ROCK TEMPERATURES IN VICTORIA. 315 
underground this is not easy to use; the insulation of the 
terminals is a troublesome matter when water vapour and 
mineral salts are everywhere, and it is moreover necessary 
to know exactly the temperature of the measuring-coils. 
This has led the author to devise a form in which the coils 
will be in an oil bath, and the terminals quite covered. 
The mode of working the Wheatstone bridge hy means 
of a temperature adjustment has, however, proved extremely 
convenient. The apparatus used for this purpose consists 
of a duplicate thermometer coil (precisely similar to the one 
in the rock) immersed in a bath of liquid, and made the 
comparison-coil of the bridge. The two equal ratio arms of 
the bridge can also be immersed in the same bath 
Then, upon slowly raising the temperature of the liquid to 
that of the rock, there will be a point at which perfect 
balance will be obtained, and the temperature can be taken 
directly from the liquid by a mercurial thermometer of 
suitable range—it is that of the rock when all the tem- 
peratures are steady. The difference of temperature be 
tween the rock and the working place in which the reading 
is made is never very great, and this much facilitates the 
working; and, should the coils be slightly out of adjust- 
ment, they may be calibrated, and a correction applied that, 
if small, is practically constant for a wide range. 
DESCRIPTION OF THE APPARATUS. 
The apparatus used by the author consists of a metallic 
vessel, 10 inches high and 5 inches in diameter, provided 
with a vulcanite top, upon which are four tall vulcanite 
pillars for the thermometer connections, and with four other 
pillars for the battery, and the galvanometer keys. All the 
important parts are thus well insulated. The coils, that are 
permanently. connected in a mass of insulating material, 
hang from the vulcanite top plate in glass tubes, the ratio 
coils wound side by side in one layer on a long reel, and the 
comparison coil also in one layer upon a long reel, being 
each enclosed in a separate tube, the tubes having metallic 
sheaths to protect them from injury. 
The vessel when in use is filled with water and warmed 
by spirit-lamp, but we in future shall use a small heating 
coil and an accumulator, as being more convenient for final 
adjustment. The temperature of the vessel is raised very 
slowly, and steadied until the bridge shows equilibrium on 
repeated zero-readings. The temperature of the bath is 
then read off by means of a sensitive mercurial ther- 
mometer. The bridge and the reversing keys stand on a 
