gas, the changes of temperature produced by the application of stresses to elastic solids which you have investigated experimentally, and the cooling effect I have proved to be produced by drawing out a liquid film which I shall have to notice particularly below? Easy enough experiments on the contact electricity of metals will answer this question. If the contact-difference diminishes as the temperature is raised, it will follow from the Second Law of Thermodynamics, by reasoning precisely corresponding with that which I applied to the liquid film in my letters to you of February 2d and February 3d, 1858,* that plates of the two metals kept in metallic communication and allowed to approach one another will experience an elevation of temperature. But if the contact-difference increases with temperature, the effect of mutual approach will be a lowering of temperature. On the former supposition, the diminution of intrinsic energy in quantities of zinc and copper, consequent on mutual approach with temperature kept constant, will be greater, and on the latter supposition less, than I have estimated above. Till the requisite experiments are made, further speculation on this subject is profitless; but whatever be the result, it cannot invalidate the conclusion that a stratum of of a centimeter thick cannot contain in its thickness many, if so much as one, molecular constituent of the mass. Besides the two reasons for limiting the smallness of atoms or molecules which I have now stated, two others are afforded by the theory of capillary attraction, and Clausius' and Maxwell's magnificent working out of the Kinetic Theory of gases. In my letters to you already referred to, I showed that the dynamic value of the heat required to prevent a bubble from cooling when stretched is rather more than half the work spent in stretching it. Hence, if we calculate the work required to stretch it to any stated extent, and multiply the result by, we have an estimate, near enough for my present purpose, of the augmentation of energy experienced by a liquid film when stretched and kept at a constant temperature. Taking 08 of a gram weight per centimeter of breadth as the capillary tension of a surface of water, and therefore 16 as that of a water bubble, I calculate (as you may verify easily) that a quantity of water extended to a thinness of ̄ ̄ ̄ of a centimeter would, if its tension remained constant, have more energy than the same mass of water in ordinary condition by about 1,100 times as much as suffices to warm it by 1° Cent. This is more than enough (as Maxwell suggested to me) to drive the liquid into vapor. Hence, if a film of of a centimeter thick can exist as liquid at all, it is perfectly certain that there cannot be many molecules in its thickness. The argument from the Kinetic Theory of gases leads me to quite a similar conclusion.

5. Comparison of Mechanical Equivalents; by PLINY EARLE CHASE, (Proc. Am. Phil. Soc., xi, 313, 1870).—The comparison of different mechanical equivalents will open a new field for investigation, which may prove to be fertile in valuable results. For * Proceedings of the Royal Society for April, 1858.

example, recent determinations, by the different methods of Thomsen and Farmer, fix the mechanical equivalent of light, in a wax candle burning 126 grains per hour, at 13.1 foot-pounds per minute, the equivalent of 1 grain being 6.213 foot-pounds. According to Dulong, the heat evolved during the combustion of 1 grain of olive oil in oxygen, is sufficient to heat 9862 grains of water 1° C. According to Favre and Silbermann, 1 grain of oil of turpentine, burned in oxygen, would heat 10,852 grains of water 1° C.

It may therefore be presumed that the total heat given out by the combustion of 1 grain of wax, is about sufficient to raise 10,000 grains or water 1° C., or 18,000 gr. 1° F. This represents a mechanical equivalence of (18,000 X 7727000) 1985.143 foot-pounds, which is 3195 times as great as the corresponding equivalent of the light given out during the combustion.

Tyndall, in his lecture on Radiation, states that the visible rays of the electric light contain about one-tenth of the total radiated heat. The relative luminous intensity of an electric lamp would therefore appear to be about 32 times as great as that of the wax candle. This ratio so nearly resembles that of solar to terrestrial superficial attraction, and the connection of electric and magnetic currents with solar radiation is so evident, that additional experiments, to furnish materials for a great variety of similar comparisons, seem desirable. While it is possible that the resemblance, in the present instance, may be accidental, the numerous harmonies between the manifestations of cosmical and molecular forces render it at least equally possible that it may have a weighty significance.

II. GEOLOGY AND MINERALOGY.

1. On a Fossil Tooth from Table Mountain; by Prof. WILLIAM P. BLAKE. (Communicated by the author for this Journal.)— The fossil tooth, found by Mr. D. T. Hughes, 1,700 feet under Table Mountain, and 300 feet below the surface, I have carefully examined and compared with specimens in the Smithsonian Institution. It proves to be a back lower molar of an equine animal of the genus Hipparion, or a closely allied genus. This genus is one of the connecting links between the Palæotherium and the horse.

