208 THE PRINCIPLES OF SCIENCE. 



heavenly bodies. Physicists have accordingly bestowed 

 a great amount of labour upon the exact estimation of 

 this density, consisting in the exact comparison of the 

 gravity of the whole globe with the gravity of some se- 

 lected body of matter, of which the mass, or what comes 

 to the same thing, the density compared with water, is 

 known more or less exactly. But this body of matter may 

 be variously chosen ; it may consist of a heavy bah 1 of 

 lead, or a mountain, or a portion of the earth's strata, and 

 the methods of experiment are so very different in these 

 different cases that they may be regarded as giving entirely 

 independent results. 



The mutual gravitation of two balls, or other small 

 objects at the earth's surface, is so exceedingly small com- 

 pared with their gravitation towards the immense mass 

 of the earth, that it is usually quite imperceptible, and 

 although asserted by Newton to exist, on the ground of 

 theory, was never detected until the end of the i8th 

 century. Michell attached two small balls to the ex- 

 tremities of a delicately suspended torsion balance, and 

 then bringing heavy balls of lead alternately to each side 

 of these small balls was able to detect a certain slight 

 deflection of the torsion balance, which was a new verifi- 

 cation of the theory of gravitation. Cavendish carried 

 out the experiment with more care, and by estimating 

 the actual gravitation of the balls by treating the torsion 

 balance as a pendulum, and then taking into account the 

 respective distances of the balls from each other and from 

 the centre of the earth, was able to assign 5*48 (or as re- 

 computed by Baily, 5*448) as the probable mean density 

 of the earth. Newton's sagacious guess to the effect that 

 the density of the earth was between five and six times 

 that of water, was thus remarkably confirmed. The 

 same kind of experiment repeated by Reich gave 5 '438. 

 Baily having again performed the experiment with every 



