Earth Movements. 
31 
which the rivers and glaciers bring about.” Professor 
Milne, a great authority on earthquakes, has noticed 
slight sway in gs of the earth which, though occupying 
a short time — from a few seconds to a fow hours — are 
still too slow to produce a shock of any kind. These 
lie calls “ earth pulsations.” They have been observed 
by means of delicate spirit levels, the bubbles of which 
move with very slight changes of level at either end 
of the instrument. At present only a few experiments 
of this kind have been made; but they toll us that the 
surface of the earth (which is apparently so firm and 
immovable) is subject to slight but frequent oscillations. 
Some think that they depend upon changes in the 
weight of the atmosphere. If this is so, the balance 
between the forces at work below the earth’s surface 
and those that operate on its surface must bo very 
easily disturbed. Still we cannot see that this is a 
seriousobjection; on the contrary, there is much reason 
to think that any slight extra weight on the surface 
such as might bo caused by an increase of the pressure 
of the atmosphere, and still more by the accumulation 
of the vast sedimentary deposits on the floor of the 
ocean, may be quite sufficient to cause a movement to 
take place. Moreover, Mr. G. IT. Darwin has shown 
that the earth’s crust daily heaves up and down under 
the attraction of the moon, in the same kind of way 
that the ocean does; so that we must give up all idea 
of the solid earth being fixed and immovable, and must 
look upon it as a flexible body, like a ball of india- 
rubber. 
