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 that the volcano on Ceres would have to be possessed of very considerable power, quite independently of whatever projective force might be necessary for the mere purpose of conveying the missile clear from the planet.

Let us suppose the most favourable case possible. In other words, let us try to conceive the circumstances under which, with the least expenditure of power, a projectile might be launched from Ceres under such conditions that it should cross the earth's track. It is obvious that this most favourable condition would be presented in the case of a volcano which was so placed on the planet as to lie exactly on the opposite side of the little globe, from that point which was foremost in its motion. For what the volcano has now to do is to abate the velocity which the missile possesses in virtue of the planetary movement, which it possesses in common with every other part of its globe. It follows that the velocity which must be imparted by the explosive power of the volcano, has to be at least three miles a second. For as the little object has a velocity of eleven miles a second in the direction perpendicular to the radius of the planet's movement, it is necessary to reduce this by three miles a second, in order to bring the actual velocity of the planet to the eight miles a second, which we have already stated to be an indispensable requirement if the object is to arrive at the earth as a meteorite. It can be easily shown that a volcano which happened to lie in any other situation than that just mentioned would have to impart an initial speed of more than three miles a second, if it were to reduce the velocity that the meteorite acquires from the planet down to the amount under which alone it would be possible for it to fall on the earth. It thus