coalition of its parts amounts to × 6,370,000 or 4,250,000 metre-tons per ton of its whole mass: the metre-ton (an ordinary gravitation unit of work) being the amount of work required to overcome, through a space of one metre, a force equal to the weight of a ton at the earth's surface; the difference of the force of gravity at different parts of the earth's surface neglected. But unless, which is very improbable, the conglomeration took place quite suddenly by the simultaneous collision of materials falling in from all sides, a large part of this energy must have been dissipated away by radiation of heat consequent on partial collisions. We must therefore look on the definite estimate 4,250,000 metre-tons per ton of the earth's mass, which expresses somewhat accurately the whole potential energy exhausted during the conglomeration, as being considerably above the greatest amount of plutonic energy due to gravitation, that can ever have existed in the earth at any one time.

38. To estimate the potential energy of chemical affinity already exhausted, or yet to be exhausted,

by the combination of the materials constituting the earth, we may remark first, that the upper crust consists chiefly of metallic oxides, but contains also a large quantity of carbonic acid and water. Now we have the following results, from two very accurate observers, regarding heat of combination-reduced so as to show the amount of heat generated per unit mass of the compound substance formed:

HEAT OF COMBINATION OF VARIOUS ELEMENTS WITH

These numbers make it, I think, very certain

that the heat of combination per ton of the average

materials of the earth would be over estimated at 3,000 units centigrade—that is, 3,000 times the quantity of heat required to raise the temperature of a ton of water by 1° cent., or, according to Joule's equivalent, 1,270,000 metre-tons of energy.

39. The number 4,250,000 previously found (37) for the amount of potential energy of gravitation exhausted in the coalition of the earth's mass, is 3 times this estimate of the potential energy of the chemical affinity of its elements. The whole amount of energy due to the two causes together is about 5 million metretons, or 13,000 thermal units centigrade, per ton of the earth's mass. This, being about 700 times as much heat as would raise the temperature of an equal mass of surface rock from 0° to 100° cent., is three and a-half times the amount stated in § 34, as an over-estimate of the whole amount of heat at present in the earth. But considering, as in § 37, how much heat must have been dissipated during the conglomeration of the materials which now constitute the earth, we are rather compelled to contract than permitted to enlarge our ideas of

the possible total of plutonic energy at present in the earth, by tracing its history backwards to its probable origin.

PART III.-NOTE ON THE METEORIC THEORY
OF THE SUN'S HEAT.

[From Report of the Glasgow Philosophical Society's
Meeting of March 24, 1869.]

40. SIR WM. THOMSON, in reply to a question from the President, Dr. Bryce, said that his contribution to the meteoric theory of solar heat had been to point out that the meteoric supply could not be perennial. In his paper "On the Mechanical Energies of the Solar System" (Trans. R.S.E., April, 1854: republished in Math. and Phys. Papers, Vol. II., pp. 1-25), he had shown that meteors falling from extraplanetary space in sufficient abundance to generate the heat emitted from the sun for the last 2000 years, must, by the augmentation they must have brought to the central mass, have caused a gradual shortening of the year of which the accumulated effect during that period must have dislocated the

seasons to the extent of a month and a half. But observation proves that there has been a dislocation of the seasons only to the extent of about an hour and three-quarters, since a certain eclipse of the moon was seen on March 19, 721 B.C., in Babylon. It is quite certain, therefore, that meteoric supply for sun heat has not within historical periods come from distant space outside the earth's orbit. He therefore found it necessary to modify the meteoric hypothesis of sun-heat-a hypothesis which he had learned from a communication by Mr. Waterston to the British Association at Hull in 1853, but which he has since found had been previously proposed by Mayer. If it is true that the heat emitted by the sun is compensated from year to year by meteors, he proved that instead of a certain quantity of meteors falling in a certain time from distant extra-planetary space, as supposed by Mayer and Waterston, a double quantity in the same time must fall from orbits inside that of Mercury. But at the same time he pointed out that observation and dynamical theory of the motions of the planets must be had recourse to, to