262 Prof. C. V. Boys. [June 20, 



Fig. 5. 



a slightly different position from that of the Curve 1, and its greatest 

 value is only just over half that of the first curve, which shows that 

 the sensibility can be nearly doubled by lowering the ball b until it 

 is too far down to be appreciably attracted by the ball which is on a 

 level with a. The five curves below 2 show the effect of lowering b 

 until it is 1, 2, 3, 4 and 5 times the length of ab below the general 

 level, and the five curves between 1 and 3 in the same way represent 

 the values of the balance in favour of a when b is at these different 

 levels. The curves when drawn are instructive in that they show 

 both the relative advantages of the various differences of levels, and 

 from the curvature at the maximum positions the practical importance 

 of correctly determining the azimuth. By reference to the number 

 on the vertical scale it is also easy to directly compare the sensibility 

 of the apparatus with any of the arrangements which have been in 

 use. For instance, calling in every case the half length of the beam 

 unity and the dimensions of the other parts by numbers in proportion, 

 we have in the apparatus of Cavendish — 



Equivalent distance between centres of large and 



small balls (8-95 inches) 0'249 



Diameter of large balls (12 inches) 0*333 



Comparative value of deflecting force 0'333 3 /0'249 2 0-596 



