As an entozoologist and correspondent of the Academy of Natural Sciences of Philadelphia, I request permission to correct an error recorded in the report of the Academy as given in your columns (at p. 500) this week. Dr. Leidy is represented as having stated that "the minu e acetabular pit or fovea at the summit of the head [of Tania mediocanellata] is not mentioned by Kuchenmeister and subsequent observers as a character of that species. I beg to remark that I both figured and described this supplementary sucker-like structure in the first edition of my small work on Tapeworms," published in 1866 (p. 33 et seq). At least two other observers have figured and described this central depression, not only in the adult but also in the measle or cysticercal stage of the worm. Even Bremser recognised it, but his description was for a time overlooked.

84, Wimpole Street, London, Oct. 21 T. S. COBBOLD

Winter Fertilisation

IN the first number of NATURE, (for Nov. 4, 1869,) I ventured on a hypothesis, founded on a series of observations, that plants which flower in the winter have their organs of reproduction specially arranged to promote self-fertilisation. The following fact, which has just come under my notice, appears to confirm this theory. Plan's belonging to the order Caryophyllacea are, as a rule, strongly protandr us (see my paper in the Journal of Botany for October 1870), the anthers discharging their pollen at so long an interval before the maturing of the stigma as to render cross-fertilisation almost inevitable. The other day, Oct. 21, I came across a late flowering patch of Stellaria aquatica Scop., in which the anthers were discha ging their pollen simultaneously with the maturing of the stigmas, each of the five styles being curled in a sirgular manner round one of the stamens, so as to bring the stigmatic surface in actual contact with the dehiscing anther. This occurred in several flowers that were just opening, and there was abundance of seminiferous capsules on the plants. ALFRED W. BENNETT

Velocity of Sound in Coal

YOUR correspondent will find in Prof. Tyndall's beautiful work on "Sound" the data required for the exact determination of its velocity in different media. I believe that in coal it will be found to be between six and seven times that in air, or about 7,000 feet per second.

If Mr. D. Joseph places his ear against the solid coal of the "rib" or side of the "heading" or gallery, at a distance of some twenty to thirty yards from a collier at work, he will hear two sounds for each blow of the workman's pick or mandril-the first being transmitted through the coal, the second more slo ly through the air, the impression being almost irresistible that two persons are at work.

This is probably the origin of the legend, common in more than one coal district, of a collier who always worked alone, did more work than his fellows, and whose diabolical assistant was often heard but not seen. C. J.

Changes in the Habits of Animals

YOUR correspondent Mr. Potts in the last number of NATURE furnishes us with a few interesting facts regarding the Kea. In a paper which I read about three years ago to the Dumfries Natural History Society, entitled "The Influence of the Human Period on the Sagacity of Animals," and subsequently in a letter published in NATURE, vol. i., on the "Mental Progress of Animals," I endeavoured to show from general considerations, and from the few facts which we possessed on this subject, that the habits and instincts of animals were not so fixed and definite as might be supposed. The general principle for which I contended was that whether we considered the globe to have received

its human inhabitants according to the laws of evolution, or in some miraculous manner, the arrival of the human race produced great modifications and changes of surrounding circumstances. These changes were in the direction of increasing the fertility of all vegetable productions capable of sustaining life, and at the same time securing their use entirely for the human family. Hence arose, in the vicinity of man, two new factors; the superior attraction of better food for all kinds of animals, and at the same time the extinction of such animals whose greed was not overruled by sufficient wariness or cunning to become successful thieves. Hence a probable gradual increase in these qualities in the animals maintaining themselves against man, Since my attention was drawn to this subject, we have had some interesting observations on modifications of swallow's nests by Pouchet, and a discussion as to the validity of his conclusions by Noulet, and now I have read with pleasure Mr. Potts's observations. Most likely the progress of development in the carnivorous habits of the Kea will meet with a check now that shepherds are alive to its depredations; but without the influence of the human period we can scarcely suppose that such development would have begun. I recollect a case of change of habits in weasels. They multiplied so thickly in a parish in the south of Dumfriesshire that some hungry philosopher among them took the initiative in sucking the blood from the cattle. Suspicion having been aroused, the fact was proved, but its discovery was fatal to the weasels, for the whole country-side arose against them, and all but extirpated them in that quarter. It is very interesting to observe what modifications are being produced in the habits of various species of sea-gulls since Glasgow, by its enormous increase of commerce, has wrought great changes in the River Clyde, filling it with all kinds of garbage. The conditions of existence having been avourable, the gull is steadily passing more and more time inland; ascending tributaries of the Clyde, and alighting in flocks on fields that used to have him very seldom.

