connected the determination of the masses of the different bodies of the solar system. And here I must observe that, on entering minutely into this subject, it is impossible to take one thing alone. When I take the theory of gravitation, I must begin by taking the evidence relating to the laws of motion, for these were described and defined long before the theory of gravitation was expounded. Now the laws of motion, in the shape in which they have been commonly expressed, are these: in the first place, if a body be started in motion, and if no force act upon it, that body will continue in motion in the same direction and with the same velocity. Of all things in the world, this is the most difficult to prove immediately. It is obvious that we cannot put a body in motion so that it shall go on in one unvaried direction, and that it shall go on for ever, for we cannot put it in motion in a place where no force will act on it; and we cannot observe it through infinite space and infinite time. This is one of those instances in which we can examine a law only in connection with other laws. We must investigate by profound mathematical process what will be the effect of combining this law with others, so that we may observe whether the results, which are produced practically, agree with the results which we have found from the mathematical process.
For instance, one of the cases which we can observe is, that where rotatory motion is continued for a very long time, and where the velocity very slowly diminishes, as the motion of a wheel or spinning top. Contrivances have been made for the purpose of spinning tops and wheels in the exhausted receivers of air pumps. They go on spinning and spinning, and the motion seems as if it would never
