266 THE SCIENTIFIC PAPERS OF 



temperature might be applied to the construction of a thermometer, 

 which would be of use in cases where a mercurial thermometer is 

 not available. 



The instrument I described has since been largely used as a 

 pyrometer for determining the temperatures of hot blasts and 

 smelting furnaces, and Professor A. "Weinhold,* using the instru- 

 ment with a differential voltameter described in my paper referred 

 to, found its indications to agree very closely with those of an 

 air thermometer within the limits of his experiments from 100 

 to 1,000 Centigrade. I am not aware, however, that any results 

 have been published of its application to measuring temperatures 

 where a much greater degree of accuracy is required, as in the 

 case of deep sea observations. My friend, Professor Agassiz, of 

 Cambridge, U.S., ordered last year for the American Government 

 an instrument designed by me for this purpose, and during the 

 autumn it was subjected to a series of tests on board the United 

 States Coast and Geodetic Survey steamer "Blake," by Commander 

 Bartlett. 



The apparatus consists essentially of a coil of wire T, which is 

 lowered by means of a cable to the required depth ; and is coupled 

 by connecting wires to form one arm of a Wheatstone's bridge. 

 The connexions of the bridge are shown in Figs. 1 and 2, Plate 2:->. 

 The arm CD is the comparison coil S made of the same wire as 

 the resistance coil T, and equal to it in resistance. This coil is 

 immersed in a copper vessel of double sides, filled with water, and 

 the temperature of the water is adjusted by adding iced or hot 

 water until the bridge is balanced. The temperature of the water 

 in the vessel is then read by a mercurial thermometer ; and this 

 will also be the temperature of the resistance coil. 



To avoid the error, which would be otherwise introduced by the 

 leads to the resistance coil, the cable was constructed of a double 

 core of insulated copper wire, protected by twisted galvanised steel 

 wire. One of the copper cores was connected to the arm BC of the 

 bridge, and the other to the arm DC, and the steel wire served as 

 the return earth connexion for both. 



The resistance coil and comparison coil were made of silk- 

 covered iron wire -15 millim. diameter, and each about 432 Ohms 



* " Annalen der Physik und Chemie," 1873, p. 225. 



