Page:The Harveian oration (electronic resource) - Royal College of Physicians, 1881 (IA b20411911).pdf/22

 not be known to us. Let us not imagine, however, that the law is the cause. I am afraid that many think and speak of the laws of gravitation as if they were the final cause of attraction. A larger body attracts a smaller one, not because it is larger, but because the principle of gravitation, whatever that may be, acts in proportion to its dimensions. This is one of the laws of its action, but the whole of these laws taken together do not explain its cause; and I conceive that it may be said generally that all laws are liable to exception; all causes operate uniformly.

No better or happier example of this difference can be cited than the suggestion of Adams of Cambridge, that the law of attraction from which the orbit of Uranus could be calculated was interfered with by some extraneous influence, which, for the time, modified the ordinary force, and produced an exception to the usual law. He calculated that a body external to it, moving in a similar orbit, attracted to the same centre, and attracting and being attracted by other bodies placed in relation to itself, would produce just such deviations from the recognised law as had been previously observed. By his calculations, he was able to indicate the exact portion of the heavens in which the planet Neptune was to be sought.

The lesson taught by this and similar instances seems to me to be this, that the cause of attraction