A

new amusement within my own recollection has been afforded the river passengers during the summer months in feeding these sea mews, &c., by throwing overboard food to them, and their increased tameness and boldness of approach in following the river steamers within the last thirty years have been frequently commented on. J. SHAW Oct. 23

THE words " aspect" and "slope" have already a use in relation to the position of planes. They indicate two elements which together fix the position. Neither of them, taken alone, can indicate the position of a plane, unless a new and artificial meaning be assigned to one or other. Thus if I speak of the "aspect" of one of the faces of a roof as southerly, I have done something but not all that is necessary, towards describing the position of that face; if I add further that the "slope" is 30° I have definitely assigned the position. Again if I speak of the "slope" of Saturn's rings as 28° (the plane of reference being ecliptic), I have done something towards the description of their position; if I add further that their "aspect" is toward such and such a degree of the sign Gemini, I fully assign their position in space. And so on.

In the preceding sentences I have used the words "slope" and "aspect" as they are already understood. I apprehend that I have also used the word "position" as it is already understood, and that no other word could properly be used in the same sense in descriptive writing. I can see no reason why "position" should be dismissed from the position it has so long occupied, nor why "aspect" and "slope" should be regarded in a new and unfamiliar aspect.

It chances that I have long since had occasion to consider the question suggested last month by Mr. Wilson. In each of twelve books which I have written during the past six years, I have had repeated occasions to consider the slope and aspect, that is, the "position" of many important astronomical planes.

Sea-water Aquaria

I HAVE read with much gusto your article upon the Crystal Palace Aquarium. I am induced by it to put forward a caution with regard to the construction of rock-work in tanks.

Several weeks ago, casually looking over a heap of Bangor slaty rock, on the road bordering the Brighton Aquarium works, and being used for the rock-work of tanks, my attention was attracted by some bright green patches upon some of the stones, which appeared to me to be carbonate of copper, but was probably silicate. Looking further at one with a lens, I imagined that I could also distinguish particles of pea cock ore. On attempting to purloin a specimen, I was very properly stopped from so criminal an act by the Cerberus in charge. I wrote to the chairman of the company, stating that, not having examined the stone, I might be only contributing a mare's nest to their zoological collection, but that if it contained much copper the fish would be in danger. I understand that upon receipt of my letter some rock was sent up to Dr. Percy, whose report, I am told, was to the effect that there was much sulphide of copper, and that the pretty green rock was therefore unfit for tank rock-work.

I think this will serve as a caution to the constructors of aquaria carefully before using it. There are so many minerals which would be deleterious that I strongly advise an analysis and report in the case of every untried rock. The accident of my passing a heap of stones has saved the company, with which I am not in the least connected except as a fervent well-wisher, from a large expenditure and a serious scrape.

to examine all material which is to be in contact with water most

Allow me to ask those who are accustomed to the management of tanks, whether hydraulic pressure upon a small and strong one would be likely to assist in maintaining life in any of the deep-sea organisms, and whether it would be useful to make recesses for those loving darkness, with the axes opposite the plate glass side, so that a bull's-eye lantern could occasionally throw light upon their actions and mode of life? Brighton, Oct. 21

MARSHALL HALL

ON HOMOPLASTIC AGREEMENTS IN

AT

PLANTS

T the recent meeting of the British Association I pointed out in a short communication the difference that existed between mimicry in animals and what has been spoken of under that name amongst plants. The distinction was sufficiently obvious, and must have occurred to everyone who had given the matter any consideration, but my object was to try to raise a discussion upon the whole subject as exhibited in plants.

