Page:Popular Astronomy - Airy - 1881.djvu/233

Rh come to an end. Now, how does this apply to the first law of motion, that a body moves in the same straight line and with the same velocity? All I can tell you is this: if it is true that each part of a body would, if unconstrained, move steadily in a straight line, and if (by the connection of the parts) each part is constrained to move in a circle, then it appears by mathematical investigation that the body will revolve with a uniform, velocity; but if it were not true, then the body would not move in a circle with a uniform velocity. There is another instance which is perhaps still more remarkable. I allude to the motion of a pendulum. The motion of a pendulum backwards and forwards is a result of the first law of motion, taken in combination with the disturbing force of gravity; this motion of the pendulum being the most permanent of all that are the subjects of ordinary experiments. If a pendulum be properly constructed, mounted with a steel edge (like that of the best balances) moving on a flat plate of hard agate, and if it be set in the exhausted receiver of an air pump, it will go on for 24 or 30 hours, without the action of anything to keep up its motion. But still it seems a very strange thing to infer, from this backward and forward motion, the law which asserts that the pendulum would go on continually in the same direction in a straight line, if there were nothing to disturb or counteract the operation of that law. Upon making the proper mathematical investigation, it is found that the only way of explaining the motion of the pendulum is, by saying that it would go on in a straight line continually, if it were not acted on by certain causes which we are able to take into account.

It is right you should understand how the matter