408 



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



of the preliminary enquiries made, and the arrangements actually 



adopted. 



12. It had been remarked in the Report of the " Lightning Expedition M 

 (Proceedings of the Royal Society, Dec. 17, 1SSS, p. 1S5; that while the 

 existence of a minimum Temperature (probably that of the bottom) at least 

 as low as 32" (0" Cent.), over a considerable area of which the depth was 

 between 500 and 600 fathoms, had been conclusively established, the 

 actual minimum might probably hare been from 2 C to 4 3 below that 

 recorded by the Thermometers employed, the pressure of 100 atmo- 

 spheres, to which their bulbs were subjected at a depth of about 535 

 fathoms*, being very likely to alter the capacity of the bulbs to that 

 extent. " In any renewal of the enquiry," it was added, Si it will be of 

 course desirable that the Thermometric apparatus used should be specially 

 protected from this source of error." 



13. So soon, therefore, as there was reason to believe that the application 

 of the Council of the Royal Society for such renewal would be acceded to 

 bv H.M. Government, steps were taken to determine the precise amount of 

 this error, and to devise the best means of preventing it. After consulta- 

 tion between the Hydrogrspher, Dr. Carpenter, and Mr. Casella (the 

 maker of Meteorological Instruments to the Admiralty), it was determined 

 that an apparatus should be constructed on the principle of the Bramah 

 Press : in which Thermometers immersed in water should be submitted to 

 hydraulic pressure, which could be gradually raised till it reached three 

 tons on the square inch, its amount being indicated by a pressure-gauge 

 as the experiment proceeded. Mr. Casella further undertook to construct 

 Thermometers with bulbs of extra thickness, in order that it might be 

 ascertained whether the error arising from external pressure (if such should 

 be proved to exist) could be kept in check by this simple expedient. The 

 question was at the same time made the subject of consideration by a Com- 

 mittee appointed by the Council of the Royal Society, as set forth in the 

 Minutes already cited (p. 402, 403); and it was determined that trial should 

 be given to a plan proposed by Dr. W. A. Miller, which consists in the 

 enclosure of the bulb of the Six's Thermometer (the form of Self-register- 

 ing Thermometer that had been found by experience best adapted to 

 Deep-sea Soundings) in a second or outer bulb, sealed around the neck of 

 the stem, — the space between the inner and outer bulbs being nearly 

 filled with alcohol, and the greater part of the air being displaced from 

 the small unfilled space, by boiling the spirit before the outer bulb is 

 sealed. In this manner the inner bulb is protected from the influence 

 of variations in external pressure upon the outer, the only effect of which is 

 to alter the capacity of the unfilled space ; whilst changes of temperature 

 in the medium surrounding the outer bulb are speedily transmitted to the 

 fluid contained within the inner, by convection through the thin stratum 



* The pressure of a column of Sea-wafer of 100 faihcms depth is 2S0 lbs. upon the 

 Kjuare inch, or w< (on for every 800 fathoms. 



