THE EARTH. 11 



vitation and its local distribution as electro-magnetic pro* 

 cesses and the chemical metamorphosis of matter. To refer 

 to extreme conditions, we may assume that if our planet had 

 only the, mass of the moon, and therefore almost six times 

 less intensity of gravity, the meteorological processes, the 

 climate, the hypsometrical relations of upheaved mountain 

 chains and the physiognomy of the vegetation would be quite 

 different from what they now are. The absolute size of our 

 planet which we are here considering, maintains its impor- 

 tance in the collective economy of nature merely by the re- 

 lations which it bears to mass and rotation ; for even in the 

 universe, if ohe dimensions of the planets, the quantitative 

 admixture of the bodies which compose them, their velo- 

 cities and distances from one another, were all to increase or 

 diminish in one and the same proportion, all the phenomena 

 depending upon relations of gravitation would remain un- 

 changed in this ideal macrocosmos, or microcosmos. 2 



a. Size, Figure, Ellipticity, and Density of the Earth. 



(Expansion of the Picture of Nature, Cosmos, vol. i, 



pp. 154164.) 



The earth has been measured and weighed in order to de- 

 termine its form, density, and mass. The accuracy which 

 has been incessantly aimed at in these terrestrial determina- 

 tions, has contributed, simultaneously with the solution of 

 the problems of astronomy, to improve instruments of mea- 

 surement, and methods of analysis. A very important part 



2 " The law of reciprocal attraction which acts inversely as the square 

 of the distance is that of emanations, proceeding from a centre. It ap- 

 pears to be the law of all those forces, whose action is perceptible at 

 sensible distances, as in the case of electrical and magnetic forces. One 

 of the remarkable properties of this law is that, if the dimensions of all 

 the bodies in the universe, together with their mutual distances and 

 their velocities, were proportionally increased or diminished, they would 

 still describe curves precisely similar to those which they now describe ; 

 so that the universe, after being thus successively reduced to the smallest 

 conceivable limits, would still always present the same appearance to 

 the observer. These appearances are consequently independent of the 

 dimensions of the universe, as in virtue of the law of the ratio which 

 exists between force and velocity, they are independent of absolute 

 movement in space." Laplace, Exposition du Syst. du Monde (Seme e"d.) ( 

 p. 385. 



