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



during the expansion on relief of pressure. The first could almost' always be avoided, 

 even in the smaller apparatus, provided the pressure was raised with sufficient steadiness, 

 and the index brought down to the mercury at starting. But the instantaneous 

 reaction, partly elastic, partly due to cooling, and on rare occasions due to leakage of 

 the pump or at the plug, after a rash stroke of the pump, sometimes left the index a 

 little above the mercury just before the next stroke. If another rash stroke followed, 

 the index might be carried still farther above the point reached by the mercury. 

 Practically, however, there is little fear of my estimates of compression having been 

 exaggerated by this process. They are much more likely to have been slightly 

 diminished by a somewhat sudden fall of pressure which, in sjiite of every care, 

 occasionally took place at the very commencement of the relief. Once or twice the 

 experiments were entirely vitiated by this cause ; but, as we had recorded the sudden 

 outrush before the plug had been removed in order to take out the piezometers, we were 

 fully warranted in rejecting the readings taken on such an occasion : — and we invariably 

 did so, whether they agreed with the less suspicious results or not. 



Another and very puzzling source of uncertainty in the use of these indices depends 

 on the fact that the amount of pressure required to move them varies from one part of 

 the tube to another, sometimes even (from day to day) in the same part of the tube : 

 — and the index thus records the final position of the top of the mercury column in 

 different jriases of distortion on different occasions. The effect of this will be to make 

 all the determinations of compression too small, and it will be more perceptible the 

 smaller the compression measured. And in sea-water, and still more in strong salt- 

 solutions, the surface-tension of the mercury changes (a slight deposit of calomel (?) 

 being produced), while the elasticity of the hairs also is much affected. But, by 

 multiplying the experiments, it has been found possible to obtain what appears a fairly 

 trustworthy set of mean values by this process. 



I discarded the use of the silvering process, which I had employed in my earlier 

 experiments, 1 partly because I found that the mercury column was liable to break, 

 especially when sea-water was used, partly from the great labour and loss of time which 

 the constant resilvering and refilling of the piezometers would have involved. This 

 process has also the special disadvantage that the substance operated on is not 

 necessarily the same in successive repetitions of the experiment. 



And the electrical process 2 which I devised for recording the accomplishment of a 

 definite amount of compression could not be employed, because it was impossible to 

 lead insulated wires into either of my compression-chambers. This was much to be 

 regretted, as I know of no method but this by which we can be absolutely certain of 

 the temperature at which the operation is conducted. 



My next difficulty was in the measurement of pressure. In my former Report I 



1 Proc. Roy. Soc. Edin., vol. xii. pp. 223, 224, 1883. 2 Appendix A to this Report. 



