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his difficulties would vanish, and he would already know the its human inhabitants according to the laws of evolution, or in proposition.

some miraculous manner, the arrival of the human race proAll that I contend for is, that the new book on geometry duced great modifications and changes of surrounding circumought to be carable of such usage. If it contains little more stances. These changes were in the direction of increasing the than the chief steps of the solutions, and those disguised (to the fertility of all vegetable productions capable of siistaining life, unpractised and tottering mind) under symbols, it will not satisfy and at the same time securing their use entirely for the human the want now felt.

A FATHER family. Hence arose, in the vicinity of man, two new factors;

the superior attrac ion of better food for all kinds of animals, The Beef Tapeworm

and at the same time the extinction of such animals whose greed

was not overruled by sufficient wariness or cunning to become As an entozoologist and correspondent of the Academy of successful thieves. Hence a probable gradual increase in these Natural Sciences of Philadelphia, I request permission to correct qualities in the animals maintaining themselves against man, an error recorded in the report of the Academy as given in your

Since my attention was drawn to this subject, we have had some columns (at p. 500) this week. Dr. Leidy is represented as

interesting observations on modifications of swallow's nests by having stated that "the minu e acetabular pit or fovea at the Pouchet, and a discussion as to the validity of his conclusions by summit of the head [of Tænia mediocanellata) is not mentioned Noulet, and now I have read with pleasure Mr. Potrs's observations, by Kuchenmeister and subsequent observers as a character of that

Most likely the progress of development in the carnivorous habits species." I beg to remark that I both figured and described of the Kea will meet with a check now that shepherds are alive this supplementary sucker-like structure in the first edition of my to its depredations ; but without the influence of the human small work on “'Tapeworms,” published in 1866 (p. 33 et seq). period we can scarcely suppose that such development would have At least two other observers have figured and described this begun. I recollect a case of change of habits in weasels. They central depression, not only in ihe aduit but also in the measle multiplied so thickly in a parish in the south of Dumfriesshire or cysticercal stage of the worm. Even Bremser recognised it, that some hungry philosopher among them took the initiative in but his descriprion was for a time overlooked.

sucking the blood frora the cattle. Suspicion having been 84, Wimpole Street, London, Oct. 21 T.S. COBBOLD 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 Winter Fertilisation

observe what modifications are being produced in the habits of

various species of sea-gulls since Glasgow, by its enormous in. In the first number of NATURE, (for Nov. 4, 1869,) I ventured crease of commerce, has wrought great changes in the River on a hypothesis, founded on a series of observations, that plants Clyde, filling it with all kinds of garbage. The conditions of which Aower in the winter have their organs of reproduction existence having been lavourable, the gull is steadily pa-sing more specially arranged to promote self-fertilisation. The following and more time inland ; ascending uibutaries of the Clyde, and fact, which has just come under my notice, appears to confirm alighting in focks on fields that used to have him very seldom. this theory. Plants belonging to the order Caryophyllacea are, А new amusement within my own recollection has been as a rule, strongly protandrı us (see my paper in the Journal of afforded the river passengers during the summer months in feedBotany for October 1870), the anthers discharging their pollen ing these sea mews, &c., by throwing overboard food to them, at so lung an interval before the maturing of the stigma as to and their increased tameness and boldness of approach in followrender cross-fertilisation almost inevitable. The other day, ) ing the river steamers within the last thirty years have been freOct. 21, I came across a laie flowering patch of Stellaria aquatica quently commented on.

J. SHAW 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 staniens, so

A Plane's Aspect as to bring the stigmatic sursace in actual contact with the dehi-cing anther. This occurred in several flowers that were just opening, Mr. LAUGHTON has hit the nail on the head. “ Aspect" is and there was abundance of seminiserous capsules on the plants. exactly the word wanted. The aspect of a plane is the direction

ALFRED W. BENNETT of its normal ; and "parallel planes are defined as those which

have the same aspect. Two aspects determine one direction, and two directions determine one aspect

. Mr. Laughton deserves Velocity of Sound in Coal

the 'hanks of geometers for suggesting so good a word. Your correspondent will find in Prof. Tyndall's beautiful Rugby, Oct. 23

J. M. WILSON work on “Sound” the data required for the exact determination of iis velociiy in different media. I believe that in coal it will be found to be between six and seven times that in air, or about The words “aspect” and “slope” have already a use in re7,000 feet per second.

