366 Proceedings of the Royal Society 
increased. Thus, according as there is a large or small percentage of 
fire-damp in a mine, so will the length of tube which best resounds 
to the tuning-fork be great or small. 
There are one or two small practical details which have given some 
trouble, but which now render the instrument very perfect. 
1. In order to give to the hand great control over the lengthening 
and shortening of the tube, a rack has been attached to the piston, 
which works in a pinion on whose axis there is a large disc with a 
milled head 3 inches in diameter. This disc has a glass face with a 
graduated scale round the circumference ; so that a fixed index marks 
with great precision the exact length of the tube. The scale is thus 
made so large that readings can be made in the feeblest light. 
2. In order that the instrument may be taken in advance of a 
lamp in places where gas is expected in large quantities, a phospho- 
rescent powder is placed in a cavity behind the graduated glass plate, 
by which means readings can be taken in the dark. 
3. To be sure that the gas or air in the tube is the same as what 
is to be found in the particular part of the mine under examination, 
I have introduced, through the piston which works the pinion, a rod 
at the upper end of which is a packed disc fitting the tube tightly. 
Previous to taking a reading this disc is, by means of a handle 
attached to the rod, driven up to the open end of the tube, and in 
being drawn back it sucks in the air from the place under observa- 
tion. It is thus, by a single turn of the handle, locked to the 
piston which works the pinion, by a bayonet joint. 
4. The temperature in a mine is generally very constant. But to 
prevent errors arising from variations in the temperature a thermo- 
meter is attached whose graduations are given m. percentages of fire- 
damp, which are to be subtracted from the percentages recorded in 
the circular scale. 
5. To test the accuracy of the scale, I have a circular trough 4 
feet diameter and 3 inches deep. This is partially filled with water, 
and a grating is placed in the water to stand upon. In the centre 
of the trough there is a hole with an inch metal tube projecting 
upwards 4 inches. To this is attached an india-rubber tube 2 feet 
long, with a mouthpiece which can be firmly attached to the mouth 
for breathing. Sitting on the stool with the mouthpiece attached, 
and the nose closed by spring pincers, a tin cover 4 feet high and 3 
