566 THE PRINCIPLES OF SCIENCE. [CHAP. 



gravitation of the globe with that of some body of matter of 

 which the mass is known in terms of the assumed unit of 

 mass. This body of matter, serving as an intermediate 

 term of comparison, may be variously chosen; it may 

 consist of a mountain, or a portion of the earth's crust, or 

 a heavy ball of metal. The method of experiment varies 

 so much according as we select one body or the other, that 

 we may be said to have three independent modes of arriving 

 at the desired result. 



The mutual gravitation of two balls is so exceedingly 

 small compared 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 observed until the end of the 1 8th 

 century. Michell attached two small balls to the extremi- 

 ties of a delicately suspended torsion balance, and then 

 bringing heavy balls of lead alternately to either side of 

 these small balls was able to detect a slight deflection of 

 the torsion balance. He thus furnished a new verification 

 of the theory of gravitation. Cavendish carried out the 

 experiment with more care, and estimated the gravitation 

 of the balls by treating the torsion balance as a pendulum ; 

 then taking into account the respective distances of the 

 balls from each other and from the centre of the earth, 

 he was able to assign 5 '48 (or as re-computed by Baily, 

 5 '448) as the probable mean density of the earth. New- 

 ton'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 Eeich gave 5*438. Baily having again per- 

 formed the experiment with every possible refinement 

 obtained a slightly higher number, 5-660. 



A different method of procedure consisted in ascertaining 

 the effect of a mountain mass in deflecting the plumb-line ; 

 for, assuming that we can determine the dimensions and 

 mean density of the mountain, the plumb-line enables us 

 to compare its mass with that of the whole earth. The moun- 

 tain Schehallien was selected for the experiment, and obser- 

 vations and calculations performed by Maskelyne, Hutton, 

 and Play fair, gave as the most probable result 4713. The 

 difference from the experimental results already mentioned 

 is considerable and is important, because the instrumental 