I fancy it is hardly sufficiently understood how commonly this agreement of facies occur in plants widely differing in other respects. I will give a few illustrations of it. Humboldt remarks ("Views of Nature," p. 351): "In all European colonies the inhabitants have been led by resemblances of physiognomy (habitus, facies) to apply the names of European forms to certain tropical plants, which bear wholly different flowers and fruits from the genera to which these designations originally referred. Everywhere in both hemispheres the northern settler has believed he could recognise alders, poplars, apple, and olive trees, being misled for the most part by the form of the leaves and the direction of the branches." Nor has the popular eye alone been deceived by these resemblances. Schleiden states ("The Plant," p. 255) that Australia has in common with Europe a very common plant, the daisy, yet Dr. Hooker has pointed out (Flora of Tasmania, pl. 47) that the plant intended by Schleiden is the very

Since I read my paper, I have met with an essay by Schouw, in which he enumerates facts of the same kind. "There is still," he says ("Earth, Plants, and Man," p. 61), "another kind of repetition which I might call habitual repetition, or denominate mimicry, if this expression was not at variance with the subjection to law which exists throughout nature, but to comprehend which our powers are often insufficient." After various illustrations he proceeds: In the genus Mutisia we have the remarkable sight of a compositous flo ver, with the tendrils of a leguminous plant." (This by an accidental coincidence burgh.) "In Begonia fuchsioides the leaves are similar was one of the instances which I, myself, used at Edinto a Fuchsia, and very different from the other forms of leaf among the begonias, and the colour of the blossom likewise reminds us of the fuchsias. We have another most striking example in certain Brazilian plants, which although possessed of perfectly developed flowers and fruits, mimic, as it were, in their leaves and stems, groups of plants of much lower rank." (He is alluding to the Podostemaceae mentioned above.) "Lacis fucoides retaken for one by a person who did not see the flowers. sembles certain seaweeds so much, that it might be misMniopsis scaturiginum strikingly resembles a Junger

mannia."

I suggested that when a plant put on the characteristic facies of a distinct natural family, it might conveniently be spoken of as a pseudomorph, having in view an obvious analogy in the case of minerals. I do not, however, now think on further consideration, that this term, although convenient, includes all the cases. In small natural families it is not always easy to recognise any general habit different families where this is the case, but having a or facies at all, and in the case of plants belonging to similar habit, it would be purely arbitrary to fix the pseudomorphism on any of them. Again all the individuals of distinct groups of plants might have a similar habit, and the same remark would apply. The difficulty is, however, got over by speaking of the plants in these cases as isomorphic.

My friend, Mr. E. R. Lankester, has pointed out to me that agreements of this kind may all come under what he has termed homoplasy (Ann. and Mag. of Natural History, July 1870). This is the explanation he gives of this expression:

"When identical or nearly similar forces, or environments, act on two or more parts of an organism which are exactly or nearly alike, the resulting modifications of the various parts will be exactly or nearly alike. Further, if, instead of similar parts in the same organism, we suppose the same forces to act on parts in two organisms, which parts are exactly or nearly alike and sometimes homogenetic, the resulting correspondences called forth in the several parts in the two organisms will be nearly or exactly alike. I propose to call this kind of agreement homo plasis or homoplasy. The fore legs have a homoplastic agreement with the hind legs, the four extremities being, in their simplest form (e.g. Proteus, which must have had ancestors with quite rudimentary hind legs), very closely similar in structure and function. . . . Homoplasy includes all cases of close resemblance of form not traceable to homogeny."

The resemblances, therefore, above described between the vegetative organ of plants with no close generic relations, may be described as homoplastic. The difficulty

having been published and described by Kunze as a species of Lomaria, a Perhaps one of the most striking is the Natal cycad Stangeria paradoxa genus of Ferns.

still, of course, remains to show how the homoplasy has been brought about. In some cases, as in the homoplastic forms of American Cactaceae and South African Euphorbias, or in the stipular bud scales of many wholly unrelated deciduous trees, the nature of the similar external conditions may possibly be made out with some correctness. Again, Dr. Seemann has pointed out that by the rivers in Nicaragua and in Viti, the vegetation, although composed of very different plants, puts on the willow form ("Dottings by the Roadside," p. 46). A phenomenon true of two distant places accidentally contrasted, might be expected to obtain more generally; at any rate, among our indigenous riparian plants Lythrum Salicaria and the willow-herb are, as their names indicate, additional illustrations. The band of vegetation that fringes a stream is always densely crowded with individual plants, and it is easy to see that elongated and vertically disposed leaves would be most advantageous, exactly as they are to the gregarious plants of meadows and plains. The homoplastic agreement of riparian plants may be therefore a direct result of selective effort due to the position in which they grow.

