464 Mr. J. Prestwich on Tables of [Jime 18, 



the Mediterranean, at a depth of 1000 to 2000 feet, a temperature, so 

 nearly right, of 55*5. 



Sir John B/oss and Admiral Spratt sometimes used Six's thermometers, 

 and at others took the temperature of the silt brought up from the bottom. 

 The former obtained readings of 28- 5 F. for Baffin's Bay, and the latter 

 of about 55 for the Grecian archipelago, agreeing therefore closely with 

 good thermometrical observations. 



Phipps used a differential overflow thermometer invented by Cavendish, 

 but it was not found to answer. This form of instrument remained in 

 abeyance until a greatly improved form of it was contrived by Wal- 

 ferdin (thermometre a deversement) in 1836. It was used by Mar- 

 tins and Bravais in the Arctic seas, and by Aime in the Mediterranean, 

 and was said to give very satisfactory results. Aime also used another 

 somewhat similar instrument, which, at a given depth, was reversed and 

 then hauled up. These instruments have the great advantage of being 

 free from errors arising from the shifting or immobility of the index. 

 It is not clear why their use was abandoned, except that they were diffi- 

 cult to construct and not generally known. 



Six described his thermometer in 1782 ; but the first person to use it 

 was Krusenstern, in 1803. It did not come into general use for deep- 

 sea observations until the Arctic voyages of Eoss and Parry, after 

 which date it was, with the exception of Lenz's and Aime's, employed 

 for that purpose on all the expeditions sent out by foreign governments, 

 as well as by our own. The necessity of protecting the instrument 

 against pressure was early insisted upon by Lenz, Arago, Biot, and 

 others ; and there is reason to believe that protected thermometers were 

 used by D'TJrville and Berard, for their observations in the same Medi- 

 terranean area show a remarkably close agreement with those recently 

 made by Dr. Carpenter, with protected instruments, at and below depths 

 of about 200 fathoms, the results being : 



D'Urville (May 1826). Berard (Nov. 1830). Carpenter (Aug. 1870). 



Surface . . . . 64-l F. Surface .... 67-l F. Surface .... 73-5 F. 



1062 ft 54-2 3189 ft 55-4 2958 ft 55-5 



3189 ft 54-7 6377 ft 55-4 7968 ft 54-7 



It was, however, on Du Petit Thouars's voyage of 1836 that the first 

 special steps were taken to protect the thermometer against pressure. 

 For that purpose an improved instrument of Bunten's was provided, 

 and this was enclosed in a strong brass cylinder. Fifty-nine observations 

 were made, of which Arago reported that 21 might be considered perfectly 

 good. Temperatures of 36, 37, and 38 F. were recorded at depths 

 (900 to 1100 brasses) in both the mid-Atlantic and mid-Pacific ; whilo 

 in one case, in taking a sounding at a depth of 12,271 feet near the 

 equator in the Pacific, the instrument came up crushed, but with the index 

 fixed at 34-8 F. (l-6 or l-7 C.), In a certain number of cases (24) the 



