MOTION, LAWS OF. The laws of motion mean those universal methods of receiving and losing motion which close attention to mechanical phenomena, coupled with strict inductive reasoning, has shown to be inherent in the constitution of matter.
If an intelligent observer, not used to inductive reasoning, nor instructed in the results of mechanics, were required to state the views which experience had taught him of the constitution of matter, as an agent or patient in the production or reception of motion, he would perhaps reply as follows : Matter seems to have no power of moving itself, though if we judge from the fall of bodies towards the earth, the phenomena of magnetism, &c., it would appear as if matter might be the cause of motion in other matter. And it seems moreover that motion is an accident of matter which diminishes and dies out of itself, if some sustaining cause be not perpetually in action; for in all cases in which the experiment can be tried, we find that moving bodies are reduced to rest by being left to themselves. The motions of the hesvenly bodies, it is true, appear to be permanent ; but we have no certain assurance that there is not a constant sustaining physical cause of this permanency.
There would be something of truth, and a good deal of falsehood, in the preceding conclusions, and it is not an easy thing to give that exhibition of the real constitution of matter which is placed beyond all doubt by the coincidence of its results with all the more complicated phenomena of nature. There is no question that those principles, to take two cases out of thousands, on which a ball can be projected almost unerringly to its mark from the mouth of a cannon, and the motions of the moon can be predicted within a small fraction of a second, are founded in truth ; but it does not therefore follow that an 4 priori demonstration of them, mathematical or experimental, can be given ; and in fact the method of presenting the laws of motion to a beginner is encumbered with serious difficulties.
We shall begin by the assumption that those laws of motion which are to be found in all works on mechanics are true ; the reason for such assumption being, that if we take them for granted, and use them as the basis of a mathematical system of mechanics, all results of that system, however many the links in the chain of deduction, are found to agree with observed phenomena in species, and as nearly in magni tude as the various resistances and disturbances will allow. In
astronomy and optics, phenomena have been predicted with all but geometrical accuracy, by deduction from principles which would certainly be false if the received laws of motion were false. In terrestrial mechanics, the number of instances is unlimited in which these laws lead to that near approximation to prediction which is fully as much as can be expected with our imperfect knowledge of data. Many hundreds of phenomena admit, upon these laws, of an explana tion which, compared with that which they could receive from any others, is as easy as the hypothesis of the motion of the earth compared with that of its stability. _ So simple are the laws of motion themselves, that many have supposed them to be necessary, in the same sense as when we say it is a necessary consequence of our conception of straightness that two straight lines cannot inclose a space. We shall mention this notion again presently : in the meanwhile we are in this situation, that while it is difficult, as a matter of reasoning, to disentangle the fundamental laws from the variety and complication of the phenomena in which their effects are exhibited, yet these laws themselves, when disengaged, are of that startling simplicity which disinclines the mind to receive them as the results of a train of deduction, and disposes it rather to think that it could have dictated them from its own previous con ceptions.
It will make some difference in our method of seeking for these laws, whether we suppose the earth to be at rest or in motion. Now the decisive proofs of the motion of the earth, as it happens, are them selves derived from certain consequences of the laws of motion. [MOTION OF THE EARTH.] We seem then to be reasoning in a vicious circle ; nor do we see any mode of escape except by establishing the truth of these laws, whether the earth be at rest or in motion. And the process will be, first to detect laws for which there is a high and almost overpowering degree of probability in their favour ; next to appeal to the above-mentioned uniform truth of the results deduced from the assumption of such laws for the conversion of this high state of probability into one of absolute demonstration.