SECT. VIII. DENSITY OF THE EARTH. 57 



cause bodies to fall through about 334*65 feet in a second. 

 Consequently, if he were habitable by human beings, they would 

 be unable to move, since their weight would be thirty times 

 as great as it is here. A man of moderate size would weigh 

 about two tons at the surface of the sun ; whereas at the surface 

 of some of the new planets he would be so light that it would be 

 impossible to stand steady, since he would only weigh a few 

 pounds. The mean density of the earth has been determined by 

 the following method. Since a comparison of the action of two 

 planets upon a third gives the ratio of the masses of these two 

 planets, it is clear that, if we can compare the effect of the 

 whole earth with the effect of any part of it, a comparison may 

 be instituted between the mass of the whole earth and the mass 

 of that part of it. Now a leaden ball was weighed against the 

 earth by comparing the effects of each upon a pendulum ; the 

 nearness of the smaller mass making it produce a sensible effect 

 as compared with that of the larger : for by the laws of attrac- 

 tion the whole earth must be considered as collected in its centre. 

 By this method it has been found that the mean density of the 

 earth is 5*660 times greater than that of water at the tempera- 

 ture of 62 of Fahrenheit's thermometer. The late Mr. Baily, 

 whose accuracy as an experimental philosopher is acknowledged, 

 was unremittingly occupied nearly four years in accomplishing 

 this very important object. In order to ascertain the mean 

 density of the earth still more perfectly,. Mr. Airy made a series 

 of experiments to compare the simultaneous oscillations of two 

 pendulums, one at the bottom of the Harton coal-pit, 1260 feet 

 deep, in Northumberland, and the other on the surface of the 

 earth immediately above it. The oscillations of the pendulums 

 were compared with an astronomical clock at each station, and 

 the time was instantaneously transmitted from one to the other 

 by a telegraphic wire. The oscillations were observed for more 

 than 100 hours continuously, when it was found that the lower 

 pendulum made 2 oscillations more in 24 hours than the upper 

 one. The experiment was repeated for the same length of time 

 with the same result ; but on this occasion the upper pendulum 

 was taken to the bottom of the mine and the lower brought to 

 the surface. From the difference between the oscillations at the 

 two stations it appears that gravitation at the bottom of the 

 mine exceeds that at the surface by the T9 -bo part, and that the 



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