14 



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



the mass in its two positions is found in exactly the manner in which 

 the difference between two weights is f onnd, except that now when three 

 successive extremities of oscillations have been observed for a resting 

 point the mass is moved from one position to the other where the weights 

 were changed in the former experiments, the clamp not being brought 

 into action. The second extremity of the oscillation which is proceed- 

 ing while the mass is moved, is observed as the first of the next three. 

 When nine or more resting points have been observed they are com- 

 bined in threes, and the mean of the resulting values of the deflection 

 n is used in the subsequent calculation. 



This deflection is, of course, less than that which would be observed 

 were there no attraction on the counterpoise, and were the out position 

 of the mass at an infinite distance. To find the factor /by which the 

 deflection n due to the change of position of the mass must be multi- 

 plied in order to reduce it to the deflection which would be observed 

 under these conditions, let AB be equal and parallel to the beam 



of the balance at the level of the counterpoise of which B is the 

 centre. Let C be the centre of the weight, D that of the mass in its 

 in position, E that of the mass in its out position. Draw BF, DF, 

 parallel to AD, AB. Let jm = the mass. 



The vertical attraction of the mass in its in position will be — 



fi _/uB¥ 

 CD" 2 BD 1 ' 



The vertical attraction in its out position will be — 



/aCD_/*BF 

 CE 3 BE 3 ' 



The difference between these is actually observed, viz. 



« f , CD 2 BF CD 3 , CD 2 BF 1 



