ON THE MEASUREMENT OF THE LUNAR DISTURBANCE OF GRAVITY. 93 



Report of the Committee, consisting of Mr. Gr. H. Uakwin, 

 Professor Sir William Thomson, Professor Tait, Professor GtRANT, 

 Dr. Siemens, Professor Purser, Professor Gr. J'orijes, and Mr. 

 Horace Dar'Win, appoiivted /«»?' the Measurement of the Lunar 

 Disturbance of Gravity. 



On an instrument for detecting and measuring small changes in the direction 

 of the force of graviti/, hy George H. Darwin, M.A., F.B.8., formerly 

 Fellow of Trinity College, Cavtbridge, and Horace Darwin, M.A., 

 Assoc. M. Inst. G.JE. 



[This report is written in the name of G. H. Darwin merely for the sake of verbal 



convenience.] 



I. Accotmt of the experiments. 



We feel some difficulty as to the form wliich this report should talce, 

 because we are still carrying on our experiments, and have, as yet, 

 arrived at no final results. As, however, we have done a good deal of 

 "woi'k, and have come to conclusions of some interest, we think it better 

 to give at once an account of our operations up to the present time, 

 rather than to defer it to the fnture. 



In November, 1878, Sir William Thomson saggested to me that 

 I should endeavour to investigate experimentally the lunar disturbance 

 of gravity, and the question of the tidal yielding of the solid earth. 

 In May, 1879, we both visited him at Glasgow, and there saw an 

 instrument, which, although roughly put togethea", he believed to contain 

 the principle by which success might perhaps be attained. The instrument 

 was erected in the Physical Laboratory of the University of Glasgow. 

 We are not in a position to give an accurate description of it, but the 

 following rough details are quite sufficient. 



A solid lead cylinder, weighing perhaps a pound or two, was suspended 

 by a fine brass wire, about 5 feet in length, from the centre of the lintel 

 or cross-beam of the solid stone gallows, wliich is erected there for the 

 purpose of pendulum experiments. A spike projected a little way out 

 of the bottom of the cylindrical weight ; a single silk fibre, several 

 inches in length, was cemented to this spike, and the other end of the 

 fibre was cemented to the edge of an ordinary galvanometer-mii-ror. 

 A second silk fibre, of equal length, was cemented to the edge of the 

 mirror at a point near to the attachment of the former fibre. The other 

 end of this second fibre was then attached to a support, which was 

 connected with the base of the stone gallows. The support was so 

 placed that it stood very near to the spike at the bottom of the pendulum, 

 and the mirror thus hung by the bifilar suspension of two silks, which 

 stood exceedingly near to one another in their upper parts. The instru- 

 ment was sci'eened from draughts by paper pasted across between the two 

 pillars of the gallows ; but at the iDottom, on one side, a pane of glass 

 was inserted, thi-ough which one could see the pendulum-bob and 

 galvanometer-mirror. 



It is obvious that a small displacement of the pendulum, in a direction 

 perpendicular to the two silks, will cause the mirror to turn about a 

 vertical axis. 



