THE MECHANISM OF NATURE t 293 



ing? Or do we find any reason to think that its independence of 

 a user has any bearing on its usefulness to those who know how 

 to use it? As it is obvious that the clock will not go unless the 

 water continues to run down hill, the assertion that it is self- 

 sustaining clearly has no better basis than our confidence that water 

 which is free to run down hill will do so; but this basis is so firm 

 that I do not suppose any one looks for or holds that he has any 

 other. 



Water runs down hill by gravitation ; and the predictions we 

 base on the stability of gravitation command our utmost confi- 

 dence. The nautical almanac, published several years in advance, 

 gives the predicted places of the sun, moon, and principal planets 

 from day to day, and in some cases, from hour to hour, through 

 the whole year. Unless gravitation is stable, these predictions are 

 worthless ; yet no one hesitates to trust his fortune and his life 

 and even the safety and honor of his country to the nautical 

 almanac. Even if this prove at fault, if, in any particular, obser- 

 vation fail to verify its predictions, no one ever dreams that its 

 principles are wrong. On the contrary, the astronomer himself, 

 after making sure that computers and printers and those who use 

 the predictions have made no mistake, uses this failure to correct 

 his estimates of the sizes and distances and velocities of the 

 heavenly bodies. Unknown planets and satellites, worlds which 

 no human eye had seen, have been deduced from the data of 

 astronomy with such exactness that the new world has been found 

 when the telescope has been turned to the designated spot. 



When we reflect upon the meaning of our confidence in gravi- 

 tation, who can wonder if some think that the clock which is 

 found to fall into a place in the same system with the facts of 

 astronomy must go on of necessity, although no words can be more 

 emphatic than those in which the men of science repudiate this 

 belief ? Huxley, for example, " anathematizes " it in the following 

 words, to which all thoughtful men of science must subscribe : 



" What is the dire necessity and ' iron ' law under which men 

 groan ? Truly, most gratuitously invented bugbears. I suppose 

 that if there be an iron law, it is that of gravitation, and if there 

 be a physical necessity, it is that a stone, unsupported, must fall 

 to the ground. But what is all we really know about the latter 



