4 



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



and another, which is to be compared with it, can be inserted in its 

 place without altering the relative positions of the planes and knife 

 edges. The counterpoise in the other pan, meanwhile, keeps the beam 

 in the same state of flexure. The pan is then undamped and the new 

 position about which it oscillates is observed. The only changes are 

 due to the change in the weight and the effect of the external disturb- 

 ing forces ; the latter we may consider as proportional to the time, if 

 sufficient precautions have been taken, and by again changing the 

 weights and again observing the position of the balance, we may 

 eliminate their effects. 



Though the method when applied to the balance does not yet give 

 such good results as Baily obtained from the torsion balance — partly, 

 I believe, because I have not yet been able to apply all his precautions 

 to remove external disturbing forces — it still gives better results than 

 would have been obtained without it. This may be seen by the numbers 

 recorded in the tables, where a progressive motion of the resting point 

 may be noticed in most cases, in the same direction, during a series of 

 experiments. Even when this is not the case, the method at once 

 shows when the disturbing forces are irregular, and when we are 

 justified in rejecting an observation on that account. 



I give in this paper two applications of the method, one to the 

 comparison of two weights, the other to the determination of the mean 

 density of the earth. The latter is given only as an example of the 

 method, but I hope shortly to continue the experiments with a large 

 bullion balance, for the construction of which I have had the honour 

 to obtain a grant from the Society. The balance is now in course of 

 construction, by Mr. Oertling, of London. 



Description of the Apparatus. 



The balance which I have employed is one of Oertling's chemical 

 balances, with a beam of nearly 16 inches, and fitted with agate planes 

 and knife edges. It will weigh up to a little more than 1 lb. To 

 protect it from sudden changes of temperature, the glass panes of the 

 case are covered with flannel, on both sides of which is pasted gilt 

 paper, with the metallic surface outwards. This case is enclosed in 

 another outer case, a large box of inch deal, lined inside and out with 

 gilt paper. The experiments have been conducted in a darkened 

 cellar under the chemical laboratory at Owens College, which was 

 kindly placed at my disposal by Professor Roscoe. As the ceilings 

 and floors of the building are of concrete, any movement near the 

 room causes a considerable vibration of the floor and walls. It was 

 necessary, therefore, to support the balance independently of the floor. 

 For this purpose, six wooden posts (A, B, C, D, E, F, fig. 1) were 

 erected resting on the ground underneath and passing freely through 

 the floor to a height of 6 feet 6 inches above it. They are connected at 



