the considerate courtesy with which they have been treated by gentlemen who differ very widely from my conclusions. 1. Regarding the use of the spectroscope in the observation of "contacts.

I think the language of Mr. Stone and some others in a discussion of the matter at a meeting of the Astronomical Society, reported in your columns in December last, implies a misapprehension. What I have proposed (and executed in the case of the moon) is to use the extinction of the bright C line in the spectrum of the chromosphere as a criterion of contact with the limb of the photosphere, not with the upper surface of the chromosphere, which would, of course, as indicated by Mr. Stone, be a perfectly worthless observation.

The advantage of the method lies in this, that it furnishes an easily apprehended phenomenon to be watched for, and gives every advantage of preparation to the observer.

With an instrument of moderate dispersive power, the slit must be normal to the sun's limb, and an accurate knowledge of the expected point of contact is required: with a more powerful instrument the slit can be placed tangential, opened somewhat widely, and thus all difficulty on this score avoided, as I have pointed out in my report. I see by a paper of Mr. Proctor's in the December number of the "Monthly Notices," that Mr. Huggins suggests the same plan.

Perhaps I may remark in passing that the idea of using the spectroscope in this manner to observe the contact of the moon with the sun's disc, was conceived before the event, so that the observation was made deliberately and by pre-arrangement-not at all accidentally, as would rather seem to be implied by one of the opening sentences in the article of Mr. Proctor's above referred to. To M. Faye, however, belongs whatever merit there may be in the method, for he proposed essentially the same thing in January 1869. But I knew nothing of this at the time of the eclipse, nor indeed till long after.

2. The self-luminosity of the Corona.

It is not impossible that the so-called corona may be complex. Some portion of its radiance may perhaps originate in our own atmosphere, although I do not yet find myself able to accord with the conclusions of Dr. Gould and Mr. Lockyer in this respect, and am strongly disposed to believe that the whole phenomenon is purely solar.

This much appears certain, however, that there exists outside of the chromosphere properly so called (ie., the envelope of red hydrogen), and as distinct from it, as it is from the photosphere, an immense atmosphere of self-luminous substance, extending to a distance of from 5' to 8' from the sun's surface, and probably much further in places-phosphorescent dust or fog in a glowing gas.

In support of this idea I adduce the photograph of Mr. Whipple, taken at Shelbyville, Ky., with an exposure of 40s On this, the photolytic corona (if I may use the expression to distinguish it from the visible corona, whose points of maximum brilliance were, according to Dr. Gould, entirely different), reaches a height of 6'. Prof. Harkness observed the 1474 line in the spectrum of the corona at a distance of nearly 5 from the sun's limb, and not near to any prominence. I do not know the precise elevation at which I saw it, but it was not less than 3' or 4'.

Indirectly, also, the idea is confirmed by the spectroscopic observation of Prof. Pickering, who used a single prism instrument, with the slit simply directed towards the sun, not attached to a telescope. He saw only three or four lines, the brightest in the green near E. Now, since this line, when observed by throwing a large image of the sun on the slit, is very faint as compared with C, D, and F, its intensity, as seen by him, can only be accounted for by supposing that the luminous area from which it was derived far exceeded that of the chromosphere and prominences.

I have noticed also that some of the observers of the Indian eclipse (Rayet and Pogson) speak of the intensity of the green line. Did they observe in the same manner as Prof. Pickering? I need hardly add that Prof. Pickering's observation of the non-polarisation of the corona concurs with what has been said. As to the faint continuous spectrum, I am sure that the reported absence of dark lines was not the result of insufficient observation.

I could not have failed to see D, E, b, 1961, F and G had they existed, for in a spectrum of similar brightness formed by a light from a cloud, not only these but many other lines are visible in my instrument. Now, the absence of some of these might, perhaps, be accounted for on the ingenious hypothesis proposed

by Mr. Lockyer, and reported in your No. of February 3; but this would not apply to D, E, or G.*

But if we admit the existence of faintly luminous solid or foggy matter near the sun, either meteoric or arising from the cloudy condensation of a non-permanent gas, the whole is at once easy of comprehension.

