1899.] SCIENCE. 101
the arithmetical mean of all the values and add to it the quantity - £ *2_ where S 2 is the sum of the squares, and S, is the sum of the cubes of the differences from the arithmetical mean.
Astronomy.
The favourite theory of the sun's heat is based on the postulate that the solar mass is of homogeneous density. Dr. See contends that if a heterogeneous mass be assumed the duration that must be assigned to the sun's heating power is much greater than in the former case ; and he has propounded the law that the absolute temperature of a nebula condensing under its own gravitation varies inversely as the radius of the contracting mass. As the greatest amount of heat is produced when the mass has reached its least dimensions and contrac- tion is about to cease, it follows that the solar temperature is still rising. This he takes to be at the present moment 8,000° C, whilst the original temperature of the central nebula when the earth was thrown off was less than 40° C. The earth on ceasing to contract had risen to about 2,000° C, which is high enough to account for all known geological facts. In addition, Dr. See, claiming to have determined the potential of a heterogeneous sphere as caused by itself, finds that the energy developed by condensation is greater than in the case of a homogeneous sphere in the ratio of 176,868 to 100,000.
As a result of M. Duner's spectroscopic observations of the sun's rotation, it appears that a point on his equator moves with a uniform velocity of 2*054 kilometres a second round an axis, the inclination of which to the axis of the ecliptic is 1812°, the longitude of the inter- section of the sun's equator with the ecliptic being + 28*00°. It is the synodic velocity, however, that is thus determined.
Messrs. Hartley and Ramage give spectroscopic reasons for believing that gallium is present in the sun.
There is little doubt that the northern and southern hemispheres of the earth have not the same curvature, though the amount of difference has yet to be ascertained. It seems probable too that her shape is tetrahedral, the result of a contractional deformation ; and Dr. Gregory has shown how geological facts uphold this theory.
Further portions of the Photographic Atlas of the Moon have been published by the Paris Observatory. The authors, MM. Loewy and Puiseux, draw some interesting conclusions from the white patches and trails that they attribute to a scattering of volcanic dust. The fact that these trails cover all the inequalities of the surface on which they lie points to their recent origin ; and their position can be ex- plained only by the supposition that they were deposited under the influence of an atmosphere agitated by variable currents. Indeed the authors contend that there must have been, at one time, a much denser atmosphere than corresponds with its theoretical distribution between the earth and the moon, which would give the latter only ri* th of the whole. They suppose that hydrogen was early lost, that other gases have been absorbed in chemical combination, and that water, unrepelled by lunar heat, has sunk into the interior. They believe that there now
