94 HEPOBT— 1881. 



A lamp and slit were arranged, as in a galvanometer, for exhibiting 

 the movement of the pendulum, by means of the beam of light reflected 

 from the mirror. 



No systematic observations were made, but we looked at the instrument 

 at various hours of the day and night, and on Sunday also, when the 

 street and railway traffic is very small. 



The reflected beam of light was found to be in incessant movement, 

 of so irregular a character that it was hardly possible to localise the 

 mean position of the spot of light on the screen, within 5 or 6 inches. 

 On returning to the instrument after several hours, we frequently found 

 that the light had wandered to quite a difi'erent part of the room, and wc 

 had sometimes to search through nearly a semicircle before finding it 

 again. 



Sir William Thomson showed us that, by standing some 10 feet away 

 from the piers, and swaying from one foot to the other, in time Avith the 

 free oscillations of the pendulum, quite a large oscillation of the spot of 

 light could be produced. Subsequent experience has taught us that con- 

 siderable precautions are necessary to avoid efl'ects of this kind, and the 

 stone piers at Glasgow did not seem to be well isolated from the floor, and 

 the top of the gallows was used as a junction for a number of electric 

 connections. 



The cause of the extreme irregularity of the movements of the pendulum 

 was obscure ; and as Sir William Thomson was of opinion that the instru- 

 ment was well worthy of careful study, we determined to undertake a 

 series of experiments at the Cavendish Laboratory at Cambridge. Wo 

 take this opportunity of recording our thanks to Lord Rayleigh' for his 

 kindness in placing rooms at our disposal, and for his constant readiness 

 to help us. 



The pressure of other employments on both of us prevented our 

 beginning operations immediately, and the length of time which we have 

 now spent over these experiments is partly referable to this cause, 

 although it is principally due to the number of difficulties to be overcome, 

 and to the quantity of apparatus which has had to be manufactured. 



In order to avoid the possibility of disturbance from terrestrial 

 magnetism, we determined that our pendulum should be made of pure 

 copper.'^ Mr. Hussey Vivian kindly gave me an introduction to Messrs. 

 Elkington, of Birmingham ; and, although it was quite out of their 

 ordinary line of business, they consented to make what we required. 

 Accordingly, they made a pair of electrolytically- deposited solid copper 

 cylinders, 5j inches long, and 2j inches in diameter. From their 

 appearance, we presume that the deposition was made on to the inside of 

 copper tubes, and we understand that it occupied six weeks to take place. 

 In November, 1879, they sent us these two heavy masses of copper, and, 

 declining any payment, courteously begged our acceptance of them. 

 Of these two cylinders we have, as yet, only used one ; but should our 

 pi-esent endeavours lead to results of interest, we shall ultimately require 

 both of them. 



Two months before the receipt of our weights, the British Association 

 had reappointed the Committee for the Lunar Disturbance of Gravity, 

 and had added our names thereto. Since that time, with the exception 



' Professor Maxwell had given us permission to use the ' pendulum room,' but we 

 had not yet begun our operations at the time of his death. 



" We now think that this was probably a superfluity of precaution. 