3. The Auroral Theory of the Corona.

At any

The objection pressed by Mr. Lockyer that the bright line 1474 is only occasionally visible, is, I think, unfounded. rate I have never failed to see it myself when looked for, and very seldom to make it visible to others when I have wished to exhibit to them. It is faint, and, like a difficult microscopic object, requires management to bring it out with five prisms; but by placing the slit tangential to the sun's disc, and giving the instrument a slight jar, it is seen to flash out as the limb passes off the slit. It is worth noting too, that it is often especially plain at portions of the limb where the chromosphere is unusually shallow and faint.

But while I think it probable that this line coincides with the aurora line reported by Prof. Winlock at 1550 of Mr. Huggins' scale, I am by no means sure of it. I understand its assigned position rested upon a single observation with a chemical spectroscope, and the probable error of such a determination cannot well be less than ten divisions of Kirchhoff's scale. I have naturally made many attempts to determine its position for myself, but have never seen it except thrice, and then not long enough at a time to complete a measurement. I am only sure that its position lies between 1460 and 1490 of Kirchhoff.

For this reason, although I do not at all abandon the hypothesis, which appears to have other elements of probability in the general appearance of the corona, the necessity of intense electrical disturbances in the solar atmosphere as the result of the powerful vertical currents known to exist there, as well as the curious responsiveness of our terrestrial magnets to solar storms; yet I do not feel in a position to urge it strongly, but rather await developments.

As to the substance which causes this line, I observe that Father Secchi, in a recent communication to the French Academy, is disposed to think it hydrogen; while Mr. Lockyer still believes it to be iron. I am in hopes that experiments now in progress may throw some light on the subject.

May I suggest, in closing this long communication, that it seems to me that valuable observations might be made at the Eclipse of next December, by fitting up telescopes with a ground glass sliding screen, upon which an image of the corona two or three inches in diameter should be thrown; the ground glass having the roughened side next the observer, so that he could sketch upon it with a lead pencil the outlines of the image, the glass being made long enough to allow of several such sketches.

The comparison of a series of such outlines would decide the question of changes in the coronal streamers, as the sketches, being simple tracings, could not but be accurate in their indications of position.

Dartmouth College, N. H., March 1

C. A. YOUNG

P.S.-I think that the position of the line reported by me as 2602 should have been 25815, an error of one revolution of the micrometer screw having been made. At any rate on two occasions since the eclipse I have seen a bright line in the latter position, and I have never been able to find one in the former.

Professor Huxley's Address

MAY I be permitted to advert to one view in connection with that part of Professor Huxley's admirable address to the Geological Society, which treats of distributional provinces inhabited by the terrestrial vertebrata, and the subsequent incorporation of these provinces into one another at different periods.

The view to which I refer is that wherein Mr. Huxley attributes the origin of the eocene types to their evolution during the Mesozoic epoch in some province which then was isolated from the European area, and their introduction by geographical changes into the European area in the interval between the Mesozoic and Tertiary epochs.

Having brought forward ten years ago the view that the Australian province was an actual and isolated remnant of the Triassic continent and of its mammalian fauna; and that the geographical distribution of organised beings pointed to the inference that other portions of the land tracts of the Mesozoic period, with their more ancient faunæ, had at different times

* Why not? [ED.]

become incorporated with the post-cretaceous continent, it is with much satisfaction that I find views for the most part so similar developed by Professor Huxley with the ability which marks all his work. Nevertheless, I venture to submit that the view I then advanced as to the period of the origin of the Eocene types is more in accordance with the facts, as far as we know them, than the hypothesis of their origin in a detached province during the Mesozoic period.

The view I advanced was that great changes in the distribution of the continents and seas took place at the close of the Palozoic, and again at the close of the Mesozoic epoch; and so far I am at one with Professor Huxley: but I inferred that the geographical changes taking place at the close of the Mesozoic epoch were accompanied by the formation of a continent extending over all the geologically known parts of the globe, whose endurance was so prolonged as to have afforded the necessary time for the evolution upon it of the Eocene types.

In support of this inference I dwelt upon the entire disappear ance of the orders Pterosauria, Enaliosauria, and Dinosauria among vertebrates, and of the Ammonitida among invertebrates; as well as upon the great extinction that took place in various other forms of life. Such a process as the one suggested by Mr. Huxley would lead us to look in Eocene strata for an intermingling of these distantly evolved types with forms belonging to the several orders just named; however much these forms might in their specific, or even in their generic characters, have been changed during the interval in which these distantly evolved types were introduced. But instead of this we find an absolute disappearance of several important orders of life, of which, from their habitat, some-especially the Pterosauria-would seem to have been independent of geographical changes simply.

