112 
DEFLECTION OF THE VERTICAL 
CH. VI 
the centre of the dimple was moved from a distance of 
fifteen feet to sixteen feet. 
It might perhaps be thought that all observation 
would be rendered impossible by the street traffic and 
by the ordinary work of the laboratory. But such 
disturbances only make tremors of very short period, 
and the spirits and water damped out quick oscilla 
tions so thoroughly, that no difference could be detected 
in the behaviour of the pendulum during the day and 
during the night. Indeed, we found that a man 
could stand close to the instrument and hit the tub 
and pedestal smart blows with a stick, without produc 
ing any sensible effect. But it was not quite easy to 
try this experiment, because there was a considerable 
disturbance on our first entering the room ; and when 
this had subsided small movements of the body pro 
duced a sensible deflection, by slight changes in the 
distribution of the experimenter’s weight. 
It is clear that we had here an instrument of 
amply sufficient delicacy to observe the lunar tide 
generating force, and yet we completely failed to do 
so. The pendulum was, in fact, always vacillating 
and changing its position by many times the amount 
of the lunar effect w r hich we sought to measure. 
An example will explain how this was : A series of 
frequent readings were taken from July 21st to 25th, 
1881, with the pendulum arranged to swing north and 
south. We found that there w 7 as a distinct diurnal 
period, with a maximum at noon, when the pendulum 
bob stood furthest northward. The path of the 
pendulum was interrupted by many minor zigzags,