126 NOTES ON THE CONFERENCE OF DELEGATES OF THE
the Secretary to the B. A. Committee dealing with this
subject, said that he would be glad to receive photographs show-
ing lightning flashes of an abnormal character. He added that he
read now and then in the newspapers accounts of the remarkable
effects produced by a whirlwind in this or that district. But he
usually found that it was then too late to have the results
photographed, the damage done having been repaired, or the
damaged object removed. In such cases he would be very grateful
to the Secretary of any local society in the district affected who
would get photographs taken at once and send them to him.
Earth tremors.—Mr. Davison, Secretary of the Earth Tremors
Committee, gave some account of the work of the Committee, and
Mr. Horace Darwin described the construction and use of the
Bifilar Pendulum, which he has invented, and a full account of
which appeared in "Nature" of July 12th, 1894. He said it was
not affected by the rapid complicated movements which took place
during an earthquake, nor by the slight tremors produced by pass-
ing carts or trains. The movements which it would measure and
register were such as would make a factory chimney lean over to one
side. Extremely small movements of this kind could be detected.
Mr. Symons, the Chairman of the Earth Tremors Committee, said
that some time ago the attention of the Committee had been directed
to certain vibrations recorded by an instrument placed at the bottom
of a deep mine in the district of Newcastle-on-Tyne. These pulsa-
tions were traced to two causes, one, the gradual settlement of the
ground in consequence of the removal of the coal, the other, the
beating of the waves upon the coast. Mr. Davison on one occasion
found pulsations were taking place which eventually turned out to
have been produced by an earthquake then going on in Greece.
They wanted information, added Mr. Symons, as to the changes
going on in connection with the faults in geological strata, and, if
possible, to get records of the alterations in the earth's crust caused
by tidal waves. The Committee had one instrument under the
charge of Mr. Davison at Birmingham, but they wished to establish
others in various parts of the British Isles.
In Essex we have no great lines of fault along the course of
which an instrument for recording earth tremors would be especially
desirable. But in the small compact area from Colchester to
Mersea, which was so severely damaged by the earthquake of 1884,
we have a tract of country in which we might expect earth tremors