1878.] 



using the Balance with great delicacy, Sfc. 



3 



more easily guarded against with, the former, by reason of the nature 

 of the experiments usually performed with it. They might, perhaps, 

 as has been suggested by Mr. Crookes, be removed from both by using 

 the instruments in a partial vacuum, in which the pressure is lowered 

 to the " neutral point," where the convection currents cease, but the 

 radiometer effects have not yet begun. But a vacuum balance 

 requires such complicated apparatus to work it, that it is perhaps 

 better to follow the course which Baily adopted in the Cavendish 

 experiment. He sought to remove the disturbing forces as much as 

 possible, and to render those remaining as nearly uniform as possible 

 in their action during a series of experiments, so that they might be 

 detected and eliminated. For this purpose the instrument was placed 

 in a darkened draughtless room, and was protected by a thick wooden 

 casing gilded on its outer surface. Most of the heat radiated from 

 the surrounding bodies was reflected from the surface of the case by 

 the gilding. The heat absorbed only slowly penetrated to the in- 

 terior, and was so gradual in its action, that, for a considerable time, 

 the effect might be supposed nearly uniform. Under this supposition 

 it was then eliminated by the following method of taking the observa- 

 tions. The resting point (that is the central position of equilibrium, 

 about which the oscillations were taking place) of the torsion rod, at the 

 ends of which were the small attracted weights, was first observed when 

 the two large masses pulled it in one direction. The masses were then 

 moved round to the opposite side, when they pulled the rod in the 

 opposite direction and the resting point was again observed. The 

 masses were then replaced in their original position and the resting 

 point was observed a third time. These three observations were made 

 at equal intervals of time ; if, then, the disturbing effect w r as uniform 

 during the time, the mean of the first and third observations gave 

 what the resting point would have been, had the rod been pulled in 

 that one direction at the same time that it was actually observed when 

 pulled in the opposite direction. The difference between the second 

 resting point and the means of the first and third might, therefore, be 

 considered as due to the attractions of the masses alone. 



In the experiments of which this paper contains an account, T have 

 endeavoured to apply this method of introducing time as an element 

 to the ordinary balance. But, before it could be properly applied, 

 it was necessary to remove the errors due to the raising of the beam 

 between successive weighings, as they could not be considered to vary 

 in any uniform way with the time. I think I have effected this satis- 

 factorily, by doing away altogether with the raising of the beam by 

 the supporting frame, between the weighings. For this purpose I 

 have introduced a clamp underneath one of the pans, which the 

 observer can bring into action at any time, to fix that pan in whatever 

 position it may be. The weight can then be removed from the pan, 



b 2 



