10 



Mr. J. H. Poynting on a method of [Nov. 21, 



the rider on the beam being the right hand one denoted by R, 

 column 1. The balance is the clamped, and the other rider is brought 

 on to the beam while the first is taken up. The resting point is again 

 observed. In No. I it was 270'05. The balance is again clamped, 

 and the first rider again brought on the beam, and in unclamping the 

 resting point again observed. In the same weighing it was 296 '75. 

 These three are sufficient to give one determination of the deflection 

 due to the transference of a rider. This will be the difference between 

 the second resting point and the mean of the first and third. For 



instance, 62 + 296 ^5 _297'18=27'13 divisions. This number is 

 2 



found in the fifth column. 



This process is continued, the resting points being combined in threes 

 till several values of the deflection due to the rider have been obtained, 

 and the mean of these is taken as the true value. This plan of com- 

 bining the resting points requires that the observations should be 

 taken at nearly equal intervals. After a little practice it will always 

 take the observer about the same time to go through the same opera- 

 tions of clamping, changing the riders, unclamping, clamping again to 

 lessen the vibrations about the new resting point, and then beginning 

 to observe, and I have considered that this was a sufficiently correct 

 method of timing the observations. 



When a series has been taken it will at once be seen whether they 

 were begun too soon after entering the room, or whether any irregular 

 disturbing force has acted. For instance, in weighing No. II, determi- 

 nation of one scale division, the first resting point is so much lower 

 than the succeeding with the same rider that evidently the balance was 

 still affected by my entrance into the room. It was, therefore, rejected. 

 Again, in weighing No. Ill determination of the difference between the 

 weights, the fourth resting point was much lower than the others with 

 the same weight in the pan. The resting points, when the other 

 weight was in the pan, showed no similar sudden drop of such magni- 

 tude. This observation was, therefore, rejected as being affected by 

 some irregular disturbance. 



When the value of the deflection is determined, the value of one 

 scale division is at once found by dividing *3282 mgm. by the number 

 of divisions of the deflection, since the charge of the sides is 

 equivalent to the addition of *3282 mgm. to one pan. 



The determination of the difference between the weights is then 

 begun. This is carried on in a precisely similar manner, the only 

 difference being, that the rod changing the weights is now turned 

 round in place of the rod changing the riders. I have usually taken a 

 greater number of observations of the difference between the weights 

 than of the deflection due to the riders, as the former is somewhat 

 more irregular than the latter. This irregularity I believe to arise 



