522 
The British Association . 
[October, 
an endless chain or thread passing over a pulley on the axles 
of the scape-wheel, and over the minute wheel of a secondary 
clock, hanging between them in a festoon which supports 
the weight by a pulley. The secondary clock gives the hours 
and minutes, and the clock without train shows the seconds. 
We thus have a clock without the errors introduced by a 
train. It is a gravity escapement without the locking 
friction. 
Prof. G. Forbes also described u An Instrument for Deter- 
mining the Quantity of Fire-Damp in Mines,” which consists 
of a resonator of various dimensions, and a tuning-fork of 
^definite pitch. The resonator is a metal tube I inch in 
diameter and 15 inches long, in which a piston slides so as 
to regulate the length of the tube. This tube is fixed in a 
block of wood, to which is attached a tuning-fork, whose 
points are just above the open end of the tube. The tuning- 
fork is sounded in every convenient way, and the piston is 
moved out and in until the proper length is found, which is 
indicated by the resonator intensifying the sound of the 
tuning-fork. With practice the length can thus be deter- 
mined with an accuracy of at least 1 in 250. But the length 
depends on the density of the gas, a light gas requiring a 
longer resonator, and, by reading off on a scale the position 
of the piston, a person can judge of its density. In this 
manner 1 or 2 per cent of fire-damp, mixed with common 
air, can be detected. Barometric pressure produces no dif- 
ference on the instrument. The temperature correction is 
made by reading off a thermometer of the proper dimen- 
sions, instead of reading off a fixed mark on the piston. 
Prof. Silvanus P. Thompson narrated several instances of 
rainbows, chiefly seen in Switzerland, when radial streaks 
of light, devoid of colour, were observed within the primary 
bow, and without the secondary bow, 
Mr. C. Meldrum read a paper “ On Sun-Spots and Rain- 
fall.” The conclusion at which the author arrived, from a 
great number of observations in all parts of the world, was 
that the maximum and minimum rainfall apparently coin- 
cided with the maximum and minimum sun-spots respectively. 
Mr. W. Morris read a paper “ On the Temperature of the 
Earth Within.” He held that the present method of deter- 
mining underground temperatures with the thermometer as 
used in air or water was quite unsatisfactory. He suggested 
the employment of pairs of chronometers, one of each pair 
to be placed at the bottom of a bore, and the other at the 
surface. 