lation to the position of planes. They indicate two elements If Mr. D. Joseph places his ear against the solid coal of the which together fix the position. Neither of them, taken alone, “rib” or side of the "heading" or gallery, at a distance of some can indicate the position of a plane, unless a new and artificial twenty to thirty yards from a collier at work, he will hear two meaning be assigned to one or other. Thus if I speak of the sounds for each blow of the workman's pick or mandril—the first “aspect " of one of the faces of a roof as southerly, I have done being transmitted through the coal, the second more slo - ly something but not all that is necessary, towards describing the through the air, the impression being almost irresistible that two posision of that face ; if I add further that the “slope" is 30° I persons are at work,

have definirely assigned the position. Again if I speak of the This is probably the origin of the legend, common in more "slope" of Saturn's rings as 28° (the plane of reference being than one coal district, of a collier who always worked alone, did | ecliptic), I have done something towards the description of their more work than his fellows, and whose diabolical assistant was position; if I add further that their “ aspect” is toward such often heard but not seen.

C. J.

and such a degree of the sign Gemini, I fully assign their posi

tion in space. And so on. Changes in the Habits of Animals

In the preceding sentences I have used the words slope” and

“aspect" as they are already understood. I apprehend that I Your correspondent Mr. Potts in the last number of NATURE | have also used the word "position" as it is already understood, furnishes us with a few interesting facts regarding the Ka. In and that no other word could properly be used in the same sense a paper which I read about three years ago to the Dumfries in descriptive writing. I can see no reason why "position" Natural History Society, entitled “The Influence of the Human should be dismissed from the position it has so long occupied, Period on the Sagacity of Animals," and subsequently in a letter nor why “ aspect” and “slope” should be regarded in a new published in NATURE, vol. i., on the “Mental Progress of and unfamiliar aspect. Animals," I endeavoured to show from general considerations, It chances that I have long since had occasion to consider the and from the few facts which we possessed on this subject, that question suggested last month by Mr. Wilson. In each of the habits and instincts of animals were not so fixed and definite twelve books which I have written during the past six years, I as might be supposed. The general principle for which I con. have had repeated occasions to consider the slope and aspect, tended was that whether we considered the globe to have received that is, the “position ” of many important astronomical planes.

Oct. 23

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In a large proportion of the essays I have written, the same sub- similar but distinct Brachycoma decipiens Hook. fil. ject of plane position has had to be con-idered and described | Again, true flowering plants belonging to the very curious am, therefore, somewhat seriously interested in opposing as well family Podostemacea have been figured as liverworts and the disuse of the word "position,” which no one can misunder. stand, as the use of the words “ aspect," "slope," "tilt," &c,

other cryptogamic plants (Berkeley, Intr. to Crypt. Bot., in a sense not at present assigned (nor properly assignable) to

p. 5). Many other instances of similar errors might be them.

Richd. A. PROCTOR

given.

Since I read my paper, I have met with an essay by

Schouw, in which he enumerates facts of the same kind. Sea-water Aquaria

“ There is still,” he says ("Earth, Plants, and Man," p. 61), I have read with much gusto your article upon the Crystal “another kind of repetition which I might call habitual Palace Aquarium. I am induced by it to put forward a caution repetition, or denominate mimicry, if this expression was with regard to the construction of rock-work in tanks.

not at variance with the subjection to law which exists Several weeks ago, casually looking over a heap of Bangor throughout nature, but to comprehend which our powers slaty rock, on the road bordering the Brighton Aquarium works,

are often insufficient.” After various illustrations he proand being used for the rock-work of tanks, my attention was attracted by some bright green patches upon some of the

ceeds :-"In the genus Mutisia we have the remarkable stones, which appeared to me to be carbonate of copper

, but sight of a compositous flo ver, with the tendrils of a was probably silicate. Looking further at one with a lens, leguminous plant.” (This by an accidental coincidence I imagined that I could also distinguish particles of peas burgh.) “ In Begonia fuchsioides the leaves are similar

was one of the instances which I, myself, used at Edincock ore.

