90 



Dr. C. W. Siemens. 



[June 15 r 



furnaces, and Professor A. Weinhold,* using the instrument 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 



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