The specimen closely resembles a fossil in the Smithsonian museum, from the Pliocene formations of the Niobrara river in Nebraska,* not only in size but in the foldings of the enamel, and particularly in the posterior part of the tooth, but it differs enough, in several particulars, to justify the belief that it is a distinct species. Dr. Leidy does not attempt to determine, specifically, the specimen from Nebraska, but considers it closely related to, if not identical specifically with, Hipparion gratum, possibly Protohippus placidus.

* Described by Prof. Leidy in his work upon the Extinct Mammalian Fauna of that region, p. 319, pl. xix, fig. 7,

The size of the Table Mountain specimen, which is considerably worn by attrition in the gravel, is: length, 11 lines; breadth upon the crown, 9 lines; breadth at the base, 10 lines; thickness, anteriorly, 4 lines, posteriorly, 2 lines.

This fossil is the first of the kind discovered west of the Rocky Mountains. It adds to the list of the fauna of the period antedating Table Mountain-a list which includes the mammoth (Elephas, from Knight's Ferry), the rhinoceros, and an animal allied to the elk. I have believed that remains of man were also found under the lava; but upon this point, after diligent inquiry, I am satisfied that the evidence is insufficient. But we now add this fossil allied to Hipparion, and I regard it as another indication that the Table Mountain beds are Pliocene, and homotaxial with those of the Bad Lands of Nebraska.

2. Cause of the Descent of Glaciers.-Rev. HENRY MOSELEY, before the Royal Society in January, 1869, in the Philosophical Magazine for the May following, and at the meeting of the Royal Institution of Great Britain, May 13, 1870, opposes the view that glaciers owe their movement mainly to gravity, and gives as the principal cause contraction and expansion due to change of temperature through the mass. He compares the movement to that of a sheet of lead on a sloping surface, in its expansion the lower edge working downward, and in its contraction the upper edge or part.

In the Phil. Mag. for July, 1870, Mr. John Ball, after alluding to the criticisms on the above theory by Mr. Wm. Mathews in the Alpine Journal for February last, shows that the supposed contraction and expansion to which Canon Moseley appeals, does not take place, and that the "crawling theory" of glacier motion is therefore unsatisfactory. He argues that the glacier is not a continuous solid mass like a sheet of metal; that the temperature of the interior, as observers have proved, is very nearly constant; that the movement is half as fast in winter as in summer; that the rate of motion is not proportioned at all to the length of the glacier as it should be by the theory; that the supposed expansion and contraction, if a fact, would exceed twenty or more times the rate of actual motion but for modifying causes,-and this is an amount of modifying intervention for the sake of the theory, sufficient to prove the theory of no value. He concludes as follows: "If I might presume to estimate the net results of this renewed discussion of the causes of glacier-motion, I should say that they are not considerable, but yet are far from worthless. Canon Moseley's experiments have added something to our knowledge, and especially those on the tenacity of ice, which have some bearing on the origin of crevasses. Of far greater importance are the observations on ice-planks made by Mr. William Mathews. The first of these, published in the Alpine Journal,' gave prominence to a fact which had long been familiar to myself, and probably to many others. I have often found that long icicles placed in an inclined position, and supported only at the upper end, will gradually resume the vertical direction, and I had, perhaps too lightly, assumed that this was a particular instance of the process by

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which ice changes its form through fracture and regelation. In Mr. Mathew's first experiment, conducted during a thaw, a thick plank of ice supported at each end was deflected at the middle through a space of 7 inches in as many hours. Although none but very minute fissures were observed, the facts did not seem to me altogether inconsistent with that explanation. In the second series of observations, made during the severe frost of February last, Mr. Mathews found that at temperatures notably below the freezing-point a plank of ice, supported as before, subsides slowly between the points of support under the sole influence of its own weight. The deflection under these circumstances was about 1 inch in twenty-four hours. Taking this observation in connection with a multitude of facts recently brought to light, and especially the researches of M. Tresca, we are led to admit that ice, in common with very many apparently rigid bodies, does possess a certain degree of plasticity which is exhibited by changes of form effected very slowly under the action of forces of moderate amount, rather than by the rapid action of more powerful agencies.*

"The admission of this conclusion may slightly modify, but will not materially alter, the views now generally held as to the causes of glacier-motion, which are mainly derived from the remarkable researches of Professor Tyndall. Whatever may be the final judgment of men of science, I feel quite sure that it will not confirm the opinion expressed by Canon Moseley in his latest publication: that "the phenomena of glacier-motion belong rather to mechanical philosophy than to physics." Every real advance that has been made toward the explanation of those phenomena has been due to the application of increased knowledge of the physical properties of glacier-ice; and if any thing be wanting to complete the explanation now generally accepted, it must be derived from such additional acquaintance with those properties as may be derived from continued observation and experiment."