On attempting to purloin a specimen, I was very properly stopped from

so criminal an act by the Cerberus to a Fuchsia, and very different from the other forms of in charge. I wrote to the chairman of the company, stating leaf among the begonias, and the colour of the blossom that, not having examined the stone, I might be only contributing likewise reminds us of the fuchsias. We have another

I a mare's nest to their zoological collection, but that if it con- most striking example in certain Brazilian plants, which tained much copper the fish would be in danger. I understand although possessed of perfectly developed flowers and that upon receipt of my letter some rock was sent up to Dr.

fruits, mimic, as it were, in their leaves and stems, groups Percy, whose report , I am told, was to the effect that there was

of plants of much lower rank.” (He is alluding to the much sulphide of copper, and that the pretty green rock was

Podostemacece mentioned above.) * Lacis fucoides retherefore unfit for tank rock-work. 1think this will serve as a caution to the constructors of aquaria taken for one by a person who did not see the flowers.

sembles certain seaweeds so much, that it might be misto examine all material which is to be in contact with water most carefully before using it. There are so many minerals which Mniopsis scaturiginum strikingly resembles a Jungerwould be dele erious that I strongly advise an analysis and report mannia.in the case of every untried rock. The accident of my passing a

I suggested that when a plant put on the characteristic heap of stones has saved the company, with which I am not in facies of a distinct natural family, it might conveniently the least connected except as a fervent well-wisher, from a large be spoken of as a pseudomorph, having in view an obvious expenditure and a serious scrape.

analogy in the case of minerals. I do not, however, now Allow me to ask those who are accustomed to the manage: think on further consideration, that this term, although ment of tanks, whether hydraulic pressure upon a small and convenient, includes all the cas-s. In small natural famistrong one wuld be likely io assist in maintaining life in any of lies it is not always easy to recognise any general habit the deep-sea organi-ms, and whether it would be useful to make recesses for those loving darkness, with the axes opposi'e the different families where this is the case, but having a

or facies at all, and in the case of plants belonging to plate gla-s side, so that a bull's-eye lantern could occasionally similar habit, it would be purely arbitrary to fix the throw light upon their actions and mode of life? Brighion, Oct. 21

MARSHALL HALL

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 ON HOMOPLASTIC AGREEMENTS IN cases as isomorphic. PLANTS

My friend, Mr. E. R, Lankester, has pointed out to me

that agreements of this kind may all come under what he AT T the recent meeting of the British Association I has termed homoplasy (Ann. and Mag. of Natural History,

pointed out in a short communication the difference July 1870). This is the explanation he gives of this that existed between mimicry in animals and what has expression :been spoken of under that name amongst plants. The

* When identical or nearly similar forces, or environdistinction was sufficiently obvious, and must have oc- , ments, act on two or more parts of an organism which are curred to everyone who had given the matter any consi- exactly or nearly alike, the resulting modifications of the deration, but my object was to try to raise a discussion various parts will be exactly or nearly alike. Further, if, upon the whole subject as exhibited in plants.

instead of similar parts in the same organism, we suppose I fancy it is hardly sufficiently understood how com- the same forces to act on parts in two organisms, which monly this agreement of facies occur in plants widely parts are exactly or nearly alike and sometimes homodiffering in other respects. I will give a few illustrations genetic, the resulting correspondences called forth in the of it. Humboldt remarks (“ Views of Nature,” p. 351) : several parts in the two organisms will be nearly or exactly “In all European colonies the inhabitants have been led alike. I propose to call this kind of agreement homo by resemblances of physiognomy (habitus, facies) to apply plasis or homoplasy. The fore legs have a homoplastic the names of European forms to certain tropical plants, agreement with the hind legs, the four extremities being, which bear wholly different flowers and fruits from the in their simplest form (c.g. Proteus, which must have had genera to which these designations originally referred. ancestors with quite rudimentary hind legs), very closely Everywhere in both hemispheres the northern settler has similar in structure and function. . . . Homoplasy inbelieved he could recognise alders, poplars, apple, and cludes all cases of close resemblance of form not traceable olive trees, being misled for the most part by the form of to homogeny.". the leaves and the direction of the branches." Nor has The resemblances, therefore, above described between the popular eye alone been deceived by these resem- the vegetative organ of plants with no close generic blances. Schleiden states (“The Plant," p. 255) that Australia relations, may be described as homoplastic. The difficulty has in common with Europe a very common plant, the daisy, yet Dr. Hooker has pointed out (Flora of Tasmania, having been published and described by Kunze as a species of Lomaria, a

Perhaps one of the most striking is the Natal cycad Stangerin paradura pl. 47) that the plant intended by Schleiden is the very genus of Ferns.

on

still, of course, remains to show how the homoplasy has group of plants. Except as an illustration of this point, been brought about. In some cases, as in the homoplastic the matter was quite irrelevant to the subject about which forms of American Cactaceæ and South African Euphor- I was speaking.

