104 



LECTURES 



conclusion, that there are so many discrepancies in the results of these 

 computations as to force upon astronomers the conviction that the 

 figure of the earth is not that of a spheroid of revolution. There 

 must be in different portions of the surface local depressions and eleva- 

 tions which are most probably due to unequal densities in the interior. 

 The surface, strictly speaking, is warped and irregular. But these 

 irregularities, though capable of being detected, are very small. 



Such are the results of two centuries of arduous scientific labor 

 upon the figure and magnitude of the earth ; labor in the field and 

 in the study, which has put in requisition the utmost skill of mechanic 

 arts, and exhausted the resources of mathematical analysis. 



LAW OP GRAVITATION. 



In my previous lecture I endeavored to explain the method of find- 

 ing the exact figure and magnitude of the earth. 



1. I shall this evening call your attention to the law of gravitation. 

 This may be termed the great law of the material universe. There 

 is no single law of nature which reveals so much, none which controls 

 and determines so many physical conditions and events, none which 

 carries us so far into. the distant future, or conducts us back so far to 

 contemplate the past. This law determines alike the fall of a pebble, 

 the mighty swell of the ocean, and the vast sweep of the comet, 

 coursing its fiery track of a thousand years. It guides every planet 

 in its orbit round the central sun, and, in the midst of inevitable and 

 endless perturbations, guarantees the final stability and harmony of 

 the system. In its immense range of applications it invites to the 

 sublimest contemplations of which the human mind is capable. 



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2. The law of gravity may be stated as follows, viz: every particle 

 of matter attracts every other particle by a force proportional to the 

 mass, and inversely as the square of the distance. 