In other cases the operation of similar external moulding influences is not so easy to trace. It might, perhaps, however, be imagined that plants would hereditarily retain the effects when the influences had ceased to operate, and no new ones had come into operation precisely adapted to obliterate the work of those that preceded them. Suppose, for example, that willows got their habit and foliage from ancestors that were exclusively riparian, then any descendant that happened to be able to tolerate situations with less abundant supplies of moisture, would not necessarily lose their characteristic foliage on that account. Such races might be expected to occur near rivers subject to periodic droughts, since under these conditions any others would be likely to perish. Under such circumstances we should have cause and effect no longer in contiguity; the riparian habit surviving the riparian situation.

I suggested at Edinburgh that possibly similar habits in plants might be bronght about by different causes. This was only a suggestion, and probably what has just been said is a truer account of the matter. At any rate the illustration I gave of my meaning has been quite misunderstood (as, for example, in the last number of the Popular Science Review). It is well known that there are a certain number of plants indigenous to the British Isles, which are found at a considerable height upon mountains and also upon the sea-shore, but not in the intervening space. In the latter situations they contain more sodium salts than in the former, and inasmuch as these salts are destructive to many plants, those that compose a strand flora must be able to tolerate them, and this of course is an advantage, because many of their competitors are poisoned off. Similarly plants of mountains must have a similar advantage over others in ability to tolerate mountain asperities of climate. Now, suppose a mountain submerged; its flora and certain portions of that of the strand come to coincide. Then if we suppose the mountain gradually to emerge, some of these plants will spread downwards under the uncovered surface, and travel over the whole of the interval that ultimately separates the mountain-top and the strand. Why, then, do they not remain there? Simply, I believe, because they are elbowed out by other plants which, nevertheless, cannot tolerate the conditions of life either on the mountain or the shore, and leave these, therefore, as refuges which they are unable to invade. It is possible that the action of similar soil constituents might help to bring about homoplastic agreements in plants. The suggestion is not, however, one that occurred to me to make. My object was simply to show how two perfectly different causes might produce the same effect, namely, that of giving immunity from competition to a small

group of plants. Except as an illustration of this point, the matter was quite irrelevant to the subject about which I was speaking. W. T. THISELTON DYER

ON THE DISCOVERY OF STEPHANURUS IN

THE UNITED STATES AND IN AUSTRALIA

THE

HE time has now arrived when a full statement of the facts relating to this interesting parasite. Stephanurus dentatus, should be made more generally known; for not only is the progress of helminthological science likely to be checked by delay in this matter, but, in the absence of definite information, the several merits of the original discoverer and describer of this entozoon are likely to be altogether ignored. I therefore record the facts and inferences in the order in which they have recently come under my notice.

On the 10th of January last, through the firm of Messrs. Groombridge. I received an undated communication from Prof. W. B. Fletcher, of Indianapolis, Indiana, U.S.A. In that letter Dr. Fletcher announces that he has "found a worm" infesting the hog, and he helps one to realise its abundance by adding that he obtained it "in nine out of ten hogs" which he examined. After recording some other important facts respecting the tissues and organs which were most infested by the parasite, Dr. Fletcher remarks that he cannot find any description of the worm in the work on Entozoa issued by the publishers above mentioned, nor in the writings of Von Siebold and Küchenmeister, and he therefore encloses specimens for my determination, requesting a reply.

