server whose researches may have been omitted, to attribute the neglect to the disturbing influence of recent events. The earth and her satellite are treated at some length, and the questions of the moon's influence on the earth's atmosphere, the winds, weather, and magnets, are fully discussed. The chapter on meteorites is very interesting. We are told, on the authority of Miller and Haidinger, that the earliest mention of meteorites is probably in Iliad xv. 18-22, where the anvils spoken of by Jupiter are supposed to refer to these phenomena. Livy mentions a shower which some think may have been a star shower; and the famous black stone in the Kaaba, at Mecca, is said to be undoubtedly a meteorite of great antiquity. Numerous analyses of meteorites are given, and tables are added containing full details of all those which are recorded to have fallen from the earliest times. There are similar tables with regard to comets and starshowers; and finally we have two well-executed plates of the appearance of different sun-spots, and a chart of part of the moon's surface. We should like to see an English edition.

The Theoretical Astronomy of Dr. Klinkerfues, director of the Royal Observatory of Göttingen, is a reproduction of lectures delivered by him in that University. This is the first part of the work, and its object is to give an explanation of the means by which the courses and positions of heavenly bodies are determined. It is not adapted to the general reader, but will prove a useful companion to the mathematician who wishes to obtain an insight into astronomical methods of calculation. Several very good figures accompany the text.

G. T. A.

Kuklos; an Experimental Investigation into the Relationship of Certain Lines. By John Harris Part I. (Montreal, 1870)

IN a review of Prof. Bretschneider's History of Early Geometry we have mentioned some clever attempts to square a circle, made at a time when this problem engaged the attention of the first mathematicians. Then, however, as at present, there existed circle squarers of a different kind, who excel only in demonstrating their own ignorance. A fine specimen is preserved by Simplikios. Some persons had heard of square numbers which are at the same time cyclical, that is to say, the last figure in the square number is the same as that of the root, as 25 and 5. Nothing, of course, could be more evident to them than that a number which is both square and cyclos must be a measure for the circle. Mr. Harris ranks almost as high, only he does not give his conclusions in quite so short a form. His book is to consist of four parts in quarto, of which the first contains merely a preface, preliminary arguments, and on the last page an introduction. In the preface the author excuses the haste in which the publication has taken place, with the remark that if his researches are of value they cannot be brought early enough before the public,-if a failure" the communication itself would not be worth the additional labour bestowed on improving its form." This latter conclusion we willingly grant. It is only to be regretted that Mr. Harris has not had the same opinion of the time he spent in writing this communication and preparing the numerous and long figures which fill ten large plates.

LETTERS TO THE EDITOR

[The Editor does not hold himself responsible for opinions expressed by his Correspondents. No notice is taken of anonymous communications.]

Pangenesis

MR. GALTON by acting upon the conclusion that the supposed gemmules supposed to be detached from the cells of the body at different periods of life in the case of the higher animals swarmed

in the blood prior to their supposed collection and union to form the reproductive element-favoured the provisional hypothesis of Pangenesis, for he indicated a not improbable manner in which the very improbable phenomena involved in the hypothesis might actually occur.

But Mr. Darwin, in NATURE for April 27th, writes to explain that he maintains that the gemmules must be "thoroughly dif fused"-I conclude, suspended in the fluids which circulate freely in every part of the very substance of all the tissues of the body. The supposed gemmules must be much more minute than the smallest particles that can be seen by the highest magnifying powers used in these days, and must be invisible to the eye when made to appear five thousand times larger than their real size. They must be capable of diffusion, and, as is suggested by Mr. Darwin, much as chemical substances are diffused. But the terms of the hypothesis would imply that the gemmules are actual particles suspended and not dissolved in the fluids.

