376 Mr. C. Vernon Boys [June 8, 



The periods and deflections were taken in the first four hours after 

 midnight, then, after a few hours' sleep, and far too soon for the tem- 

 perature to have quieted down, I took the period with the counter- 

 weight, but was only able to give ten minutes, as I had to catch a 

 train in order to be able to give my midday lecture at South Kensington. 

 It is not surprising that under such conditions a diiference of 1 part 

 in 600 should arise. There is a difference of about the same order of 

 magnitude between the earlier experiments and the favourable four. 

 There is one point about the figures that I should like to mention. 

 No results were calculated till long after the completion of the last 

 experiment. Had I known how the figures were coming out, it would 

 have been impossible to have been biassed in taking the periods and 

 deflections. Even the calculating boys would not have been quick 

 enough to discover whether the observed elongations were such as 

 would give a definite point of rest. I made my observations, and 

 the figures were copied at once in ink into the books, where after- 

 wards they left my hands and were ground out by the calculating 

 machine. The agreement, such as it is, between my results is 

 therefore in no way the effect of bias, for I had no notion till last 

 May what they would be. 



My conclusion is that the force with which two spheres weighing 

 a gramme each, with their centres 1 centimeter apart, attract one 

 another, is 6*6576 x 10~ 8 dynes, and that the mean density of the 

 earth is 5*5270 times that of water. I do not think the fourth 

 significant figure can be more than one or, at the outside, two parts 

 in error. 



It is evident, from what I have already said, that this work is of 

 more than one-man power. Of necessity I am under obligations in 

 many quarters. In the first place, the Department of Science and 

 Art have made it possible for me to carry out the experiment by 

 enabling me to make use of apparatus of my own design. This 

 belongs to the Science Museum, where I hope in time to set it up so 

 that visitors who are interested may observe for themselves the gravi- 

 tational attraction between small masses. Prof. Clifton, as I have 

 already stated, has given me undisturbed possession of his best ob- 

 serving room, his only good underground room, for the last four years. 

 The late Prof. Pritchard lent me an astronomical clock. Prof. 

 Viriamu Jones enabled me to calibrate the small glass scale on his 

 Whitworth measuring machine ; and Mr. Chaney did the same for 

 my weights. I would specially refer to the pains that were taken by 

 Mr. Pye, of the Cambridge Scientific Instrument Company, to carry 

 out every detail as I wished it, and to the highly skilled work of 

 Mr. Colebrook, to which I have already referred. Finally, I am under 

 great obligations to Mr. Starling, of the Eoyal College of Science, 

 who performed the necessarily tedious calculations. 



In conclusion, I have only to say that while I have during the 

 last five years steadily and persistently pursued this one object with 

 the fixed determination to carry it through at any cost, in sjrite of any 



