appears from the consideration of the orbit of Ceres, and of the orbit of the earth, that a velocity of three miles a second would be demanded by dynamical considerations quite independently of whatever additional speed the missile, should receive, in order to carry it free from the attraction of the globe on which the projective agent was situated. No doubt, as Ceres is small, this last might be, as we have said above, a velocity of moderate dimensions, attainable, in all probability, by ordinary artillery. But the velocity which has to be imparted on the other account is so considerable that no matter how small the mass of Ceres may be, a volcano of a projective power of at least three miles a second would be demanded. We thus see that there is no alleviation of the difficulty gained by locating the volcano on one of the minor planets.
Quite independently of this there is a line of reasoning which demonstrates that in all probability meteorites could not have come from any planet situated where Ceres is. It must not be forgotten that the track which the earth pursues in its annual progress round the sun is only an extremely fine line when viewed from the distance of Ceres. This consideration shows that it is only under exceptional circumstances that a meteorite projected from Ceres, even if it had speed enough, should ever tumble on our globe. The question is one in the theory of probabilities. In another part of this volume I have illustrated the importance which the theory of probabilities has for the astronomer. To the cases which are there given I may now add that which is connected with our present argument.
Let us imagine that Ceres was covered with volcanoes; suppose that these volcanoes were from time to time