It is not very encouraging to those who work, to discover after having performed numerous and well-devised series of difficult, of a hypothesis, that they have been investigating a shadow, and

laborious, and troublesome experiments honestly to test the value

to be then informed that the results they have obtained have little or no bearing on the question at issue. The "experiments are extremely curious," says Mr. Darwin, and the experimenter "deserves the highest credit for his ingenuity and perseverance." It would, of course, be possible to remove from one animal portions of tissue which, according to the hypothesis, must contain the supposed gemmules, if they exist, and graft the pieces of tissue upon another. If the experiment was successful, and the offspring exhibited any of the characters of the variety from which the graft was taken, the opponents of Pangenesis would admit the doctrine at once, but if the results were again of a negative kind, would Mr. Darwin consider that his hypothesis had "received its death blow?" It would certainly be as easy to defend it as it is at this time. Nor do I believe it possible to obtain a series of experimental results which would lead belief in hypothetical gemmules, which cannot be rendered evithe supporters of Pangenesis to abandon the hypothesis. A firm dent to the senses, is not likely to be shaken.

Depend upon it, neither the well-devised experiments of Mr. Galton, nor any other experiments that may be devised, will overthrow this doctrine. The provisional hypothesis of pangenesis is perfectly safe, and will withstand every attack that may be made. It cannot be successfully assailed. Like many favoured hypotheses of these days, it can neither be proved to be true ground his belief in the truth of this and other doctrines upon the nor positively shown to be false, and it is open to anyone to fact that they have not been and cannot be disproved. For undoubtedly gemmules may be formed in the manner supposed; if formed, they may be detached; if detached, they may pass through the tissues; they may then collect together, and may form reproductive elements. Each one of the countless millions of sperm elements produced in such profusion during so many years of life may, indeed, be formed by the union of millions of gemmules which, after meandering through the various textures of the body, marshal themselves in order in one particular locality. From the vast company thus supposed to have collected, we may conceive, by the light of imagination, the formation of regiments composed of multitudes of individual gemmules of the same kind; and further, it is not difficult to imagine that each individual gemmule of every regiment may move away and unite with thousands and tens of thousands of others, to form at length that marvellous compound and complex speck of matter less than the oo of an inch in diameter, which constitutes the active material of each small reproductive particle. This is one way in which the properties of the spermatozoon may be accounted for. Nor is it beyond the power of the imagination to picture the orderly arrar gement and rearrangement of such vast hosts of potential molecules as is supposed. No confusion, no jostling of one another, no struggling would be seen, for each molecule takes its appointed place, in obedience to its own properties, knowing of course the position it is to occupy in the complex ranks at each different period of its life's progress, and, never ambitious of discharging a higher function than that which it is destined to fulfil, performs the important office of transmitting certain peculiarities, important or trivial, useful or useless, from the existing to a new being.

* In NATURE for May 1st, Mr Francis Galton very properly remarks that the term Mr. Darwin should have employed is "dispersion" not "diffusin," and there are other critical remarks which appear to me equally just.

We may be led from the consideration of the broad facts nature to conceptions of the most abstract kind, without being conscious of the slightest gap between the facts of Science and the creations of the Imagination. In these days the utmost skill is often displayed in hiding and ignoring or denying the hiatus by which the arguments deduced from the results of observation and experiment are separated from those which are based upon the fictions of the fancy. But, unhappily, the gulf cannot be filled up, or bridged over. It may be obscured by mists and clouds, but, though it be lost for a time, it is sure to be rediscovered and its limits studied by the curious and unphilosophical. Nowadays analogical argument is employed very freely without any attempt to show, in the first place, that there is any real analogy between the facts upon which the reasoning is based. order to convince people that a hypothetical gemmule may move long distances through all sorts of tissues, it is only necessary to show that actual matter, millions of times as large, does burrow a short distance through certain textures. Mr. Darwin remarks that it cannot be objected "that the gemmules could not pass through tissues or cell-walls, for the contents of each pollen grain have to pass through the coats both of the pollen tube and embryonic sack."

