CH. VI 
WARPING OF THE SOIL 
111 
into the field of view of the telescope. He then placed 
a heavy weight on the floor where he had been stand 
ing ; this of course drove the image out of the field 
of view, but after he had left the room the image of 
the flame was found to be in the field. 
We ultimately found, even when no special pains 
had been taken to render the instrument sensitive, 
that if one of us was in the room, and stood at about 
sixteen feet south of the instrument with his feet 
about a foot apart, and slowly shifted his weight from 
one foot to the other, a distinct change was produced 
in the image of the gas-flame, and of course in the 
position of the little mirror, from which the image 
was derived by reflection. It may he well to consider 
for a moment the meaning of this result. If one 
presses with a finger on a flat slab of jelly, a sort of 
dimple is produced, and if a pin were sticking upright 
in the jelly near the dimple, it would tilt slightly 
towards the finger. Now this is like what we were 
observing, for the jelly represents the soil, and the 
tilt of the pin corresponds to that of the pendulum. 
But the scale of the displacement is very different, for 
our pendulum stood on a block of stone weighing 
nearly a ton which rested on the native gravel at two 
feet below the level of the floor, and the slabs of the 
floor were removed from all round the pendulum. The 
dimple produced by a weight of 140 lb. on the stone- 
paved floor must have been pretty small, and the 
slope of the sides of that dimple at sixteen feet must 
have been excessively slight; hut we were here virtually 
observing the change of slope at the instrument, when