UNDERGROUND TEMPERATURE. 
19 
There are two thermometers now construc- 
ted especially for the committee by the firm 
of Negretti and Zambra. Fig. 3 is the slow 
action thermometer for taking direct earth 
temperatures. The bulb of this instrument 
is shown in its glass sheath surrounded by a 
good non-conducting substance, as suggested 
by Professor Everett. The thermometer i^ 
lowered down to the desired depth, in a 
copper case, and allowed to remain for a 
stated time (in the case of my observations 
for 48 hours.) It is then drawn up and the 
reading taken, but should the time occupied 
j in raising the instrument exceed 4 ^, or at the 
most, 5 minutes, the reading would not, I 
think, be reliable. 
Fig. 4 represents the maximum ther- 
mometer. The mercury bulb is at the top, 
and on the temperature rising, the mercury 
of course expands, and leaks out into the 
glass tube; on gently tapping or shaking the thermometer, it 
falls down to the column in the bottom of the tube : the height 
of the whole represents the temperature.^ This thermometer is 
available for any maximum temperatures, and will not fail to give 
satisfaction ; the same may be said of the slow acting minimum 
thermometer. 
1 I am indebted to Messrs. Negretti & Zambra, of Holborn Viaduct, for the 
use of the stereo of the thermometers. 
c a 