W. T. THISELTON DYER bias, or in the stipular bud scales of many wholly unrelated deciduous trees, the nature of the similar external | ON THE DISCOVERY OF STEPHANURUS IN conditions may possibly be made out with some correctness. Again, Dr. Seemann has pointed out ihat by the rivers in

THE UNITED STATES AND IN AUSTRALIA Nicaragua and in Viti, the vegetation, although composed

THE

CHE time has now arrived when a full statement of of very different plants, puts the willow form the facts relating to this interesting parasite. Stepha(“ Dottings by the Roadside," p. 46). A phenomenon true nurus dentatus, should be made more generally known ; of two distant places accidentally contrasted, might be for not only is the progress of helminthological science expected to obtain more generally; at any rate, among our likely to be checked by delay in this matter, but, in the indigenous riparian plants Lythrum Salicaria and the absence of definite information, the several merits of the willow-herb are, as their names indicate, additional illus- original discoverer and describer of this entozoon are trations. The band of vegetation that fringes a stream likely to be altogether ignored. I therefore record the facts is always densely crowded with individual plants, and it is and inferences in the order in which they have recently easy to see that elongated and vertically disposed leaves come under my notice. would be most advantageous, exactly as they are to the On the roth of January last, through the firm of Messrs. gregarious plants of meadows and plains. The homo

Groombridge. I received an undated communication from plastic agreement of riparian plants may be therefore a Prof. W. B. Fletcher

, of Indianapolis, Indiana, U.S.A. direct result of selective effort due to the position in which In that letter Dr. Fletcher announces that he has " found they grow.

a worm” infesting the hog, and he helps one to realise its In other cases the operation of similar external mould- abundance by adding that he obtained it"in nine out of ten ing influences is not so easy to trace. It might, perhaps, hogs” which he examined. After recording some other however, be imagined that plants would hereditarily re. important facts respecting the tissues and organs which tain the effects when the influences had ceased to

were most infested by the parasite, Dr. Fletcher remarks operate, and no new ones had come into operation pre- that he cannot find any description of the worm in the cisely adapted to obliterate the work of those that preceded work on Entozoa issued by the publishers above menthem. Suppose, for example, that willows got their habit tioned, nor in the writings of Von Siebold and Küchenand foliage from ancestors that were exclusively riparian, meister, and he therefore encloses specimens for my then any descendant that happened to be able to tolerate determination, requesting a reply. situations with less abundant supplies of moisture, would As I have already stated in my first letter recorded in not necessarily lose their characteristic foliage on that the British Medical Journal (for January 14, p. 50, where account. Such races might be expected to occur near many other particulars are given which I need not here rivers subject to periodic droughts, since under these con- recapitulate) I was instantly struck with the "strongyloid ditions any others would be likely to perish. Under such character” of the fragmentary and shrivelled up specimens, circumstances we should have cause and effect no longer and I may also add that it at once occurred to me that I in contiguity; the riparian habit surviving the riparian had had some previous acquaintance with a scientific situation.