In

He might have advanced in his support the fact of fungi traversing tissues, of entozoa of various kinds burrowing long distances through the textures of the living body, and many wellknown instances of a similar kind. But such facts do not strengthen the hypothesis of Pangenesis in the slightest degree. They were known before it was advanced, and the objection controverted has not been raised in the form indicated. We know that

a thing infinitely larger than the hypothetical gemmule does pass through tissues, but do the gemmules really exist, and do they pass through? Certainly, if they exist, they may pass, but, as I have indicated, there are other matters invalidating the hypothesis besides the question of the gemmules traversing the tissues. Pangenetic gemmules might pass everywhere. They might leave the body, collect in the atmosphere and coalesce, and the compound particle formed might easily wriggle itself back again into the organism through the chinks between the cuticular cells. Such gemmules might move anywhere, up and down and in and out through any cell wall. They might pervade solids and fluids and gases. The pangenetic gemmule cannot be seen or tested, neither can its presence or absence be proved in any way. The phenomena adduced by Mr. Darwin in support of his hypothesis can be demonstrated; but the pangenetic gemmules are of the imagination alone, and the analogy between the actual facts and the supposed facts is surely but an analogy of the imagination. The facts alluded to no more support the pangenetic hypothesis than does the demonstration of living germs in the air support the hypothesis of life in the blue sky. It is possible to supply many arguments stronger than those adduced in support of the hypothesis, nay, perhaps, stronger than any Mr. Darwin himself has yet advanced in favour of Pangenesis; but yet other considerations appear to me greatly to preponderate against the acceptance of the doctrine. Mr. Darwin admits that "from presenting so many vulnerable points" the life of his hypothesis "is always in jeopardy;" but is it not this very jeopardy which lends interest and enchantment to many a hypothesis, and sustains it in the estimation of those who delight in conjectural information and scientific speculation? LIONEL S. BEALE

MR. DARWIN, in his letter to NATURE of April the 27th, says: "The fundamental laws of growth, reproduction, inheritance, &c., are so closely similar throughout the whole organic kingdom that the means by which the gemmules (assuming for the moment their existence) are diffused through the body, would probably be the same in all beings, therefore the means can hardly be diffusion through the blood." Now, if in the vegetable kingdom pangenetic gemmules are able freely to be "diffused" from cell to cell by endosmosis, we should expect that in the case of grafts, where certainly such diffusion goes on between the cells of the stock and the scion, a bud borne upon the graft would certainly be affected by the gemmules arising in the root and stem of the stock. Yet we all know that the pips from a pear grafted on a quince stock will not give rise to a hybrid between a pear and a quince, neither will the stone of a peach which has been grafted on a plum stock grow into a tree whose stock bears plums, while the extremities of its branches bear peaches. A. C. RANYARD

Noises at Sea off Greytown

IN NATURE, vol. ii. p. 25, Mr. Dennehy gave an interesting account of a peculiar vibration, accompanied by sound, which is perceivable at night on board all (?) iron steamers which anchor off Greytown, Central America; and in subsequent pages I have read with great interest various speculations as to its origin, which is ascribed (1, the probable solution) to troops of Scianoids (with reservation) by Mr. Kingsley (p. 46); (2) to musical fish or shells, by Messrs. Evans and Lindsay (pp. 46 and 356); and (3) to gas-escape from vegetable mud and sand, by Mr. Malet (p. 47); whilst Mr. Dennehy himself suggests the possibility of some galvanic agency.

I remarked upon this vibratory phenomenon in a communication published in the Field newspaper of October 26th, 1867, signed Ubique," after having heard it myself when on board the Royal Mail steamer Danube (Capt. Reeks) during the nights of the 12th, 13th, 14th, and 15th of May, 1867; the new moon occurring on the 4th of the same month. As my statement serves to confirm Mr. Dennehy's report, I may be forgiven for giving it in full."

