30 LAW OF GRAVITATION, 



obeys, enables us to conclude that the sun and distant 

 planets are consolidated masses like this earth. We 

 find that they have gravitating power, and by com- 

 paring this influence with that exerted by the earth, we 

 are enabled to weigh the mass of one planet against 

 another. In the balance of the astronomer, it is as 

 easy to poise the remote star, as it is for the engineer 

 to calculate the weight of the iron tunnel of the Menai 

 Straits, or any other mechanical structure. Thus through- 

 out the universe the balance of gravitating force is un- 

 erringly sustained. If one of the most remote of those 

 gems of light, which flicker at midnight in the dark 

 distance of the starry vault, was, by any power, removed 

 from its place, the disturbance of these delicately ba- 

 lanced mysteries would be felt through all the created 

 systems of worlds. 



From the peculiarity of the laws which this power 

 called gravity obeys, it has been inferred that it acts 

 from centres of force ; it is proved that its power dimi- 

 nishes in the inverse ratio of the square of the distance, 

 and that the gravitating power of every material body 

 is in the direct proportion of its mass. In astronomical 

 calculations we have first to learn the mass of our 

 earth. Experiment informs us that the density of our 

 hardest rock is not above 2'8; but from the enormous 

 pressure to which matter must be subjected, at great 

 depths from the surface, the weight of the superincum- 

 bent mass constantly increasing, it is quite certain 

 that the earth's density must be far more than this. 

 Maskelyne determined the attraction of large masses 

 by a plummet and line on the mountain Schehallion.* 



* Delambve dates the commencement of modern astronomical 

 observation in its most perfect form from Maskelyne, who was the 

 first who gave what is now called a standard catalogue (A.D. 1790) 

 of stars; that is, a number of stars observed with such frequency 

 and accuracy, that their places serve as standard points of the 

 heavens. His suggestion of the Nautical Almanack, and his su- 



