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 be exerted to check its upward movement, and consequently the speed will but slowly abate. It, therefore, follows that this force must be put forth for a long time before the upward movement is entirely neutralised, and during this long time the body will have attained a correspondingly high elevation.

It can, indeed, be demonstrated that there is a critical velocity for every globe, such that if a body be projected upwards with that critical velocity or any greater one, it will be permitted to escape altogether.

When we know the mass of the globe as well as its radius, we are enabled to calculate what this critical velocity has to be. To take, for instance, the case of the earth. It can be shown that for any globe possessing the weight of the earth, and the exact size of the earth, the velocity with which a missile would have to be shot upwards so as to effect its escape must be about seven miles a second. If the velocity with which the body started on its vertical ascent were less than seven miles a second, then after reaching a height