410 



Messrs. Carpenter, Jeffreys, and Thomson [Nov. 18, 



by experiments devised for the purpose, that this is the true account of 

 it, and that the inner bulb of these Thermometers is not altered in capacity 

 in any appreciable degree by a pressure reaching to three tons on the square 

 inch *. This pressure was found to send up the maximum index of the best 

 unprotected Thermometers made upon the Admiralty pattern as much as 10°; 

 whilst a pressure of 2| tons on the square inch sent up the index of an 

 ordinary Phillips's maximum mercurial thermometer no less than ll/ 0, 5. 



14. A considerable number of Thermometers by different makers, includ- 

 ing six protected according to Dr. Miller's plan (all of them previously 

 tested in the Pressure-apparatus), were supplied to the 1 Porcupine' by the 

 Meteorological Department ; and during its earlier Cruises numerous compa- 

 rative observations were made at different depths, with the view of deter- 

 mining the differences between the protected and various forms of unpro- 

 tected Thermometers at gradationally increasing depths, — such differences 

 being here of course due to pressure only. The records of these observa- 

 tions, having been transmitted to the Admiralty, were carefully reduced to 

 curves by Capt. Davis, and compared with their differences at correspond- 

 ing pressures in the Pressure-apparatus ; with the result of showing (when 

 due allowance was made for the small increment of Temperature in the 

 experiments) such a close conformity, that it became obvious that the 

 protected Miller- Casella thermometers might be thoroughly relied on for 

 indicating the true temperature within 1° Fahr. under any pressure not ex- 

 ceeding that to which they had been tested, — this being equivalent to that 

 of a column of Sea-water 2400 fathoms (4389 metres) deep. This happens 

 to be almost exactly that of the deepest Sounding taken in the * Porcupine' 

 Expedition, which was 2435 fathoms (4453 metres). 



15. The thorough reliableness of this instrument having been thus demon- 

 strated, it was considered unnecessary to carry the comparative observa- 

 tions further ; and in the last Cruise two protected Thermometers were 

 alone employed. The excellence of these instruments may be inferred 

 from the fact that they never differed more than a few tenths of a degree 

 (Fahr.)t, and that after having travelled vertically downwards and upwards 

 with the Sounding-apparatus to a total amount of nearly a hundred miles 

 (one of the Soundings having been taken at a depth of nearly three miles, and 



* [The results of a more elaborate series of investigations subsequently carried on by 

 Staff-Commander Davis, -which were communicated to the Koyal Society, May 19, 1870, 

 lead him to believe that the small elevation above alluded to is not entirely accounted for 

 by increment of Temperature, and that it consequently indicates that some influence is 

 still exerted on the inner bulb by Pressure on the outer. The elevation thus produced, 

 however, does not in any case amount to 1° (Fahr.) ; and the error can be reduced to a 

 scale, by the application of which it can be easily corrected.] 



f These small differences are probably due to slight differences in the rate at which 

 the instruments took the temperature of the water around ; in which case the lower read- 

 ing would be the most correct. — Before leaving Belfast on the Third Cruise, Prof. Wyville 

 Thomson tested the condition of these Thermometers by immersing them in ice ; and 

 they both recorded exactly 32°. 



