1894.] on the Newtonian Constant of Gravitation. 373 



I manage to reserve Sunday nights, from midnight to six or eight in 

 the morning, for observations of deflection and period. All the other 

 operations can be carried on in the daytime. Sunday is the only 

 night that is suitable, as the railway companies spend every other 

 night shunting and making up trains about a mile away, and this 

 causes such a continuous clatter and vibration, that hours of work 

 may be lost. A passing train does not seem so injurious; but, 

 fortunately for me, most of the observations were made during the 

 coal strike, and fewer trains than usual were running. However, 

 though I may escape from the rattling traffic of St. Giles by working 

 at night, and on Sunday nights am not so badly affected by the trains, 

 I am still not sure of quiet even when there is no wind. For 

 instance, at a quarter to four on Monday morning, Sept. 10, 1893, I 

 was recording chronographically the passage of every ten divisions. 

 Everything was quite quiet, and at the particular moment the marks 

 on the drum recurred at intervals of about three seconds. Suddenly 

 there was a violent non-vibrating lurch of fifteen divisions, or 

 150 units, which is enormously greater than anything that either 

 trains or traffic could produce ; of course I could make no further 

 record. The time of the last mark was, allowing for the known 

 error of the clock, 15k. 44m. 14* 3s. This was entered the same day 

 in my note-book as an earthquake, and in Tuesday's Standard I read 

 an account of a violent earthquake in Eoumania at about the same 

 time. Mr. Charles Davison informs me that the shock was recorded 

 at Bucharest at 15h. 40m. 35s., but that the epicentrum must have 

 been some distance from there. Exact particulars, it seems, cannot be 

 obtained. Though it would appear that the rate of travel of the 

 shock is unusually high, there never was any doubt that what I 

 observed was an earthquake, and it is practically certain that it was 

 the Roumanian earthquake. 



Owing to the viscosity of the air, which limits the time during 

 which an observation for period can be made to about 40 minutes, 

 on account of the resistance that the slowly moving mirror and gold 

 balls experience in their passage through it, I made one exj>eriment 

 with a view of reducing this difficulty, by the use of an atmosphere of 

 pure dry hydrogen gas, which possesses a viscosity only half that of 

 air. I found that on this account a great advantage could be gained ; 

 but this was more than counterbalanced by the difficulty of getting up 

 a sufficient swing in the gas, and of efficiently controlling the mirror. 

 At the same time, I think that if I had had time to provide means for 

 feeding the gas into the tube without entering the corner, and at the 

 same time were to prevent diffusion at the lower screw, that a little 

 trouble in this direction would be well rewarded. Meantime I found 

 within the limits of error, which were greater than without the hydro- 

 gen, that the deflection and the period corrected for the diminished 

 damping were the same. The chief interest of this experiment lies 

 in the fact that it revealed an action unknown to me, and I believe to 

 others, that a thin plane glass mirror, silvered and lacquered on one 