descrip!ion of the worm. Proceeding, therefore, to turn I suggested at Edinburgh that possibly similar habits over a series of helminthological memoirs, for many of in plants might be bronght about by different causes. This which I stand indebted to the late veteran, Dr. K. M. was only a suggestion, and probably what has just been Diesing, of Vienna, I soon had the good fortune to find said is a truer account of the matter. At any rate the the desired record. The memoir in question forms part illustration I gave of my meaning has been quite mis- of the “Annalen des Wiener Museums” for 1839, the full understood (as, for example, in the last number of the title being “Neue Gattungen von Binnenwürmern, nebst Popular Science Review). It is well known that there einen Nachtrage zur Monographie der Amphistomen.” are a certain number of plants indigenous to the British As this work is probably Jittle, if at all, known in the Isles, which are found at a considerable height upon countries now necessarily most interested in the history mountains and also upon the sea-shore, but not in the of this entozoon, I cannot, perhaps, do better than interve ning space. In the latter situations they contain transcribe Dr. Diesing's brief notice of the original dismore sodium salts than in the former, and inasmuch as covery, together with his description of the external these salts are destructive to many plants, those that characters presented by the worm. After naming the compose a strand flora must be able to tolerate them, and parasite Stephanurus, on account of the coronet-like this of course is an advantage, because many of their figure of the tail of the male, and giving a technical competitors are poisoned off. Similarly plants of moun- description of the species, he continues as follows :-"At tains must have a similar advantage over others in ability | Barra do Rio Negro, on the 24th of March, 1834, Natterer to tolerate mountain asperities of climate. Now, suppose discovered this peculiar genus occurring singly or several a mountain submerged; its flora and certain portions of together in capsules situated amongst the layers of fat, in that of the strand come to coincide. Then if we suppose a Chinese race of Sus scrofa domestica. Placed in water the mountain gradually to emerge, some of these plants or in spirits of wine, they stretched themselves considerably, will spread downwaids under the uncovered surface, and almost all moved up and down." and irave over the whole of the interval that ultimately “1 he males measure from ten to thirteen lines in length, separates the mountain-top and the strand. Why, then, the females from fitteen to eighteen lines, the former being do they not remain there? Simply, I believe, because scarcely a line in breadth at ihe middle of the body, whilst they are elbowed out by other plants which, nevertheless, the latter are almost a line-and-a-half in thickness. The cannot tolerate the conditions of life either on the moun- curved body thickens towards the tail, is transversely tain or the shore, and leave these, therefore, as refuges ringed, and when viewed with a penetrating lens, is seen which they are unable to invade. It is possible that the to be furnished with integumentary pores. The oral action of similar soil constituents might help to bring aperture opens widely, and is almost circular ; it is supabout homoplastic agreements in plants. The sug- plied with six marginal teeth, two of which, standing gestion is not, however, one that occurred to me to opposed to one another, are larger and stronger than the make. My object was simply to show how two perfectly rest. The tail of the male, when evenly spread out, is different causes might produce the same effect, namely, surrounded by a crown of five lancet-shaped flaps ; ihe that of giving immunity from competition to a small combined flaps being connected together from base to

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apex by means of a delicate transparent membrane. The made investigations in some Texas cattle (being cut up in single spiculum situated at the extreme end of the tail, our market) which damaged their sale a few years ago." projects slightly forwards, being surrounded by three In a third letter Dr. Fletcher tells me that greater facilities skittle-shaped bodies. The tail of the female is curved for examining the carcases of hogs had since been upon itsell, rounded off, and drawn out at the extreme accorded him through the liberality of a Liverpool firm of end into a straight, beak-shaped point, whilst to both sides pork packers, who had already killed 75,000 hogs during of the stumpy caudal extremity of the body, short vesicular the summer season, i.e., up to the date of the first week in elevations are attached. The female generative opening July. In hot weather the slaughtering is conducted in iceoccurs at the cominencement of the second half of the houses. body.

These practical observations by Dr. Fletcher appear to " Judging by its external characters this genus is most me to be of the highest importance, even though it should closely allied to Strongylus."

eventually turn out that there is no immediate connection The above description is supplemented by a more between the occurrence of Stephanurus and the hog lengthened account of the internal organisation of the cholera epidemics. That this opinion rests upon subworm ; this part of the record displaying in an espe- stantial data seems probable from the circumstance that cial manner those powers of accurate observation which we have now not only received evidence of the occurrence so fully characterised the great systematist in helmin- of Stephanurus in Australia, but we are further apprised thology prior to the time when he was deprived of his that the pigs which harbour it die of the disease super. eye-sight.

