100 THE VOYAGE OF H.M.S. CHALLENGER. 



sections sent home in the preliminary reports continued, however, to be constructed with 

 temperatures " corrected for pressure," in order that they might be comparable with 

 those that had gone before, although thus far it had become evident that the ther- 

 mometers, in so far as they were used as minimum instruments, were sufficiently protected 

 by the outside bulb against the effects of pressure, and that in consequence their readings 

 at great depths were not affected by any sensible error due to this cause. 



With a view of finding out the true effect of pressure on the readings of the protected 

 thermometers, Sir Wyville Thomson, on the return of the Expedition, requested Professor 

 Tait to investigate the whole question, and handed over to him about thirty of the ther- 

 mometers, which had been used during the cruise, and also the hydraulic pressure apparatus 

 constructed in 1872, which had been on board during the voyage. The results of the 

 investigation have been published in extenso in Appendix A to Vol. II. of this Narrative. 1 



Professor Tait commenced by remarking that a correction so large as that given by 

 Captain J. E. Davis for the maximum index, if it were to be applied at all, must be applied 

 with but little diminution to the minimum index also. So that the question is a serious 

 one. He then tested the pressure apparatus, but found it to be in many respects unsuit- 

 able for the work he contemplated. 



1. It was capable of holding only two thermometers at once ; and, when two were 

 inserted, there was no room for other necessary apparatus, such as pressure gauges, &c. 



2. The Bourdon-gauge attached to it was graduated only to four tons weight per 

 square inch, while it was desirable to carry the pressure to six tons at least. 



3. When compared with an air-manometer inserted in the pressure cylinder, this 

 gauge proved to be very inaccurate. 



4. Even with the moderate pressures which had been applied to it, the cylinder was 

 not deemed perfectly safe, and had in consequence been strengthened (?) by massive 

 rings of Swedish iron clamped round it. 



Professor Tait therefore informed Sir Wyville Thomson, that if the experiments were 

 to be conducted in the Edinburgh University Buildings, it was essential that a stronger 

 and much more capacious pressure cylinder should be procured ; and suggested that it 

 should be constructed on the principle of the Fraser gun. 



In the spring of 1879 the new instrument 2 (weighing nearly three tons) arrived in 

 Edinburgh from the Koyal Gun Factory at Woolwich, and was erected on a mass of 

 concrete embedded in the ground below the floor of one of the basement rooms in the 

 College. As the gas-engine belonging to the Physical Laboratory happened to be fixed in 

 a neighbouring cellar, the requisite shafting was put up to connect it with the pump ; 



1 The Pressure Errors of the Challenger Thermometers, by Professor P. G. Tait, Narr. Chall. Exp., vol. ii., 

 Appendix A. 



2 Described and figured in the' above mentioned paper, where will be found a full description of the experiments and 

 their results, also an account of the various new forms of gauges employed for the accurate measurement of pressure. 