As I have already stated in my first letter recorded in the British Medical Journal (for January 14, p. 50, where many other particulars are given which I need not here recapitulate) I was instantly struck with the "strongyloid character" of the fragmentary and shrivelled up specimens, and I may also add that it at once occurred to me that I had had some previous acquaintance with a scientific description of the worm. Proceeding, therefore, to turn over a series of helminthological memoirs, for many of which I stand indebted to the late veteran, Dr. K. M. Diesing, of Vienna, I soon had the good fortune to find the desired record. The memoir in question forms part of the "Annalen des Wiener Museums" for 1839, the full title being "Neue Gattungen von Binnenwürmern, nebst einen Nachtrage zur Monographie der Amphistomen."

As this work is probably little, if at all, known in the countries now necessarily most interested in the history of this entozoon, I cannot, perhaps, do better than transcribe Dr. Diesing's brief notice of the original discovery, together with his description of the external characters presented by the worm. After naming the parasite Stephanurus, on account of the coronet-like figure of the tail of the male, and giving a technical description of the species, he continues as follows :-"At Barra do Rio Negro, on the 24th of March, 1834, Natterer discovered this peculiar genus occurring singly or several together in capsules situated amongst the layers of fat, in a Chinese race of Sus scrofa domestica. Placed in water or in spirits of wine, they stretched themselves considerably, and almost all moved up and down."

"The males measure from ten to thirteen lines in length, the females from fifteen to eighteen lines, the former being scarcely a line in breadth at the middle of the body, whilst the latter are almost a line-and-a-half in thickness. The curved body thickens towards the tail, is transversely ringed, and when viewed with a penetrating lens, is seen to be furnished with integumentary pores. The oral aperture opens widely, and is almost circular; it is supplied with six marginal teeth, two of which, standing opposed to one another, are larger and stronger than the rest. The tail of the male, when evenly spread out, is surrounded by a crown of five lancet-shaped flaps; the combined flaps being connected together from base to

apex by means of a delicate transparent membrane. The single spiculum situated at the extreme end of the tail, projects slightly forwards, being surrounded by three skittle-shaped bodies. The tail of the female is curved upon itself, rounded off, and drawn out at the extreme end into a straight, beak-shaped point, whilst to both sides of the stumpy caudal extremity of the body, short vesicular elevations are attached. The female generative opening occurs at the commencement of the second half of the body.

"Judging by its external characters this genus is most closely allied to Strongylus."

The above description is supplemented by a more lengthened account of the internal organisation of the worm; this part of the record displaying in an especial manner those powers of accurate observation which so fully characterised the great systematist in helminthology prior to the time when he was deprived of his eye-sight.

Having communicated to Prof. Fletcher my views respecting the true history and identification of Stephanurus, he was pleased to supply me with some further particulars. Thus, (after receiving my reply) in his second communication (dated from Indianapolis, February 22), he says: "I at once renewed my researches, and was rewarded by finding the little saw-like teeth, upon a six-sided jaw, and, if I mistake not, two larger teeth or hooks. I also removed the lungs, heart, and liver, entire, from several hogs (just killed by shooting in the head) and found the worm, as before stated, in the liver, in all the hepatic vessels, and also in the vena cava. In some cases I found the eggs in abundance in the pelvis of the kidney, and in the urine, even when I could discover no cysts or worms about them."

Dr. Fletcher then alludes to the circumstance that he had since his first letter to me placed himself in correspondence with Prof. Verrill, who, it appears, had previously examined the worm. Prof. Fletcher also obligingly enclosed Prof. Verrill's paper, extracted from the American Journal of Science and Arts of September 1870, and, in so far as I may be guided by its contents, it would now appear that the very first specimens which were obtained in the United States were the "five" examples sent by Dr. M. C. White, of New Haven, U.S., to Prof. Verrill, who adds:-" In the second instance, at Middleton, Conn., Dr. N. Cressy found large numbers of the worms in the fat about the kidneys of a young Suffolk pig, brought from New Jersey."

The title of Prof. Verrill's paper is, "Description of Sclerostoma pinguicola, a new species of entozoa from the hog."