induced by their presence. As I have already stated, in Having communicated to Prof. Fletcher my views re- my second letter, published in the pages of the British specting the true history and identification of Stephanurus, Medical Journal, our earliest intelligence on this point he was pleased to supply me with some further particulars. rests upon the evidence furnished by a series of unnamed Thus, (after receiving my reply) in his second communi- slides transmitted from Sydney to the President of the cation (dated from Indianapolis, February 22), he says : Royal Microscopical Society of London. Through the "I at once renewed my researches, and was rewarded by kindness of the Society's able Secretary, Mr. Slack, F.G.S., finding the little saw-like teeth, upon a six-sided jaw, and, I was permitted to examine, identify, and name all the if I mistake not, two larger teeth or hooks. I also re- specimens, and it was then that I recognised Stephanurus moved the lungs, heart, and liver, entire, from several amongst the number. hogs (just killed by shooting in the head) and found the On the 4th inst. Dr. Morris's paper, which accompanied worm, as before stated, in the liver, in all the hepatic the specimens, was read to the Society. In that paper the vessels, and also in the vena cava. In some cases I found | author, like Prof. Verrill, expresses his belief that he has the eggs in abundance in the pelvis of the kidney, and in found a new entozoon, "its habitat being the fat surroundthe urine, even when I could discover no cysts or worms ing the kidney of the pig.” He speaks of it as occurring about them."

both in the “ free and encysted state, the encysted being Dr. Fletcher then alludes to the circumstance that its final stage of existence, and, he adds, “its solid parts he had since his first letter to me placed himself in ultimately disappear, leaving a greyish brown fluid concorrespondence with Prof. Verrill, who, it appears, taining thousands of eggs."

. Those who desire further had previously examined the worm. Prof. Fletcher particulars in reference to the parasitism of pigs and sheep also obligingly enclosed Prof. Verrill's paper, extracted in Australia should consult Dr. Morris's paper, which will from the American Journal of Science and Arts appear in the forthcoming November number of the of September 1870, and, in so far as I may be guided by Alonthly Microscopical Journal. Dr. Morris speaks of its contents, it would now appear that the very first the pigs as dying from some mysterious disease, and specimens which were obtained in the United States were thinks “it is possible that this worm or its brocd may be the “five” examples sent by Dr. M. C. White, of New the cause.” In some cases their death takes place quite Haven, U.S., to Prof. Verrill, who adds :-" In the second suddenly, and this he supposes to be due to peritonitis set instance, at Middleton, Conn., Dr. N. Cressy found large / up by the swarming and migrations of the progeny: Be numbers of the worms in the fat about the kidneys of a this as it may, it is interesting to notice the remarkable young Suffolk pig, brought from New Jersey.”

corsespondency of the conclusions arrived at by Dr. The title of Prof. Verrill's paper is, “ Description of Fletcher and Dr. Morris independently. It will probably Sclerostoma pinguicola, a new species of entozoa from the not be difficult to ascertain hereafter whether or not the hog."

maladies respectively termed “Hog Cholera” and At this point I pause to remark on some of the more “Mysterious Disease" are one and the same disorder ; practical questions connected with Stephanurus, for it but whatever happens in this respect, it is now quite clear must be quite obvious that so large a parasite, compara- that this parasite, hitherto little regarded, and for many tively speaking, must, when present in great numbers, years past persistently overlooked, is extraordinarily pregive rise to a great amount of disease, even if it should valent in the United States, and, perhaps, equally so in not ordinarily prove fatal. Dr. Fletcher, indeed, does not | Australia, it being further evident that its presence in the hesitate to write as follows :-“It is my opinion that this flesh of swine is capable of producing both disease and parasite is the cause, in some way, of the hog cholera, death. The statement of the worthy American farmer which has created such sad havoc within the past ten that the swallowing of infested flesh by a pig does not years, over the pork-producing parts of America. One necessarily involve the pig-eating hog in a bad attack of farmer told me a few days ago that within a month his a so-called “Cholera disease” requires to be further loss alone from this cause was over one hundred head ; tested, and it also remains to be proven whether or not and sometimes, in one neighbourhood, in a few days time, the Stephanurus be capable of passing through all its thousands have perished, although this season is not a developmental changes from the egg to the adult form cholera year, as our farmers say. °I advised one farmer to within the body of the bearer without having at some burn or bury the dead animals ; but he informed me that time or other gained access to the outer world. The comhe believed that fewer hogs die of the disease after eating paratively large size of the ova, which I find to be about the dead animals than those kept from them. Unforto", or more than four times the size of that of Trichina, tunately, in this state there is no law guarding the spread is not without significance; but as yet we are unacquainted of disease, neither is there any reward of reputation or with the larval stages of growth. If no intermediary gain for pursuing any investigation that would bring pork bearers are necessary to its development, we ought not to and beet packers into disrepute. I myself could not get have to wait long for a complete record of the life-history a pig's kidney or beef's liver in our city market, because I of Stephanurus dentatus.

