32 
Geology of Sydney. 
Slight movements of rather a different kind have 
been noticed, to which the name of te earth-tremors ” 
has been given. These are very slight jarrings or 
quiverings of the earth, too slight to be observed by 
our unaided senses, but rendered visible by means of 
very delicate pendulums and other contrivances. Now, 
wherever such observations have been made, it has 
been discovered that the earth is constantly quivering 
as if it were a lump of jelly. In Italy, where this 
subject has been very carefully studied, the tremors 
that are continually going on are found to vary con- 
siderably in strength ; for instance, when the weather 
is very disturbed and unsettled, the movements of the 
pendulum are often much greater. Again, before an 
earthquake, the instrument shows that the tremors are 
more frequent and violent. 
Another way of observing these curious little 
movements is by burying microphones in the ground. 
The microphone is a little instrument invented of late 
years, which is capable of enormously magnifying the 
very slightest sounds, such as our ears will not detect. 
By its means one can hear, as someone said, “ the 
tramp of a fly’s foot,” if he will be so obliging and walk 
over it. It has thus been proved in Italy that the 
earth sends forth a confused medley of sounds caused 
by little crackings and snappings in the rocks below 
our feet . 1 
The facts referred to so far will convince most 
people as to the instability of the so-called solid crust. 
1 Hutchinson. “ Story of the nills,” p 193. 
