114 Mr. Baily's Experiments "with the Torsion-rod 



of ascertaining whether the results would be affected by such 

 a variation : these were platina, lead, zinc, glass, ivory, and 

 hollow brass, varying from 1^ inch to 2^ inches in diameter. 

 The mode of suspension was also diversified, with a similar 

 view: iron, copper, brass, and silk were successively used, 

 not only single, but also double, similar to the bifilar mode 

 suggested by Gauss* for certain magnetical experiments. The 

 mean weight of each of the great balls (or masses) was 

 2,663,282 grains, or about 380| pounds avoirdupois, as de- 

 termined by the accurate weights and scales of the Bank of 

 England. And the weight of each of the small balls varied 

 from 1950 to 23,742 grains. The length of the suspension- 

 line was 60 inches ; and the length of the torsion-rod (between 

 the centres of the two balls affixed thereto) was nearly '80 

 inches. The torsion-rod was made of fine deal, of an uniform 

 shape throughout its whole length, and weighed only about 

 2300 grains. Another torsion-rod was afterwards made, for 

 some special experiments, the weight of which was nearly ten 

 times as great : it consisted of a solid brass rod, and was oc- 

 casionally used without any balls attached to the ends. 



The torsion-rod and the suspension-lines were screened by 

 a mahogany box, constructed exactly similar in form to that 

 used by Cavendish, but supported from the ceiling in a very 

 firm manner, and unconnected with the floor or any other part 

 of the surrounding apparatus. Every precaution was taken 

 to secure the torsion-rod from the influence of any sudden 

 or partial change of temperature ; and also to insure the sta- 

 bility and firmness of the support to which it was attached. 

 The author's remarks on this subject are worthy of notice : 

 for he says, " In order to satisfy myself on this point, at the 

 time of the original construction of the apparatus, I made 

 various attempts to create a sensible disturbance in the mo- 

 tion of the torsion-rod, by causing the doors to be frequently 

 and violently slammed — by jumping heavily on the floor 

 of the room — and also above the ceiling — and in other differ- 

 ent ways, having a similar tendency ; but in no instance could 

 I observe the least effect upon the lateral motion of the rod. 

 I have also frequently tried the same experiment, when dif- 

 ferent visitors were present, since the apparatus has been com- 

 pleted ; and have moreover many times not only accidentally, 

 but also designedly, made a regular series of experiments for 

 determining the density of the earth, during the most violent 

 storms that I have ever witnessed, when the wind has been so 

 boisterous, and blowing in such gusts, that the house has been 



[* See Taylor's Scientific Memoirs, vol. ii. p. 252. — Edit.] 



