LOCAL VARIATIONS AND VIBRATIONS OF EARTH’S SURFACE, 53 
amounting to 00292” would occur from time to time ; and as this 
took place very frequently, and w ag ara than the lunar effect, 
‘there was no hope of eliminating the latt 
I think this conclusion is to be i: for although there 
can be no doubt about the accuracy of the figures, yet had their 
spot of light been made to record all its changes, it is probable that 
in many cases it would have been possible to separate one effect 
from the other as depicted in the automatic record. My experience 
at Lake George strengthens this view of the question. 
Mr. Darwin has given in the report a most valuable epitome of 
the labours of other experimenters in the same field, and I wil 
ask your attention to several of the items which have special 
significance in reference to what I have to bring before you to- 
night. 
It states that M. Bouguet zn la Greye made observations on a 
free pendulum when he was t Campbell’s Island, in lat. 52° 34’ 
south, and he found that the “se swell of the ocean breaking on 
the island caused a change in the vertical of 1-1” ; and that 
d’Abbadie, i in order to test these changes, built in 1863 a very 
massive stone cone 26 feet high, ‘which Fested on the solid rock 
about 24 feet from the surface of the ground ; in this cone was a 
— hole 3 feet in diameter, and this was carried down into the 
6 feet 6 inches, making the depth of the well-hole 32 feet 6 
that nothing could be seen init. At times he saw sudden changes 
of the vertical, amounting to 0-49, and even as much as 0°65, 
Do cataean earthquakes were frequently seen, in some of which 
the changes were so great that the reflected image was carried out 
of the field of view, and as the tide rose and the biases of water 
accumulated on the eu 430 yards distant, he could distinctly 
see the vertical’ line chan 
During the years 1867 * 1872 he found the plumb-line deviate 
northwards during the latter months of the year, in every yea 
except 1872, when it deviated to the south. The greatest sudden 
ie saw amounted to 2-4” in the dination of the vertical in 
1 hours 
M. Pla antamour, who worked for years in the same field, used a 
very delicate level fixed in a cellar, and at right angles to the 
ridian. The observations revealed a diurnal oscillation, in 
—- the east end of the level was highest about 5°30 p.m., ‘and 
the change once ran up to 8:4”, 11:2”, and 15-75” on three successive. 
_ days. In addition to these ‘diurnal motions, he noted a gradual 
_ rise in the east end, and he remarks that the east piers of transit 
instruments rise in the same way, but not to so great an extent. . 
