1891.] Density of the Earth and the Gravitation Constant. 41 



so that it could be brought in turn immediately under either of the 

 suspended attracted masses. A balancing mass of half the weight, 

 and at double the distance from the centre of the turntable on the other 

 side, was found necessary, so that the centre of gravity should be in 

 the axis of rotation. Before this was used, the ground level was 

 seriously altered by the rotation of the turntable.. The attraction of 

 the balancing mass was calculated and allowed for. 



The alteration in the weights of the attracted masses, due to the 

 motion of the attracting masses from one side to the other, was the 

 quantity to be measured. When this was determined in the lower 

 position of the attracted masses they were raised to about double the 

 distance, and the attraction again determined. The difference elimi- 

 nated the pull on the beam, suspending wires, &c. To lessen the effect 

 of want of homogeneity or sphericity in the masses, or of want of 

 symmetry in the turntable, the masses were all inverted and changed 

 over each to the other side, and the weighings repeated. 



The position of the beam was determined by the reflection of a 

 scale in a mirror used with " double suspension." The mirror was 

 suspended by two silk threads, one attached to the end of the 

 ordinary pointer about 60 cm. below the central knife edge, the other 

 parallel to it, being attached to a fixed support. The mirror turned 

 through an angle about 150 times as great as that through which the 

 beam turned, and one scale division corresponded to an angle of tilt 

 in the beam of about 2/15ths of a second. 



The value of a scale division was determined by the use of two 

 equal centigram riders which could be placed on or taken off wire 

 frames representing the scale pans of a small subsidiary beam, 2'5 cm. 

 long, fixed parallel to and at the centre of the large beam. When one 

 rider was placed on one supporting frame the other was at the same 

 instant lifted off the other frame. 



The balance was left free throughout a series of weighings, and no 

 moving parts of the apparatus were connected with the case. 



The values obtained are as follows : 



The gravitation constant G = - 8 . 



Mean density of the earth A = 5'4934. 



In the paper a description is given of a new form of cathetometer 

 used to measure the diameters of the masses. 



