LOCAL VARIATIONS AND VIBRATIONS OF EARTH’S SURFACE. 55 
Cavallere used ten pendulums of graduate lengths, and found 
that sometimes one of the 2 coca and sometimes another was 
The position of the sun and moon n hav ve some influence, but the 
seismic motions are ee frequent when the barometer is low. 
he maximum disturbance occurs near the winter solstice, and the 
minimum near Sie summer solstic 
r. Darwin continues—“ I Save seen nothing which shows that 
M. Fimaiaeices takes any special precautions with re gard to the 
weight of the observer’s body, and it would be interesting to learn 
perature of the level itself. The slow molecular changes in glass 
render levels untrustworthy for comparison at considerable inter- 
vals of time. Although we must admire M. Plantamour’s indomi- 
table ——— it is to be regretted that his mode of observa- 
tion is by means of levels; and we are compelled to regard, at 
least provisionally, these enormous changes of level either as a local 
pheno enon, OF as due to systematic error in the mode of obser- 
vatio 
I ‘ae quoted a few of the results already obtained, by way of 
introduction to what I have to say this evening, and indicate 
the importance of such investigations in reference to astro cal 
work ; for if the astronomer must recognize the fact oneitih by the 
geologist and his own observations that the earth’s surface is no 
longer to be trusted when he requires a foundation for his instru- 
delay, ascertain the character and amount of these small changes, 
and in this way eliminate another disturbing element from hi 
investigations ; and I have therefore determined to bring before 
You may remember that in my address to the Society two 
months since I expressed the hope that the ate record of the 
height of the water in Lake George would reveal the effects of 
some of these terrestrial motions. e Seittuinihal used cannot 
have the extreme sensitiveness to minute vibrations which 
Darwin’s reflecting mirror and other instruments for the like pur- 
se have; but then it is so placed that all such changes 
magnified by the relatively enormous extent over which it extends 
its sensitive part, if I may so express what I mean ; for any change 
in gravity, or the direction of the vertical, is not seen as it affects 
the base of a small instrument a few feet square, but as it affects 
a surface 20 miles long and 5 to 6 miles wide. Barometric and 
wind chan, too, so difficult to see in other instruments, at once 
become evident here by their effects on such a large body of water, 