Mr. Huxley intimates that he is led to his view by arguments which he had previously used to demonstrate the necessity of the existence of all the Eocene types in some period antecedent to the Eocene; but may we not suppose that the interval thus marked by the disappearance of so many great orders was vast enough even for this evolution? Indeed so much did this great extinction weigh upon me that even the intervention of a vast lapse of time seemed scarcely sufficient; and I felt driven to suppose that these geographical changes in some degree altered the general conditions under which life had previously existed; and that this alteration, while stimulating evolution on the newly formed continent, contributed to the great extinction which marks the intra-cretaceous and Tertiary epoch.

Subtle as are the causes which have brought into existence the various types of being, those which have produced their extinction are not less so; though they have not yet received that attention which has been directed to the origin of species. I feel how crude were the suggestions I offered in 1860 to explain this great extinction, and how wide a field of conjecture upon the subject is left open; for these orders of life were not only various in their habitat, but equally various in their food. We may

imagine the extinction of a species to take place from failure of

its food, from destruction by enemies, or-and I think this may be a cause more potent than any other, especially with forms possessing great fecundity-by a failure of the reproductive function; just as among men families diminish and eventually leave no descendants. Be the causes, however, what they may, this great extinction requires us, I submit, to suppose the occurrence of an interval of time as great, and accompanied by changes of conditions as complete, as any that we can urge as necessary for the evolution of the Eocene types, Moreover, the cretaceous period itself, whose terrestrial fauna is as yet unknown, may, so far as we yet know to the contrary, have witnessed in the European area the commencement of, or even some progress in, the evolution of the Eocene types. Brentwood, March 10

SEARLES V. WOOD, Jun.

Transactions of the Royal Society of Edinburgh WITH reference to G.'s letter in the last number of NATURE, I have merely to observe that (as you will see by the accom

panying list) the Transactions of the Royal Society of Edinburgh are regularly sent to no less than twenty-three different societies, institutions, or museums in London alone-besides being sent

"On the probable events which succeeded the close of the Cretaceous period;" read before the Geological Society on February 1st, 1860. The publication of the paper, beyond a brief abstract, having been denied by the Council, the desideratum was kindly supplied by Dr. Francis; and the paper in extenso is given in the Philosophical Magazine of March, April, and May, 186a; the title having been changed to "The form and distribution of the Land Tracts during the Secondary and Tertiary periods, &c."

to many Honorary and Ordinary Fellows residing there. As regards the special case of the British Museum, I have in my possession at this moment their acknowledgments of receipt of the successive parts of our Transactions up to March 1809, and expect immediately to hear that they have received our last published Part. J. H. BALFOUR, Sec. R. S. Edin.

Euclid as a Text-Book

"THE first four books of Euclid: or the principal properties of triangles, and of squares and other parallelograms treated geometrically: the principal properties of the circle and its inscribed and circumscribed figures treated geometrically." Such is the wording of the programme put forth by the University of London, of the Mathematical portion of the examination for matriculation candidates. Whether the papers have ever been drawn up in accordance with it I cannot say, but certainly my experience for the last four or five years has led me to believe that the alternative side has, of late, at least, been altogether ignored.

The slightest inspection of recent papers will show that they are constructed on the Euclidean type, and so long as Euclid was generally taught in schools, I think rightly so. But that such a course should now be persisted in (with such latitude as the programme provides) is hard upon those establishments which have taken up the modern views of the subject, such as those so ably advocated by Professor Hirst, and Mr. J. M. Wilson of Rugby. It can hardly be thought that so advanced an examining body as the London University will continue to act as an obstructive for non-encouragement is almost tanta. mount to tabooing the subject; and the practical result of per sistence, I fear, will be this, that the course pursued will press unfairly upon those schools in which (as in University College School, where Wright's Geometry is now the text-book) Euclid has been almost discarded. Boys are required to study in their school work this modern geometry, founded on French mathematical works; and yet, seeing what value is set upon the same in the examination papers I am discussing, feel themselves constrained to read Euclid that their prospects of good places may be enhanced.

