506 
ME. W. CROOKES ON ATTRACTION AND 
weights would themselves become warm, and since the action I sought to establish 
was only likely to be due to a difference in temperature between the hot body on one 
arm of the balance and the cold weights on the other arm, both being under the in- 
fluence of the same force of gravity, I endeavoured to obtain the desired results by 
means of a spring-balance, one in which the variations of gravity should be measured, 
not against gravity itself, but against the tension of a spring. 
17. I tried many forms of spring-balance, and obtained with them results which, at 
the time, I thought sufficiently satisfactory. The sources of error were, however, so 
numerous, and the manipulation was so difficult, that I ultimately gave up that form 
of balance in favour of the one usually employed. 
In order to obtain a very high degree of rarefaction, without the trouble and 
uncertainty attending the use of an ordinary air-pump, it was necessary to have the 
balance sufficiently small to enable it to be exhausted by the Sprengel pump. 
Before proceeding to the forms of apparatus finally adopted and the experiments 
made therewith, it will, I think, be useful, for the sake of other experimentalists, if I 
briefly describe some of the arrangements successively tried and rejected, with the 
reasons for so doing. 
18. A light beam ( a b , fig. 1) was made of two pieces of fine flattened brass wire 
Kg. 1. 
120 millims. long, 5 millims. apart at the point of suspension, and joined together at 
each end. A pair of very fine needle-points, one on each side of the beam, represented 
knife-edges, and worked on glass plates cemented horizontally to upright pillars. The 
