EQUIPMENT. АТ 
the descent of the thermometer. Such an apparatus has been 
devised by Sir William Siemens. In the Bakerian Lecture 
for 1871, he showed that the principle of the variation with 
the temperature of the electrical resistance of a conductor 
might be applied to the construction of a thermometer, which 
would be of use in cases where a mercurial thermometer was 
not available. The instrument he described has since been 
largely used as a pyrometer for determining the temperatures 
of hot blasts ара smelting furnaces; and it has been found 
that its indications agree very closely with those of an air-ther- 
mometer. He devised asimilar instrument for measuring tem- 
peratures where a much greater degree of 
accuracy is required, as in the case of deep- 
sea observations; and during the autumn of 
1881, this deep-sea electric thermometer was 
subjected to a series of tests on board the 
“ Blake,” by Commander Bartlett. 
The apparatus consists essentially of a coil 
of silk-covered iron-wire (Fig. 13), fifteen mil- 
limetres diameter, and about four hundred 
and thirty-two ohms resistance, attached to 
an insulated cable by which it can be low- 
ered to the required depth, and connected so 
as to form one arm of a Wheatstone bridge. 
The corresponding arm of the bridge is 
formed by a second coil, made precisely 
similar to the former one and of equal re- 
sistance. This coil is immersed in a copper 
vessel filled with water, and the temperature 
of the water is adjusted by adding iced 7 
or hot water until the bridge is balanced. Me edic oe ind, 
The temperature of the water in the vessel ise eae dr 
is then read by a mercurial thermometer, 
and corresponds with the temperature of the resistance-coil. 
To avoid the error which would otherwise be introduced by 
the leads of the resistance-coil, the cable is constructed of a 
double core of insulated copper-wire, protected by twisted gal- 
vanized steel-wire. One of the copper cores is connected with 