I am disposed to believe that "something will shortly he done," but the reform, though it ought rightly to commence here, ought not to stop here. Every examining body, if a fair field is to be given to the students of modern geometry, should put forth a scheme similar to that which heads my letter, and not merely put it forth "as a sop to Cerberus," but act upon it and let it be a reality.

University College School

R. TUCKER

MECHANICAL PROPERTIES OF ICE, AND THEIR RELATION TO GLACIER MOTION

A FEW weeks ago I prepared for the February number of the Alpine Journal a review of the contributions made by the Rev. Canon Moseley to the theory of glacier motion, which have appeared at various times during the last fifteen years in the Proceedings of the Royal Society and the Philosophical Magasine. Some new facts having come to my knowledge since the publi cation of my paper, I venture to recur to the subject, and to invite discussion upon those memoirs of Canon Moseley in which he endeavours to prove that the descent of glaciers by their weight alone is a mechanical impossibility.§ The arguments he advances in support of this conclusion may be epitomised as follows:

If a transverse section of a glacier were to be made, the ice would be found to be moving differently at every point of it. The velocity is greater at the surface than deeper down, at the centre of the surface than the edges. There is a constant displacement of the particles of ice over one the resistance known as shearing force. By the property another, and alongside one another, to which is opposed of ice called regelation, where a surface so sheared is * In his college lectures, and lectures to ladies at St. George's Hall, &c. "Euclid as a text-book of Elementary Geometry" (read before the London Mathematical Society, and printed in the Educational Times, Sept. 1868), and in his "Elementary Geometry."

1 Almost. In consequence of pressure from without, arising from the ar cumstances with which my letter deals, Euclid is again read in one class $Proceedings of the Royal Society, Jan. 7, 1869. Philos. Mag, May 1869. Philos. Mag., Jan. 1870.

brought into contact with a similar surface, it unites with it so as to form one continuous mass. Between the resistance to shearing and the force which tends to bring the glacier down there must be a mechanical relation, so that if the shearing resistance were greater, the force would be insufficient to cause the descent. By a series of experiments upon cylinders of ice inserted in a cylindrical hole bored through two pieces of wood perpendicularly to the surface along which the one was made to slide upon the other, it was found that the force necessary to part the ice along the sliding surface varied from 75 to 119 lbs. per square inch. Canon Moseley has calculated that for the Mer de Glace to descend by its own weight, its shear per square inch cannot exceed 13193 lbs., and that to produce the actual motion with a shear of 75 lbs. per square inch, a force in aid of the weight and thirty-four times as great must be called into existence, and applied in the direction of motion. For such a force to be produced by the weight of the glacier alone the density of ice would require to be increased more than 400 times.

In this reasoning Canon Moseley has neglected, as it appears to ine, the capability of ice when in a state of deliquescence to slide along a surface of small inclination, as demonstrated by the well-known experiment of William Hopkins. It is, however, not the motion of a block of ice as a whole, but the differential motions of its particles that we have now to consider. It occurred to me that the Canon's arguments upon this branch of the question might be put to an easy practical test by subjecting a block of ice to a strain produced by its own gravitation, and observing its behaviour under this condition, and I was fortunate in obtaining the assistance of my friend Mr. A. F. Osler, F.R.S., in carrying out the experiment.

A plank of ice 6 inches in width and 2 inches in thickness was sawn from the frozen surface of a pond, and supported at each end by bearers exactly six feet apart. From the moment it was placed in position it began to sink and continued to do so until it touched the surface over which it was supported, drawing the bearers with it, so as to make their upper ends converge. At its lowest point it appeared bent at a sharp angle, and it was rigid in its altered form. The total deflection was 7 inches, which had been effected in about as many hours under the influence of a thaw, during which the plank diminished slightly in width and thickness. On observing the under surface of the plank near the point of flexure, I noticed a number of very minute fissures extending a short distance into the ice, but they certainly were not sufficient to account for the flexure of the plank.