At this point I pause to remark on some of the more practical questions connected with Stephanurus, for it must be quite obvious that so large a parasite, comparatively speaking, must, when present in great numbers, give rise to a great amount of disease, even if it should not ordinarily prove fatal. Dr. Fletcher, indeed, does not hesitate to write as follows:-" It is my opinion that this parasite is the cause, in some way, of the hog cholera, which has created such sad havoc within the past ten years, over the pork-producing parts of America. One farmer told me a few days ago that within a month his loss alone from this cause was over one hundred head; and sometimes, in one neighbourhood, in a few days time, thousands have perished, although this season is not a cholera year, as our farmers say. I advised one farmer to burn or bury the dead animals; but he informed me that he believed that fewer hogs die of the disease after eating the dead animals than those kept from them. Unfortunately, in this State there is no law guarding the spread of disease, neither is there any reward of reputation or gain for pursuing any investigation that would bring pork and beef packers into disrepute. I myself could not get a pig's kidney or beef's liver in our city market, because I

made investigations in some Texas cattle (being cut up in our market) which damaged their sale a few years ago." In a third letter Dr. Fletcher tells me that greater facilities for examining the carcases of hogs had since been accorded him through the liberality of a Liverpool firm of pork packers, who had already killed 75,000 hogs during the summer season, i.e., up to the date of the first week in July. In hot weather the slaughtering is conducted in icehouses.

These practical observations by Dr. Fletcher appear to me to be of the highest importance, even though it should eventually turn out that there is no immediate connection between the occurrence of Stephanurus and the hog cholera epidemics. That this opinion rests upon substantial data seems probable from the circumstance that we have now not only received evidence of the occurrence of Stephanurus in Australia, but we are further apprised that the pigs which harbour it die of the disease super. induced by their presence. As I have already stated, in my second letter, published in the pages of the British Medical Journal, our earliest intelligence on this point rests upon the evidence furnished by a series of unnamed slides transmitted from Sydney to the President of the Royal Microscopical Society of London. Through the kindness of the Society's able Secretary, Mr. Slack, F.G.S., I was permitted to examine, identify, and name all the specimens, and it was then that I recognised Stephanurus amongst the number.

On the 4th inst. Dr. Morris's paper, which accompanied the specimens, was read to the Society. In that paper the author, like Prof. Verrill, expresses his belief that he has found a new entozoon, "its habitat being the fat surrounding the kidney of the pig." He speaks of it as occurring both in the free and encysted state, the encysted being its final stage of existence," and, he adds, "its solid parts ultimately disappear, leaving a greyish brown fluid containing thousands of eggs." Those who desire further particulars in reference to the parasitism of pigs and sheep in Australia should consult Dr. Morris's paper, which will appear in the forthcoming November number of the Monthly Microscopical Journal. Dr. Morris speaks of the pigs as dying from some mysterious disease, and thinks "it is possible that this worm or its broed may be the cause." In some cases their death takes place quite suddenly, and this he supposes to be due to peritonitis set up by the swarming and migrations of the progeny. Be this as it may, it is interesting to notice the remarkable corsespondency of the conclusions arrived at by Dr. Fletcher and Dr. Morris independently. It will probably not be difficult to ascertain hereafter whether or not the maladies respectively termed "Hog Cholera" and "Mysterious Disease" are one and the same disorder; but whatever happens in this respect, it is now quite clear that this parasite, hitherto little regarded, and for many years past persistently overlooked, is extraordinarily prevalent in the United States, and, perhaps, equally so in Australia, it being further evident that its presence in the flesh of swine is capable of producing both disease and death. The statement of the worthy American farmer that the swallowing of infested flesh by a pig does not necessarily involve the pig-eating hog in a bad attack of a so-called "Cholera disease" requires to be further tested, and it also remains to be proven whether or not the Stephanurus be capable of passing through all its developmental changes from the egg to the adult form within the body of the bearer without having at some time or other gained access to the outer world. The comparatively large size of the ova, which I find to be about

", or more than four times the size of that of Trichina, is not without significance; but as yet we are unacquainted with the larval stages of growth. If no intermediary bearers are necessary to its development, we ought not to have to wait long for a complete record of the life-history of Stephanurus dentatus. T. S. COBBOLD