On

After giving an account of the sudden appearance of a huge white shark in the deep sea when a man fell overboard, I proceeded to state as follows:-"On embarking on board the Danube steamer, lying at anchor in the roadstead off Greytown on the 12th May, 1867, I was informed that the ship was haunted by most curious noises at night since she had arrived, and that the superstitious black sailors were much frightened at what they thought must be a ghost. The captain and officers could make nothing of it, and it afforded a great matter for discussion. inquiry I found out that other iron ships had been similarly affected. Curiously enough this noise was only heard at night, and at certain hours. Some attributed it to fish, suckers, turtle, &c., others to the change of tide or current; but no satisfactory conclusion could be arrived at. When night came on there was no mistake about the noise; it was quite loud enough to awaken me, and could be heard distinctly all over the ship. It was not dissimilar to the high monotone of an Æolian harp, and the noise was evidently caused by the vibration of the plates of the iron hull, which could be sensibly perceived to vibrate. What caused this peculiar vibration? Not the change of current and tide, because, if so, it would be heard by day. Like everything else that we cannot explain, I suppose we must put it down to electricity, magnetism, &c. If this should meet the eye of any

of the officers of the above-mentioned steamer, or others who have noticed this phenomenon, I should be glad to hear whether this effect still continues, or if any satisfactory conclusion has yet been arrived at. I may add that from the hold of the vessel the grunts of the toad-fish could be distinctly heard. I hope that the above notice may lead to some answers from your various correspondents."

This brief notice drew forth a rejoinder from a correspondent (November 23, 1867) who had noticed a somewhat similar

sound.

"The singular sound noticed by Ubique,' I have also heard without knowing its origin. One moonlight night in 1854, on board a steamer anchored near the Tavoy river (Tenasserim) we were struck by an extraordinary noise which appeared to proceed from the shore about a quarter of a mile off, or from the water in that direction. It was something like the sound of a stocking loom, but shriller, and lasted perhaps five or six seconds, producing a sensible concussion on the ear like the piercing scream of the cicada; and this gave an impression as if the vessel itself were trembling, or reverberating from the sound. One or two Burmans on board said simply, the noise was produced by 'fishes,' but of what kind they did not describe. It was repeated two or three times. I never heard it before or after the occasion referred to, nor have I ever met with any allusion to this singular phenomenon until I perused 'Ubique's' communication in the Field of the 26th ult. The steamer in my case, I should add, was a wooden one."

Mr. Evans, in his letter, speaks of the rapid silting up of Greytown harbour this still continues, and the passage over the bar, which is continually shifting, is often a matter of great difficulty, and indeed often so dangerous that the Royal Mail Company will not undertake to allow their own boats to land, and passengers have to land in the local canoes at their own risk. The Nicaraguan Government, however, propose to carry out Mr. Shepherd's plan of diverting the waters of the San Juan river from the Colorado mouth to the Greytown channel, hoping thereby to scour the harbour clear.

Mr. F. J. Evans also refers to the vast amount of animal life, and mentions the quantities of sharks and alligators which abound in and about Greytown Harbour. I can fully corroborate this, although I believe that what Mr. Evans terms alligators are really crocodiles (Molinia Americana), I should be glad to have certain information on this point: when not actually visible, their proximity is made evident by a powerful odour of musk. The most notable, however, of the denizens of these waters, besides the turtle, is the Atlantic manatee, which Columbus mistook for a mermaid, and which Agassiz terms the modern representative of the Dinotherium. The Mosquito Indians on the Indian, Rama, and Blewfields rivers are great adepts at harpooning this paradoxical mammal, and its flesh salted is a staple article of food all along these coasts, being not unlike to ship's pork.

P.S.-When at anchor off Greytown, also in the Danube steamer, during the night of February 15, 1867, (moon eleven days old) there was no vibration or noise perceived, but then there was a tremendous swell breaking with high surf on the bar, and the vessel rolling heavily. It would be interesting to overhaul the logs of the Royal Mail Company's vessels which have been at Greytown, in order to discover the periods of these vibrations, but I am afraid that no observations have been recorded in their books.

