Page:The evolution of worlds - Lowell.djvu/172

140 step on the great underlying principles of physics. One of these is the conservation of the moment of momentum. This expression embodies one of the grandest generalizations of cosmic mechanics. The very phrase is fittingly sonorous, with something of that religious sublimity which the dear old lady said she found such a consolation in the biblical word Mesopotamia. Indeed the idea is grand for its very simplicity. Momentum means the quantity of motion in a body. It is the speed into the number of particles or the mass. Moment of momentum denotes the rotatory power of it round an axis. Now the curious and interesting thing about this quantity is that it can neither be diminished nor increased. It is an abstraction from which nothing can be abstracted—but results. It is the one unalterable thing in a universe of change. What it was in the beginning in a system, that it forever remains. Because of this unchangeableness we can use it very effectively for purposes of deduction. One of these is in connection with that other great principle of physics, the conservation of energy. By the mutual action of particles on one another, by contraction, by tidal pulls, and so on, some energy of motion is constantly being changed into heat and thus dissipated away. Energy of motion, therefore, is slowly being lost to the system, and the only stable state for the bodies composing it is when their energy of motion has