1869.] on the Scientific Exploration of the Beep Sea. 409 



7»" 



A 



■roo 

 ■go 



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of alcohol interposed between the two*. Several Thermometers were con- 

 structed upon this plan by Mr. Casella ; and these Fig. 2. 

 were tested in the pressure-apparatus, together 

 with various instruments of the ordinary construc- 

 tion, as well as with instruments constructed by 

 Mr. Casella with bulbs of extra thickness. A pre- 

 liminary trial having indicated (1) that the effect 

 of hydraulic pressure upon ordinary Thermometers 

 (as shown by the rise of the maximum index) is 

 always very considerable, though varying in 

 amount according to the construction of the in- 

 strument, (2) that this effect cannot be pre- 

 vented by an increase in the thickness of the bulb, 

 and (3) that the rise of the maximum index in 

 thermometers protected according to Dr. Miller's 

 plan was comparatively trifling, — a series of com- 

 parisons between the "protected" and the "un- 

 protected" instruments was very carefully con- 

 ducted under the direction of Staff- Commander 

 Davis of the Hydrographic Office ; who, having 

 had experience in Thermometric Soundings in Sir 

 James C. Ross's Antarctic Expedition, felt spe- 

 cially interested in the determination of this ques- 

 tion. In these experiments the difference between 

 the ordinary unprotected Thermometers con- 

 structed by Mr. Casella for the Admiralty (by 

 which the Temperature- Soundings had been taken 

 in the * Lightning ' Expedition), and protected 

 Thermometers constructed on the same pattern in 

 every other respect, was carefully noted at gradually 

 increasing pressures, so as to determine the amount 

 of such difference at depths respectively corre- 

 sponding to these pressures. The question whether 

 the small elevation of the maximum index observed in the protected Ther- 

 mometers is fairly attributable to an actual increment in the temperature 

 of the water in which they are immersed, consequent upon the com- 

 pression to which it is subjected during the experiment, was carefully 

 considered by Dr. Miller (Proceedings, toe. cit.), who satisfied himself, 



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■50 



* See Dr. Miller's " Note upon a Self-registering Thermometer adapted to Deep-sea 

 Soundings," in Proceedings of Royal Society, June 17, 1869, p. 482. The same 

 principle had been previously applied in Thermometers constructed under the direction 

 of Admiral Fitzroy, the space between the two bulbs, however, being occupied by 

 mercury instead of spirit. But owing to some imperfection in tbe construction of these 

 instruments, their performance was not satisfactory, and they were found very liable to 

 fracture. 