Mechanical Equivalence of Heat

You will see from the proceedings of the Literary and Philosophical Society at Manchester, that, since the discussion there, Dr. Joule has definitely abandoned the reasonings in his famous paper on the mechanical force of electro-magnetism, steam, and horses. I have now had time to test the facts and experiments of this new theory, and find it, as I hope soon to show in detail, as untenable as his former one. Indeed, I am sure that the mechanical equivalence of heat must soon be generally abandoned as inconsistent with facts. You will see that the April number of the "Review of Popular Science," has definitely pronounced a decision in my favour; and I am sure you will soon be convinced yourself that your own first reviewer of my article in the Quarterly Journal of Science was more reasonable than your second. H. HIGHTON

Aurora by Daylight

AN additional well-authenticated instance of this very rare but indisputable phenomenon, may, perhaps, be thought worthy of

insertion.

In the Transactions of the Royal Irish Academy for 1788 (embodied in "Memoirs of Science and the Arts," 1798), is "An Account of an Aurora Borealis seen in full Sunshine, by the Rev. Henry Ussher, D.D.," which opens in the ensuing

manner :

"The following phenomenon being very uncommon, if not entirely new, I think it worth communicating to the Academy, principally with a view to learn whether any other person has observed a similar one at any time :

"On Saturday night, May 24, 1788, there was a very bright aurora borealis, the coruscating rays of which united, as usual, in the pole of the dipping needle. I have always observed that an aurora borealis renders the stars remarkably unsteady in the telescope. The next morning, about eleven, finding the stars flutter much, I examined the state of the sky, and saw whitish rays ascending from every part of the horizon, all tending to the pole of the dipping needle, where at their union they formed a small thin and white canopy, similar to the luminous one exhibited by an aurora at night. These rays coruscated or shivered from the horizon to their point of union. These effects were distinctly seen by three different people, and their point of union marked separately by each of them."""

The Coronal Rifts

T. W. WEBB

THE enclosed extract of a letter from Captain Tupman, who observed the Eclipse of December last through the finder of Prof. Harkness's telescope at Syracuse, may interest some of your readers :

"It is a singluar feature in all the photographs that the 'rifts'

are so wide and distinct. They are actinic rifts. As seen in the telescope simply the corona had no such rifts. I watched it during the whole 105 seconds; such a feature would, of course, have struck me instantly. I actually pointed Prof. Harkness's spectroscope in the rifts as being bright parts of the corona!" A. C. RANYARD

The Name "Britain "

As "C. L. N. F." has in your last well answered the letter of "A. R. H.," I have now only to reply to Mr. Hyde Clarke's letter, in which he says I should find it difficult in my derivation of "Britannia" and "tin" "to explain on the same basis the conformable names of the countries and rivers which he mentions, inasmuch as "these names are not explainable in Phoenician, because they were given long before the Phoenicians entered on the stage of history."

His paper read before the Anthropological Institute I have not seen, but as the learned" Bochart and other authors have considered the name "Britain" to have been derived from the tin which the Phoenicians exported from Cornwall more than 3,000 years ago (Num. xxxi. 22), and as no one will venture to say that tin was not then the name of this metal in the most ancient Cornish as well as in the Phoenician language, from which it proceeded, I do not think I can fairly be called upon to go into the "difficult task suggested by Mr. Clarke.

The original name of our island I have imagined to be Bretin ("Tin Mount"), that being at first exclusively the name of the mount from which the Cornish tin was exported by the Phoenicians, and it is highly probable that the same name was afterwards given by these ancient traders to the entire island, of which the mount was only a part, for it was Britain that gave them nearly all their tin, and its most beautiful natural object known to them was St. Michael's Mount.