BALL ON MECHANICS*

THE object of this book is to "prove the elementary laws of mechanics by means of experiments "-a method the exact opposite of that generally adopted. According to the usual method, a few very general principles are assumed as derived from experimental data, a group of intermediate principles is then obtained deductively, by the aid of which the action of forces in particular cases can be analysed. The particular cases may be such as have an interest from their bearing on practical questions, but they are only examples of a general method applicable to innumerable other cases. There are therefore two distinct objects for which mechanical experiments may be made-viz, either to verify the fundamental principles, or to verify the deductions drawn in particular cases. Experiments of the former kind are absolutely essential to the existence of the science. Unless, for instance, the conditions of the action of the force of friction are determined by experiment, no deductions as to cases into which that force enters have any but a theoretical value. The same is true in all similar cases; such questions as, whether quantity of matter is proportional to weight, whether gravity at a given station is sensibly a constant force, whether the elasticity of solid bodies follows Hooke's law, and if so within what limits, can be answered by experiment only. Such questions, on the other hand, as the tension of a tie-rod under given circumstances, the relation between the weights which keep a given lever at rest,

We will now give Mr. Ball's experiment in illustration of the same question :-"A piece of pine BC, 3' 6" long and 1" x 1" in section (Fig. 2) is capable of turning round its support at the bottom B by means of a joint or hinge; it is held up by a tie AC 3' long, which is attached to the support exactly above the joint. AB is 1' long. From the point C a wire descends, having a hook at the end, on which a weight can be hung. The tie is attached to the spring balance, the index of which shows the strain. The spring balance is supported by a wire strainer, by turning the nut of which the length of the wire can be shortened or lengthened as occasion requires. This is necessary, because when different weights are suspended from the hook the spring is stretched more or less, and the screw is then employed to keep the entire length of the tie at 3'. The remainder of the tie consists of copper wire" (p. 29). Mr. Ball then goes on to notice that when a weight of 20lbs. is placed on the hook, the strain, as determined by the spring balance, is 60lbs., thus verifying the analysis of the case given above.

As an example of an experiment of the former class we will take the following,-it is the form in which Mr. Ball

B

FIG. 1.

the relation between the power and the weight in a block and tackle, the form of the surface of a revolving liquid, admit of exact answers by deduction from the proper data, and, of course, the answers may be tested by experiment. Such experiments clearly have a different object from those of the former class. They have, indeed, this in common, that experiments of the latter kind also serve to verify fundamental principles, but they do so indirectly. It is, however, from the teacher's point of view that their value will be found greatest. In teaching the elementary parts of mechanics perhaps the greatest difficulty experienced is to make the learner feel that the diagrams drawn on the black board represent facts, that, for instance, the conclusion deduced from a triangle is really applicable to a crane. Put the experiment side by side with the deduction, and it will be seen that the experiment cannot fail to bring home to the mind of the learner that his reasoning relates to things and not merely to abstractions.

Let CB (Fig. 1) represent the jib or strut, and AB the tierod of a crane, the line AC being vertical. Let a weight P hang from A, and let it be required to determine the forces transmitted through the tie and the jib. P can be resolved into two forces acting along BC and AB produced, and an inspection of the figure will show that these forces bear to P the same ratio that the lines BC and AB bear to AC, and that the force along BC is a thrust, and that along AB a tension. This analysis is perfectly general.

FIG. 2.

gives Galileo's experiment of dropping bodies from the top of the Tower of Pisa. The figure (Fig.3) is so perfect that it scarcely requires explanation. So long as the current is in action, the horse-shoe G is magnetic, and a ball of iron F remains suspended from it. When the current is broken G is no longer magnetic and F falls. In this manner, by including the wire round both horse-shoes in the circuit, a ball of iron and one of wood, into which a flat-headed nail has been driven, can be kept suspended, and then by breaking the circuit they can be let fall at exactly the same instant, they are seen to reach the cushion at the same instant, and are thus shown to fall through equal spaces in the same time. Mr. Ball describes and discusses the experiment at some length, and shows how it proves that at a given station the attraction of gravitation on different bodies is proportional to their masses.

The above examples will give a better notion both of the contents and illustrations of the book than any long description. We may say, however, that the book contains a clear and correct exposition of the first principles of mechanics, and illustrates, by well-chosen experiments, all the points in the subject that can be fairly called elementary. The figures reproduce all the circumstances of the experiments with so much exactness that with