266 



Prof. C. V. Boys. 



[June 20, 



taken, but during the observations there was an almost continuous 

 tramp of art students above, producing a perceptible tremor, besides 

 which two vehicles passed, and coals were twice shovelled in the coal 

 cellar, which is separated from the vault in which the observations 

 were made by only a four and a half inch brick wall. The result of all 

 this was a nearly perpetual tremor, which produced a rapid oscilla- 

 tion of the scale on the cross-wire, extending over a little more than 

 1 mm. This increased the difficulty of taking the readings, but to 

 what extent it introduced error I shall not be able to tell until I can 

 make observations in a proper place. 



In spite of these disturbances, the agreement between the deflec- 

 tions deduced from the several sets of observations and between the 

 periods is far greater than I had hoped to obtain, even under the 

 most favourable conditions. In order to show how well the instru- 

 ment behaved, I have copied from my note-book the whole series of 

 figures of one set, which sufficiently explain themselves. 



tion. 



15 



53 

 22 

 47 

 27 



43 

 30 

 40 

 32 

 39 

 33 



Ampli- 

 tude. 



38 



15 



30 



•72 



24 



80 



20 



•00 



16 



•12 



12 



•98 



10 



•46 



8 



•38 



6 



77 



5 



•47 



4 



•45 



Decre- 

 ment. 



0-8066 



True 

 position 

 of rest. 



•18 



•20 

 •21 

 •20 

 •22 

 •21 

 •22 

 •24 

 •24 

 •26 

 •26 



Time of 

 transit of 

 36-09. 



9 

 11 



12 

 13 

 15 

 16 

 17 

 19 

 20 

 21 



Correction 

 for transit 

 of true 

 position 

 of rest. 



+ 

 -0 

 + 

 -0 

 + 

 -0 

 + 

 -0 



+ 1 

 -1 

 + 1 



•08 

 •18 

 •24 

 •28 

 •41 

 •47 

 •63 

 •91 

 •13 

 •58 

 •94 



It will be noticed that the true position of rest is slightly rising in 

 value, and this rise was found to continue at the rate of 0*36 cm. an 

 hour during the whole course of the experiment, and to be the same 

 when the large masses were in the positive or negative position. 

 The motion was perfectly uniform, and in no way interfered with the 

 accuracy of the experiments. It was due, I believe, to the shellac 

 fastening of the fibre, for I find that immediately after a fibre has 

 been attached this movement is very noticeable, but after a few days 

 it almost entirely ceases ; it is, moreover, chiefly evident when the 



