CHAP. VII.] DEEP-SEA TEMPERATURES. 293 



ment.^ A modification of Phillip's maximum ther- 

 mometer devised by Sir William Thomson, in which 

 the thermometer is entirely encased in an outer 

 shell of glass partly filled with alcohol, appears to 

 have the smallest error of all. 



A neat modification of Breguet's metallic ther- 

 mometer was designed by Joseph Saxton, Esq., of 

 the U.S. Office of Weights and Measures, for the 

 use of the U.S. Coast Survey. A riband of 

 platinum and one of silver are soldered with silver 

 solder to an intermediate jplate of gold, and the 

 compound riband is coiled round a central axis of 

 brass, with the silver within. Silver is the most 

 expansible of the metals under the influence of 

 heat, and platinum nearly the least. Gold holds an 

 intermediate place, and its intervention between the 

 platinum and silver moderates the strain, and pre- 

 vents the coil from cracking. The lower end of 

 the coil is fixed to the brazen axis, while the upper 



1 In Messrs. Negretti and Zambra's list of. meteorological instruments 

 published in 1864, a deep-sea thermometer on this plan is mentioned 

 (p. 90) : " The thermometers constructed for this purpose do not differ 

 materially from those usually made under the denomination of Six's 

 thermometers, except in the following most important particulars : — 

 The usual Six's thermometers have a central reservoir or cylinder 

 containing alcohol ; this reservoir, which is the only portion of the 

 instrument likely to be affected by pressure, has been, in Negretti and 

 Zambra's new instrument, superseded by a strong outer cylinder of 

 glass, containing mercury and rarefied air. By this means the portion 

 of the instrument susceptible of compression has been so strengthened, 

 that no amount of pressure can possibly make the instrument vary." 

 Some obscurity is introduced into this passage by the use of the word 

 'superseded;' but I am assured by Messrs. Negretti and Zambra that 

 in principle this instrument was exactly the same as that devised by 

 Professor Miller and constructed by Mr. Casella. 



