52 LOCAL VARIATIONS AND VIBRATIONS OF EARTH’S SURFACE. 
providing apparatus to show the win variation in its position 
with reference to the vertical. This was done by attaching the 
bottom of the pendulum to a small mirror hung on two silk fibres, 
the means of connection being a single fibre of silk. The a 
experiments showed how exceedingly sensitive this arrangem 
ae and that it was useless to attempt anything in the cdeacery 
at Glasgow, owing to the emmnie of a busy city, the want of 
pene appliances, and the imperfections of the first instrument. 
All these difficulties were overcome and the instrument set up at 
Cambridge, and finally made so sensitive that it would reveal a 
motion in the pendulum equal to an angle of 0°01’, or an inch in 
quite capable of doing the work before them ; but most unexpected 
difficulties arose. Nothing could be done if any one was in the 
room with the apparatus ; if the observer lifted one foot off the 
ground the spot of — veg flying on the scale, showing that a 
slight change of weight on the surface of the ground near the 
instrument affected the senisl of the ground and thence the pen- 
dulum. To avoid this a telescope was put through the wall and 
the changes observed through it. After this it seemed probable 
that they would be able to attain the desired object, and actually 
side. But here the city traffic, railways, &c., and other minor 
vibrations due probably to earth tremors, became so troublesome 
that it was impossible to distinguish certainly the lunar effects 
from the others. At times it would be possible to get readings 
satisfactorily, and at others the observers saw a distinct diurnal 
period, with a maximum about noon when the pendulum stood 
init 1 northwards. The path of the pendulum was, however, 
interrupted by many minor zigzags, and it would sometimes, when 
abrupt change and moved northwards, and the following day the 
ce of the image was greater than usual, = then a few days 
later it was quite remarkable for its steadine 
Still there was hope that with an ileal a the 
results of their experience, and an observatory in a deep mine or 
at a distance from a sg they might be able still to measure the 
moon’s effect. Finally, however, the attempt was given up as 
‘hopeless, because Mr. Darwin, by computation, proved that the 
variations of atmospheric a in extreme cases, 7.e., where the 
difference amounted to 2 inches, would cause a — in the 
earth’s on een eee vertical 