3. The North American Lakes considered as Chronometers of Post-glacial time; by Dr. EDMUND ANDREWS. 24 pp. roy. 8vo. (Trans. Acad. Sci. Chicago, vol. 11).-Dr. Andrews discusses in this paper the nature of the post-glacial deposits on the shores of Lake Michigan, especially in the vicinity of Chicago, their extent, the areas of the several beaches, the erosion these beaches have undergone, the width of the subaqueous plateau formed along the border of the lake out of the material removed in the erosion, and the amount of sand moved in the process; and from the elements thus obtained, arrives at the following conclusions: (1.) The upper beach began to form immediately after the Boulder Drift period, and continued to accrete for about 900 years. No animal fossils have yet been found in it.

(2.) The waters then fell suddenly to about their present level, where they remained till a thin bed of peat accreted on the marshy slope vacated by the waves. I have not been able to collect data for a calculation of this first low-water period, but from the posi

* In Dana's Manual of Geology, p. 673, this plasticity is recognized among the means of motion on the ground of observations, similar to the above, made by Kane in his "Arctic Explorations."

tion of the soil-bed in the eastern dunes, I incline to think it lasted 500 or 1000 years.

(3.) The water rose again, submerging for a short time the upper beach, but soon fell to the line of the middle one, where it remained about 1,600 or 2000 years. This period appears to be cotemporary with the Loess.

(4.) The water, which had already slowly fallen some feet, now retired more rapidly to near its present level, which it has maintained with only moderate fluctuations ever since.

(5.) The total time of all these deposits appears to be somewhere between 5,300 and 7,500 years.

The discussion is an interesting and important one. Some uncertainties in the calculations occur to us; but without a special examination of the region we are not at present prepared to mention any but the following. The author writes as if he supposed that sand in the course of transportation always remained sand. He observes that "the sand movement in the lake is confined to the shore line," as proved by the fact that "there is no sand in deep water," not recognizing the well-known geological fact that sands on coasts are always undergoing wear through the attrition of grain upon grain under the action of waves and currents, and that while the finer material made by this attrition is floated off to deeper waters, the coarser is left behind in such cases near or on the shores.

3. Fossils in the Mineral veins of the Carboniferous Limestone of Great Britain.-A paper on this subject by Mr. CHARLES MOORE, (Rep. Brit. Assoc. for 1869, p. 360), contains notices of numerous fossils in the Lead mines of the Carboniferous limestone. In the walls of the Charterhouse Lead-mine, in the Mendip range, 270 feet from the surface, over 80 species of Liassic fossils were obtained by him, and more than 30 of Carboniferous. The Liassic included a Chara, wood in the form of jet, Rhizopods, Pentacrinites, a Cidaris and other Echinoderms, Serpula, claws of Crustacea, many Mollusks, remains of about 10 species of fishes of the genera Acrodus, Hybodus, Lepidotus, &c., and a tooth of an Ichthyosaur; and among the Carboniferous, there were species of Helix, Hydrobia, Planorbis, Proserpina, Valvata, Vertigo, all either land or freshwater Mollusks, also 11 species of Ostracoids, besides Mollusks, Serpulæ, Encrinites, Corals and Conodonts. A А similar range of facts was observed in connection with other lead mines. We cite the following general remarks.

Whilst the various mines and mineral deposits I have examined have certain species in common, it may be said that they have each special paleontological features of their own.

In the Keld-Head Mines organic remains are very abundant at about 450 feet from the surface, amongst which are many Foraminifera, chiefly of the genus Involutina, of which there are six species, and univalves of about twelve genera, the freshwater species Valvata anomala Moore, and Planorbis Mendipensis

AM. JOUR. SCI.-SECOND SERIES, VOL. L, No. 149.-SEPT., 1870.