ON GEOLOGICAL TIME. 41 



are just as great a disgrace to the mechanical 

 genius of Europe and America as chronometer 

 watches are a credit. Astronomical clocks go 

 only about two or three times as well as pocket 

 chronometer watches ; although the latter, from the 

 continual agitations to which they are exposed, 

 are in very disadvantageous circumstances. When 

 they shall be made two or three hundred times 

 as good as they are, we shall have an instrument 

 which, for use during a few centuries, will be a 

 superior time-keeper to the earth ; and it will 

 not then be necessary to set the clock by the 

 stars, but we shall test the earth's motion by the 

 clock. However, that is only in anticipation. 

 Perhaps we may not live to see that use of the 

 clock. In the meantime we are obliged to put 

 up with the earth and stars as a means for regu- 

 lating our clocks. Failing a good clock to check 

 the earth by, we have to take the best we can 

 find and apply corrections to it. The moon is 

 a very unequal time-keeper, but by prodigious 

 labour, carried out by Newton, Clairaut, Laplace, 

 Plana, Hansen, Adams, and Delaunay, the errors 



