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



instrument : unless, indeed, there be a strain produced on the protected bulb (altering its 

 volume) by the wry-neckedness of the protecting shell. 



Now, as a rule, till quite recently, practical workers in glass supposed that no effect 

 at all would be produced by pressure upon an ordinary thermometer stem, simply because 

 the external diameter is so much greater than the internal ; and, in fact, so little was the 

 nature of the effects of hydrostatic pressure known to practical glass-blowers that one of 

 Mr Casella's workmen undertook in 1869 to furnish Captain Davis with thermometers 

 whose bulbs should be so thick as to " defy compression " ! It will be seen presently 

 that such an idea is entirely absurd : that, however thick is an unprotected thermometer, 

 it will still have its indications altered by compression, and very nearly as much as a 

 thinner one, unless that be extremely thin. So far as the Challenger instruments are 

 concerned, the only effect that can be expected to be produced directly by pressure is 

 the diminution of the bore and length of the narrow tube, and the consequent forcing of 

 the liquid which occupies it to fill a greater length in it. I made at starting a rough 

 calculation of the amount of effect of this kind which was to be expected ; taking average 

 data as to the compressibility and rigidity of glass. I found it to be a small fraction 

 only of a degree for each ton-weight of pressure, except on those thermometers which 

 had very short degrees. It was clear to me, therefore, that (unless the wry-neckedness 

 already mentioned was the cause) the larger part of Captain Davis' result was not due to 

 pressure directly. 



Wholly protected Instruments. Their Defect. 



For the purpose of comparison with the Challenger instruments, so far as regards the 

 effect on the unprotected stem, Sir Wyville Thomson sent me two mercury thermometers 

 constructed after Sir William Thomson's device. In these instruments the whole, bulb 

 and stem alike, is enclosed in a strong glass tube, nearly filled with alcohol. The effects 

 of pressure on these instruments were very much smaller than on the thermometers of the 

 Challenger. This result was so unexpected that I at first thought it due to defects in the 

 new instruments. But, as will be seen later, it is quite consistent with the final result 

 of my investigations. It is, however, very difficult to obtain good results from these 

 instruments under the circumstances in which I was working. Their recording adjust- 

 ment is constructed on a new plan, in which a little portion of mercury is detached from 

 the rest ; and separated from it by a small quantity of air, which does not move it until 

 compressed to a definite amount. To set the index before an observation, the instru- 

 ment has to be swung round somewhat sharply at arm's length. It was scarcely ever 

 possible under these circumstances to adjust it to the temperature of the water in the 

 press. The indices in the Challenger thermometers, on the other hand, consist each of a 

 piece of enamel with a couple of hairs attached to it so as to fix itself in the tube and 