T. S. COBBOLD

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BALL ON MECHANICS*

We will now give Mr. Ball's experiment in illustration of

the same question :-“A piece of pine BC, 3'6" long THE HE object of this book is to “prove the elementary and 1" x 1" in section (Fig. 2)is capable of turning roundits

laws of mechanics by means of experiments ”-a support at the bottom B by means of a joint or hinge ; it method the exact opposite of that generally adopted. is held up by a tie AC 3' long, which is attached to the According to the usual method, a few very general prin- support exactly above the joint. AB is i' long. From ciples are assumed as derived from experimental data, a the point C a wire descends, having a hook at the end, group of intermediate principles is then obtained deduc- on which a weight can be hung. The tie is attached 10 tively, by the aid of which the action of forces in particular the spring balance, the index of which shows the strain

. cases can be analysed. The particular cases may be such The spring balance is supported by a wire strainer, by as have an interest from their bearing on practical ques- turning the nut of which the length of the wire can be tions, but they are only examples of a general method shortened or lengthened as occasion requires. This is applicable to innumerable other cases. There are necessary, because when different weights are suspended therefore two distinct objects for which mechanical from the hook the spring is stretched more or less, and experiments may be made-viz, either to verify the the screw is then employed to keep the entire length of fundamental principles, or to verify the deductions the tie a! 3'. The remainder of the tie consists of copper drawn in particular cases. Experiments of the former wire” (p. 29). Mr. Ball then goes on to notice that when kind are absolutely essential to the existence of the a weight of 20lbs. is placed on the hook, the strain, as science. Unless, for instance, the conditions of the determined by the spring balance, is bolbs., thus verifying action of the force of friction are determined by ex- the analysis of the case given above. periment, no deductions as to cases into which that As an example of an experiment of the former class we force enters have any but a theoretical value. The same will take the following, -it is the form in which Mr. Ball 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,

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Fig. 2.

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,

2. 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 funda- gives Galileo's experiment of dropping bodies from the mental principles, but they do so indirectly. It is, however, top of the Tower of Pisa. The figure (Fig.3) is so perfect that from the teacher's point of view that their value will be found it scarcely requires explanation. So long as the current is greatest. In teaching the elementary parts of mechanics in action, the horse-shoe G is magnetic, and a ball of iron perhaps the greatest difficulty experienced is to make the F remains suspended from it. When the current is learner feel that the diagrams drawn on the black board re- broken G is no longer magnetic and F falls. In this present facts, that, for instance, theconclusion deduced from manner, by including the wire round both horse-shoes in à triangle is really applicable to a crane. Put the ex- the circuit, a ball of iron and one of wood, into which a periment side by side with the deduction, and it will be flat-headed nail has been driven, can be kept suspended, seen that the experiment cannot fail to bring home to the and then by breaking the circuit they can be let fall at mind of the learner that his reasoning relates to things exactly the same instant, they are seen to reach the cushion and not merely to abstractions.

at the same instant, and are thus shown to fall through Let CB (Fig. 1) represent the jib or strut, and AB the tie- equal spaces in the same time. Mr. Ball describes and rod of a crane, the line AC being vertical. Let a weight P discusses the experiment at some length, and shows how hang from A, and let it be required to determine the forces it proves that at a given station the attraction of gravitatransmitted through the tie and the jib. P can be re- tion on different bodies is proportional to their masses. solved into two forces acting along BC and AB produced, The above examples will give a better notion buth of and an inspection of the figure will show that these forces the contents and illustrations of the book than any long bear to P the same ratio that the lines BC and AB bear description. We may say, however, that the book conto AC, and that the force along BC is a thrust, and that tains a clear and correct exposition of the first principles along AB a tension. This analysis is perfectly general. of mechanics, and illustrates, by well-chosen experiments

, * Experimental Mechanics : a Course of Lectures delivered at the Royal elementary. The figures reproduce all the circumstances

all the points in the subject that can be fairly called College of Science for Ireland. By Robert Stawell Ball, M.A. With Illustrations. (London and New York: Macmillan and Co., 1871.)

of the experiments with so much exactness that with

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