The question at once suggested itself, was the change of form in the ice plank due to fracture and regelation? I did not think it was, but the experiment was not decisive. Some weeks afterwards an opportunity occurred of trying it under other conditions. During the last frost we cut out another ice-plank. Its length was 6 feet 9 inches, its width varied from 61 to 6 inches, and its thickness was 1 inches. Two large bricks, of a width exceeding that of the plank, were set up on end, on a horizontal surface, exactly 6 feet apart, and the plank was laid upon them at five p.m. on the 12th of February. At 315 p.m. on the 13th it was continuously curved from end to end, so that it only rested on the edges of the bearers, and the middle point of its upper surface was deflected 1 inches below the line joining its two extremities. The temperature was 26°F. The curved plank was perfectly rigid, as was proved by taking it off the bearers and inverting it. I examined it again on the two subsequent days with the following results :

|

During the whole of this interval, in which the temperature never rose above the freezing point, there was no indication of fracture in the plank, nor did the optical continuity of the ice suffer the slightest interruption. On the 15th it began to thaw, and the bearers having become frozen to the ground, and the plank to the bearers, the suspended portion was unable to yield to the strain produced by its gravitation; and when I re-visited the plank on the afternoon of the 15th, it was broken into half-a-dozen pieces.

These experiments were very rough and imperfect; we intend to renew them on some future occasion, and to conduct them with much greater care and proper mechanical appliances, when we hope to be able to bend an ice-plank double, without destroying its continuity. The following conclusions may fairly be drawn from them :

1.-A mass of ice may change its form under strains produced by the gravitation of its particles, without becoming fractured, and without returning to its original form when the strain ceases.

2.-The change of form takes place at temperatures both below and above the freezing point, but is greatly accelerated in the latter case.

I shall not now attempt to discuss the nature of the molecular displacements to which the change of form is due. Their occurrence is indisputable; whether or not they are to be dignified by the name of shearing is a mere verbal question of little moment. In a very able paper in the Philosophical Magazine for March 1869, Mr. James Croll adduces good reasons for believing that when a mass of ice has a deliquescent surface, its molecules may experience repeated momentary losses of their shearing force. While, therefore, he admits the conclusiveness of Canon Moseley's reasonings for temperatures below freezing, he conceives that ice at all higher temperatures may shear by its own gravitation. It is evident that the former concession in Canon Moseley's favour cannot now be maintained, and that the point to which our experimental researches should be directed is not what amount of force will suddenly rend asunder the molecules of ice beyond the sphere of their mutual attractions, but what amount of force will produce molecular displacement within that sphere, with time allowed for its operation.

If we conceive an ice-plank, instead of being placed horizontally between bearers, to be laid with its narrowest face upon a plane of small inclination, with its upper edge horizontal, and its ends confined between vertical walls converging in the direction of motion, with its under surface deliquescent, so that friction would almost be annihilated; and if we further imagine the diminution of gravity due to resolution along the plane to be compensated by increasing the length or diminishing the thickness of the plank, the plank would alter its form in a way presenting a striking resemblance to the actual movement of a glacier. Its central portions would move more rapidly than its lateral ones; its surface more rapidly than its base; and when the strain upon its particles exceeded their cohesive power, it would fracture obliquely to the axis of the channel.

If the conclusions drawn from the experiments above described are legitimate, plasticity must be admitted by the side of sliding, and fracture and regelation as one of the constituent elements of the theory of glacier motion, and a more important place in that theory must be assigned to the views of the late Principal Forbes than has for some years been conceded to them.

WM. MATHEWS, Jun.

bright lines, which I had fully expected to see in the lower strata of the corona, I opened the jaws of the slit, and repeatedly adjusted by the finder, but without effect. What I saw was undoubtedly a continuous spectrum, and I saw no lines.* There may have been dark lines, of course, but with so faint a spectrum and the jaws of the slit wide apart, they might escape notice.

We next have the spectrum of the Great Horn :-
One line in the red was so beautiful that it needed an effort to

turn my attention to anything else; there was a line in the orange not so well defined, and one in the green which seemed

of the green line coincides with that of the brightest line in 6, instead of the mean of the three, which I read as a verification; the line near to F was in all probability F itself; E was certainly not seen by me. The line in the blue it is useless from my data to speculate upon, I must hope that some one else has identified it.

It is pleasing to point out how very nearly Major Tennant's observations, as now given, approximate to the true state of the case, which we can now determine any day that the sun shines. He must be entirely congratulated

on the degree of success of his spectroscopic observations observations made, according to his report, under difficulties which he ought not to have encountered. Major Tennant's evidence in favour of the continuous spectrum of the corona has been entirely confirmed by the observations since made in America.

The photographic results we may introduce by a wood

culum of nine inches aperture, mounted by Mr. Browning, with which the photographs were taken.