62 LOCAL VARIATIONS AND VIBRATIONS OF EARTH’S SURFACE. 
protect it. This rock face must be much warmer in summer 
than in winter, the extreme range probably being from 40° to 120°, 
and the difference would make an appreciable expansion of the 
surface stone, but this would not extend to any considerable depth, 
for I have earth theses in this rock to 19 feet, and the summer 
effe 
safely assume that the poem: temperature does not affect the 
western face of the rocks deeper than 20 feet ; and this expansion 
cannot affect the transit instrument, because there are vertical 
joints in the stone running N. and S., and these would prevent 
any expansion in the western section ‘affecting the others. I am 
satisfied, after careful examination of the conditions, that the sun’s 
heat cannot, through this western rock face, affect the transit 
instrument, but in in pec 3 facts before you it was necessary to 
mention the local conditio 
The suggestion t en special instrument for recording all 
these motions will aastaons give rise to many suggestions how 
best todo it. The record must be continuous, and, from the minute- 
ness of some of a motions, photography anne seem to be the 
best means of registration, although the expense and trouble of it 
stand very much in the way of its general adoption. The best 
form of the Taco apparatus seems at first sight to be an iron 
one buried in the ground, and carrying a telescope so mounted 
to the mercury and thence up again on to a sensitive surface 
could be done in the way of investigation, and one naturally 
turns to see if the records slide collected contain any useful 
data. 
The ocean freely receives and transmits impulses, and we 
may find some records of these on the tide sheets. Every one 
knows how the sea sometimes rises without visible cause, and 
from a distant gale, but there are so hich come from other 
causes and have distinct characteristics as they record them- 
selves on the tide-gauge. metimes a single wave of this kind 
comes in and often many of them together, and we call them 
earthquake waves, but fail often to hear of any earthquake to 
give rise to them. As the discussion must eventually include these 
waves, I append a list ofthe dates when they have been recorded 
in Sydney. Some of these, beyond doubt, come from earthquakes. 
Most of the members present will remember the terrible earth- 
quake at Iquique on the 10th of May, 1877, and the remarkable 
waves which followed it, and were recorded on our coast and ti 
gauges. 
