190 REPORT OF THE CALIFORNIA EARTHQUAKE COMMISSION. 
instruments would be, on putting the origin of codrdinates at the axis of rotation (see 
equations (132) and (134)), 
€ 




Ve, de nll a? 
2 « —1 + n*e¢, — — —2 — a) 
Gat an ee ae ; 
and (144) 
ae, des nl @ , nil dw 
—3 +2 = 2c, — ——# »' =0 
FT ale TEAM dea OY py 7 EG FT 
on adding these two equations, the tilt disappears; and on subtracting one from the other, 
the vertical linear displacement disappears. The two instruments record the displace- 
ment and rotation of the same point, and therefore the separation of these two involves 
no supposition as to the motions of points at some distance apart. 
Prince Galitzin has described another method of measuring comparatively rapid tilts. 
He has shown that a bar hung by wires of equal length, attached to its ends, the wires 
themselves being fastened to the support at different heights, so that the bar hangs in an 
inclined position, will be rotated around a vertical plane by a tilt at right angles to the 
plane of the wires, and this roéation will not be affected by the swinging of the bar as a 
pendulum. This is a modification of the bifilar pendulum, designed by Mr. Horace 
Darwin for the study of slow earth-tilts.’ 


1 See C. Davison, Bifilar Pendulum for Measuring Earth-Tilts. Nature, 1894, vol. L, pp. 246-249. 
