PHYSICS. 335 



angles to the semicircular arm, the third wheel being- at the other ex- 

 tremity of this axle, and rolling on the sphere. A graduated circle on 

 the axis, about which the semicircular arm moves, enables it to be ad- 

 justed for any latitude. Normally the pendulum-axis is in the prolonga- 

 tion of the sphere-axis. On turning the sphere the third wheel rolls 

 round its equator, rotating the plane of oscillation of the pendulum in 

 the same time as the sphere rotates, but in the inverse direction. Turn- 

 ing the semicircular arm 90° the axis of oscillation passes through the 

 equator and the wheel rests on the pole of the sphere ; hence, when the 

 latter rotates, the plane of oscillation does not change. In intermedi- 

 ate positions the change of plane is proportional to the sine of the lati- 

 tude, and the rapidity of the change may be read off on a graduated 

 circle placed beneath the jiendulum model. {J. Phys., September, 1881, 

 X, 401.) 



G. H. and H. Darwin communicated to the York meeting of the 

 British Association, on behalf of a committee appointed for the meas- 

 urement of the lunar disturbance of gravity, the results of their experi- 

 ments thus far made. In 1879 Sir William Thomson had erected at Glas- 

 gow an apparatus consisting of a solid lead cylinder suspended by a 

 fine brass wire five feet long from the cross-beam of the stone supports 

 used for pendulum exi)eriments. From the bottom of the weight a rod 

 projected, to which was fastened a single fiber, of silk attached to the 

 edge of a small mirror. A second fiber, also attached to the same point 

 of the mirror, was fastened to a supi)ort near the rod, so that the mirror 

 was supported by a bitilar suspension, such that a minute motion of 

 the pendulum would cause considerably rotation of the mirror. A lamp 

 and slit were arranged for the readings. But the spot of light was 

 found to be in incessant movement, so irregular that the mean position 

 could not be fixed within 6 inches. The authors, after seeing this ar- 

 rangement, constructed, in 1880, at Cambridge, a modification of it. The 

 pendulum, suspended by the two wires, was hung in aliquid, and an ap- 

 paratus was attached to it for giving it a known but very minute dis- 

 placement. Thej' found that it was subject tp a diurnal oscillation, being 

 farthest north at j). m. and larthest south at a. m. It was so sen- 

 sitive that it showed distinctly the slight pressure on tbe stone gallows 

 exerted by tlio finger. Water poured on the ground tilted the whole 

 structure over, and minute changes of temj)erature j^roduced marked 

 effects. One foot of displacement of the spot corresponded to one second 

 of arc in the direction of the plumb-line. In 1881 a new instrument was 

 made, in which a copper tube formed at the same time the support for 

 the pendulum and the envelop for containing the liquid. The whole 

 was immersed in a large mass of water, and the observations were taken 

 from outside of the room by means of a telescope. The diurnal changes 

 and the slow change were observed in the present case, and also periods 

 of several days in which the pendulum was in a state of continual agi- 



