for determining the Mean Density of the Earth. 115 



shaken to its centre. But in no instance have I ever seen 

 the least disturbance in the lateral motion of the torsion-rod, 

 nor any difference produced in the results of the experiments. 

 I have thought it proper to make these remarks and thus to 

 place them on record, because some persons at first ha- 

 zarded an opinion that the place which I had selected might 

 not be quite adapted for experiments of so delicate a nature. 

 But a moment's consideration will convince a person conver- 

 sant with the subject, that no dancing motion of the suspension- 

 line (even if it did exist) would tend to produce an irregular 

 lateral or angular motion in the torsion-rod ; and this is the 

 only anomalous motion we need guard against. 



" There is also another remarkable circumstance connected 

 with this subject, which I think it requisite likewise here to 

 place on record. When the torsion-rod has been in a state 

 of repose, I have frequently shaken the torsion-box, by rapidly 

 moving the ends backward and forward from side to side fifty 

 or sixty times, and even more : but I could never discover, 

 that this disturbance of the box caused the least motion in 

 the torsion-rod, which still retained its stationary position. 

 This experiment has been witnessed at various times by se- 

 veral scientific persons. Yet, notwithstanding this torpid 

 state of the torsion-rod, if the slightest change of temperature 

 be applied near the side of the torsion -box, or if either side 

 near the balls be sprinkled with a little spirit of wine, the tor- 

 sion-rod is immediately put in motion and the resting-point 

 undergoes a rapid change." 



Notwithstanding these favourable circumstances the author 

 at first met with certain irregularities and discordances, which 

 he Tound it difficult to remove ; and which appear to have 

 been experienced also by Cavendish and Reich,— caused, as 

 it is presumed, by variations in the temperature of the room 

 in which the experiments were carried on. Cavendish chose 

 an out-house in his garden at Clapham Common ; and, having 

 constructed his apparatus tsoitJmi the building, he moved the 

 masses by means of ropes passing through a hole in the wall, 

 and observed the torsion-rod, by means of a telescope fixed 

 in an ante-room on the outside. The general temperature of 

 the interior was therefore probably uniform during the time 

 that he was occupied in any one set of experiments : but it is 

 scarcely to be expected that a building of this kind, and in 

 such a situation, would preserve, the same uniform tempera- 

 ture for twenty-four successive hours : especially at the season 

 which he selected for his operations. Reich pursued a similar 

 plan, but under circumstances apparently more favourable ; 

 for he selected a dark cellar, where the temperature was not 



12 



