Page:In the high heavens.djvu/316

 surface of a globe as large as many of the globes in space would only suffice to carry the body to a height of one mile, or even less. It is, therefore, understood that the elevation to which the missile is capable of soaring depends not alone on the efficiency of the cannon from which it has been projected, but on the mass of the globe on which that cannon is placed.

Nor is it indeed only the mass of the globe which is concerned in the matter. It will easily be seen that the diameter of the body must enter as a significant element. Suppose that there were two globes equal in mass, but that in one the materials were of a lighter specific gravity than in the other, and that consequently the globes were of unequal dimensions. Then the attractions of the two globes exercised upon bodies on their respective surfaces would be very different. In the one case the attracted object would be further away from the centre of the globe than in the other, and though the masses of the globes may have been the same yet as the attraction varies inversely as the square of the distance, it would be less on the surface of the greater body than it is on the smaller.

To give an illustration of what I mean, let us suppose the case of two globes equal in weight, but one of which was made of platinum and the other of granite. Platinum is nearly eight times as heavy as granite, it therefore follows that as the globes are of equal weight, the granite globe must be about eight times as bulky as the metallic globe, and this being so it can easily be shown that the radius of the larger globe must be double that of the smaller. There would thus be considerable difference in the gravitation which would