mists, and those conclusions certainly are not supported by such evidence as we already have as to the production of fogs on a great scale, however much indicated by experiments in the laboratory. It is stated that, "It having been also shown that all forms of combustion, perfect and imperfect, are producers of fog nuclei, it is concluded that it is hopeless to expect that, adopting more perfect forms of combustion than those at present in use, we shall thereby diminish the frequency, persistency, or density of our town fogs." Now, first as to frequency: what are the facts with regard to localities differing in their methods or materials for producing heat? Every one living in or near London knows that fogs, thick mists, and dark days are far more frequent within than without its circumference, and experiment has shown that sunshine is both less frequent and much less intense within the metropolis. And, according to Mr. Aitken's theory, something of the same kind ought to be observed wherever large quantities of fuel are burned, whether smokeless or not. Thus, the large towns of the Continent, where wood and charcoal are in general use, would have their peculiar urban fogs. But they are free from any fogs beyond those which are common to the country. And Paris, before coal was much used, ought to have been distinguished by more frequent fogs than the surrounding country. But it was not so marked out. No oasis

of fog prevailed there when the sun shone brightly beyond its precincts, as in our own capital. And Philadelphia, which burns

Next, as to persistency. Early in the morning of January 31 last, in some districts of London the fog extended considerably above the tops of the houses, in others only about 10 or 20 feet from the ground in any intensity. Where the fog extended high the smoke mixed with it and produced a yellow fog, but where it remained low the smoke escaped into the upper air and drifted away, leaving a white fog below, so pure as to be a very unusual phenomenon at 10 a. m. in a London street. Now it was remarkable, that wherever the white fog prevailed in the morning, the sun soon obtained the mastery and dispelled it more or less, but in the smoke-obscured districts a dark yellow fog continued throughout the day.

White fogs may doubtless be exceedingly dense. But will not an admixture of smoke increase its density?

A humid atmosphere is not necessary for the production of mist and haze. The frequent long-continued prevalence of blue haze over the whole country, not excepting the east coasts, in the driest east winds of spring, would be a subject deserving investigation. They sometimes extend to a height much above the tops of our highest mountains. Experiments such as those of Mr. Aitken will, we may hope, ultimately solve this problem of meteorology.

Low Temperature

R. RUSSELL

THE reading of the thermometer here last night, January 15, 16, was the lowest ever recorded at this observatory in the course of thirty-three years. The reading was 4°6 F., the previous minimum having occurred on December 24, 1860, when the mercury stood at 6°7 F. S. J. PERRY

Stonyhurst Observatory, January 16

A "Natural" Experiment in Polarised Light BREAK off a plate of ice and hold it between the sky and a pool of water. Its reflected image will show the beautiful colours due to polarised light. The incident rays should come from a part of the sky about 90° from the sun, and reflection should take place at the polarising angle for water, and the plate will probably require adjusting to bring out the maximum effect. Water, vaporous, solid, and liquid, thus furnishes us with polariser, crystal, and analyser I do not remember to have read any account of this very simple experiment, for which Nature provides all the materials. CHAS. T. WHITMELL

9, Beech Grove, Harrogate, January 10

STATICS AND DYNAMICS OF SKATING

MANY years ago, when skating was but in its infancy. skates were made of bone, and if they could be made to stay on the feet they were considered to answer their purpose sufficiently well.

More recently iron runners with wooden beds came into use, and accuracy of adjustment on the foot, horizontally and longitudinally, was made easier by means of leather straps and a screw passing into the heel of the boot; and these adjustments, made haphazard, were quite sufficient for the skating of those days, namely forward skating.

Within the last twenty years however skating has made enormous strides, back skating becoming an essential qualification of a finished skater; and hence not only more perfect forms of skate are demanded from the maker, but also the adjustment of them on the boot becomes an important part of his duty.

There are three points to be attended to in the adjustment of the skate, besides the obvious one of placing the skate medially on the foot.

1. Height of foot off the ice where the greatest breadth of the sole of boot occurs.

2. Height of foot off ice at the heel.

First. The height of the foot from the ice should be such as will enable the skater to lean over sufficiently when on a curve, and such that he may be able to get a powerful enough stroke. If he is too low the edge of the boot will come in contact with the ice in leaning overt and also in taking a stroke: a fall ensuing in the firs, case, and a disagreeable and dangerous overstrain in the second. To avoid these the sole of the boot should subtend an angle at the bottom of the runner of about 96 deg. i.e. for a sole 3 inches broad the edge of the runner should be 1 inch from the sole, instead of varying from 1 to 1 inch, which are the heights of skates commonly met with.

This angle of 96 deg. will be found to clear the ice in both striking and leaning over for most skaters, and any greater height than is given by this angle should not be used, as it is not necessary, and only throws an additional

strain on the ankle.

and soles are equally prominent, but high heels must be sunk into the skate-woods." This was quite correct at that time, when back skating was little practised, and when the skate which was then worn was made very flat, in fact almost straight at and near the heel. Now, by

universal consent for figure-skating, the iron is made a segment of a single circle from toe to heel, 7 feet being the radius. Yet, notwithstanding these changes, Vandervell and Witham, as lately as January, 1880, in their "Figure Skating," recommend the very same parallelism of the foot to the skate instead of parallelism of the top of the blade to the ice, as it should be for modern skating, as I shall subsequently show.

In Fig. 1 is shown the result of adopting Dove's or Vandervell and Witham's position, i.e. no heel. It might be thought that a person standing on a curve would balance comfortably at the middle of the curve, but this cannot be, for a person standing naturally on a level surface does not distribute the weight of his body equally over the length of his foot, but by far the greater part comes on the heel, and therefore the centre of pressure of his body is nearer the heel than the toe, and consequently if he is standing on a curve the curve must roll up in front and down behind till the upward pressure of the ice just passes through the centre of pressure of his body. The point of contact of the skate on the ice will therefore not only be much behind the centre of the

his body when standing on a level surface, as he now