There being other islands close to Britain, the Romans gave the name Britannia indiscriminately to them all. When they spoke of Britain as dissociated from its contiguous islands, they called it either Britannia or Insula Britannica, which is synonymous with νῆσος Βρεττανικη. This word, BpETTAVIкn, used at first adjectively by the Greeks, had in the time of Diodorus Siculus become a substantive, so that he uses it as such when describing the daily insulated port or mount called sometimes Iktin (Tin Port), and sometimes Bretin (Tin Mount), adjacent to BPETTAVIKη, to which port or mount at low water the tin was carried from the mainland for sale and exportation. The following is the passage :-εἰς την νησον προκειμένην μεν της Βρεττανικής ὀνομαζομένην δε Ικτιν.

Plymouth, May 6

RICHARD EDMONDS

*** We cannot print any more letters on this subject. —ED.

The Sensation of Colour

PROF. CLERK MAXWELL in his valuable paper on Colour in NATURE (vol. iv. p. 13) commits himself to the opinion that there must be three distinct sets of retinal nerves, one for each of the three primary sensations of colour. It is obvious that demonstrative proof or disproof of this is unattainable we can only reason analogically. The analogy of the ear is in favour of such an opinion, so far as it goes; for there appears to be proof, or probability almost amounting to proof, that sounds of different pitch are conveyed to the brain by different nerves. But the ear resembles the other organs of sense less than they resemble each other; and there is surely no reason for thinking that there are distinct nerves of smell for every distinct kind of smell, or distinct nerves of taste for every distinct kind of taste. Nor I believe is there the slightest proof of nerves for the sensation of heat distinct from those of touch.

the other one is "Felis 14. Femur Right side." There is no record in the catalogue by whom they were presented, nor of any of the circumstances of their gisement. The specimens, in fact, have no history whatever, and I can only say that I found them in close juxtaposition with a large series of red-deer bones from Holderness, with which they perfectly agree in their mineralogical condition. I have no doubt that they are bonâ fide from the Holderness Peat.

Their identification as bones of F. leo (variety spelaa) is also Hull Royal Institution

certain.

C. CARTER BLAKE

Eozoon Canadense

SINCE reading some of the communications on the Eozoön, which have appeared from time to time in NATURE, I have felt constrained briefly to give the results of my examination of the "Eozoic" limestone in Eastern Massachusetts. I am the more disposed to do this, hoping that a new line of investigation will be suggested to observers in other localities.

Last autumn I visited for the first time the quarries of

"Eozoon" limestone in Chelmsford, under the guidance of my friend Mr. Burbank, of Lowell, Massachusetts, who has furnished many microscopists with specimens for sections. Having been long engaged in the study of the foliated series of rocks, and having years ago discovered indubitable evidence that portions of the included limestone are of vaporous origin, I was prepared to recognise the same feature in the Chelmsford "Eozoic" rock. I was accordingly not surprised on examination to find, what the advocates of the organic nature of the Eozoon seem never to have suspected, that the limestone in question is not a "sedimentary rock;" that it occupies, or rather occupied, (for it has been for the most part removed) pockets or ovenshaped cavities, which were once plainly overarched by gneiss; that it is foliated, there being a regular succession of leaf-like layers from the walls toward the centres of the cavities, witness to which is borne by a like succession of different minerals; that in some places it ramifies the surrounding rock in a vein-like way, while in others it exactly conforms with the most abrupt irregularities of surface; that in one locality, which I have repeatedly examined, it conforms with the uneven portions of a mass of syenite, with which it is so associated as to reveal its more recent origin; and that, therefore, it is not of nummulitic derivation, but was deposited in a vein-like form, the materials having been probably forced up into the cavities from below while in a vaporous state.

Such, in few words, is the result of my examination—a result which tends to show that the "Eozoon" of Eastern Massichusets is not organic, and that thus it belongs to the department of Mineralogy, and not to that of Paleontology. Waving additional particulars for the present, I may simply add that I propose in due time to give a detailed exposition of the relations of this famous "Eozoic" rock.

