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



Half-a-degree Fahrenheit per mile of depth may seem to be a matter of very little 

 consequence ; but when we recollect that some of the Challenger soundings were made 

 at depths nearly approaching six miles, we find that we have sometimes to deal with a 

 correction of 3 F., enough to modify seriously our theories of ocean circulation. For 

 it can never be too strongly impressed on the student of science that there is no such 

 thing as greatness or smallness in itself; what is very small relatively to one class of 

 quantities may be very great relatively to another and different one. All the tempera- 

 ture differences, except near the surface of the sea, though important in their conse- 

 quences, are very small relatively to differences of temperature in the atmosphere ; but, 

 just because they are so small, small errors in the determination of their values are 

 important : thus it was imperative to decide whether the corrections assigned by Captain 

 Davis are necessary. 



At first sight one might think that by far the best way of conducting an inquiry of the 

 kind would be to carry it out under circumstances nearly the same as those of the Challenger 

 observations. No doubt, if we had at hand a coal-pit or mine-shaft full of water, and of 

 six miles or so in depth, we might make the experiments without the aid of presses, and 

 under circumstances far more favourable than those in which I was obliged to operate. 

 The reasons for this statement will appear presently. There are great objections to 

 making test-observations at sea. The Challenger observations themselves had, of course, 

 to be made at sea, but to make under similar circumstances experiments for the purpose 

 of determining corrections would be a perfectly hopeless attempt. The circumstances 

 under which thermometers are let down and drawn up again at sea are extremely 

 unfavourable to accuracy of observation. I had, therefore, to content myself with such 

 conditions as could be procured by means of hydrostatic presses. 



Construction of the Thermometers. 



I will now say a word or two about the construction of the thermometers themselves ; 

 and I shall thus have an opportunity of pointing out some of the peculiarities of con- 

 struction to which I have traced the greater part of the very large effects obtained by 

 Captain Davis, and given by him as corrections which required to be made. 



The Challenger thermometers are all of the Six pattern : there is a highly expansible 

 liquid in the large bulb, which projects to a certain extent into the narrow U-tube. 

 Then there is a column of mercury occupying the bend of the U and part of each stem. 

 Above that, on the maximum side, there is some more of the sensitive liquid ; and at the 

 ends of the mercury column are the maximum and minimum indices, each containing a 

 piece of steel, so that they can be set by means of an external magnet. The large bulb 

 on which the temperature effects are mainly produced is protected by an exterior shell of 

 glass strong enough to resist a pressure of at least 5000 fathoms of sea- water ; that is to 