I

THICKNESS OF THE EARTH'S CRUST SEE that at p. 296 of your journal for February last, which has recently reached Calcutta, you print a lecture by Mr. David Forbes "On the Nature of the Earth's Interior," in which reference is made to the Mr. W. Hopkins's method of determining whether the thickness of the earth's crust is great or small when compared with the whole radius, and to M. Delaunay's objection to it.

The lecturer refers to me as having approved of Mr. Hopkins's method, which I always have done and do still, and then makes the following apparently crushing remarks to annihilate Mr. Hopkins and all who approve of his method and of the result to which it leads, viz., that the crust is very thick. He says:—“ M. Delaunay, an authority equally eminent as a mathematician and an astronomer, was induced to undertake the reconsideration of the problem; a labour (!) which has resulted in altogether reversing the above decision and demonstrating the complete fallacy of the premises upon which so much elaborate reasoning had been expended."

As the lecturer had condescended to mention my name in connection with the subject, I wonder why he has taken no notice of my letter in reply to M. Delaunay, which was printed in your journal for July 1870, six months before the lecture was delivered, and which also appeared about the same time in the Philosophical Magazine and the Geological Magazine. In this I showed that M. Delaunay had evidently misconceived the problem, and that Mr. Hopkins's method is altogether unaffected by his remarks.

So much has been said about profound mathematical calculations in connection with Mr. Hopkins's investigation, that I conceive many have shrunk from attempting to understand the question at issue, from a feeling that they would not be able to comprehend it were they to attempt to do so. But this is quite a mistake. Anyone with an ordinary degree of knowledge of popular astronomy and of mechanical action is quite competent to form a good opinion on the point in dispute. What Mr. Hopkins did may be divided into two parts. He first conceived an idea, which was to be the basis of his calculation; and then he made his calculation. It is the calculation that calls for the "profound mathematics." But it is not this that is the matter of dispute. It is the idea, on which the calculation is based, which M. Delaunay calls in question. I think I can make the matter sufficiently plain to your readers to enable them to form their own opinion.

Everyone having a knowledge of popular astronomy is aware that the earth revolves round an axis, which is fixed in the earth's solid crust, but shifts very slowly in space, producing what has been known ever since the days of Hipparchus by the name Precession. On this fact as his ground-work Mr. Hopkins reasoned as follows; and so got to his idea, which formed the basis of his calculation. Suppose the earth has a solid crust, the interior being filled up with fluid. If the axis remained steady in space and the crust revolved round it uniformly, no doubt, although the crust and fluid may have moved differently at one time, yet in the lapse of ages friction and viscosity in the fluid would cause the fluid at last to revolve with the crust just as if the whole were one solid mass. This being the case, suppose a slight horizontal push is given to the two poles, in opposite directions, so as slightly to shift the axis in space; what would happen? The revolving crust, by this new and additional motion, would slip over the surface of the revolving fluid, through a small space proportionate to the push given to the poles. The fluid could not possibly acquire in an instant this new motion, however small it might be, because the fluid is not rigidly connected with the crust. Suppose a second, and a third, and a succession of slight horizontal pushes to be given to the poles in a continually altering direction, the effect will be that the revolving crust will be continually slipping over the revolving fluid which has not time to acquire these new motions given instantaneously to the solid crust. These successive slight pushes given to the poles, and so to the solid crust, represent the unceasing action upon the crust of the force which causes the motion of precession in the earth's axis, and arises from the attraction of the sun and moon on the protuberant parts of the earth about the equator.

Mr. Hopkins having reasoned thus far, went a step farther, and so came to his fundamental idea. He saw that the thinner the crust the smaller would be the mass which the disturbing force producing precession would have to move, and therefore the greater would be the motion caused, that is, the precession. Here, then, he discerned a connecting link between the amount of precession of the earth's axis and the thickness of the earth's crust. This was the idea I have aliuded to.

Starting from this idea he entered upon a profound calculation and obtained a formula, which gives the thickness in terms of the amount of precession. This amount is a matter of observation; and the thickness can therefore be deduced by the formula from the observed pre