516 



Major J. Herschel. 



constructed by the author in (or about) the year 1859, which, how- 

 ever, was abandoned, although it enabled him " to predict approxi- 

 mately the depth that would be found on the use of the sounding 

 line." In this instrument the specific gravity of mercury would seem 

 to have been measured by determining the column necessary to 

 balance the pressure of a fixed quantity of confined air. The rise of 

 the surface under a change of gravity was magnified by a lighter 

 supernatant liquid being forced up a narrow tube. The principle, in 

 this case, is analogous to the former typical one only if we regard the 

 compressed air as acting the part of a spring. It is obviously 

 dependent on the possibility of guarding against, or exactly allowing 

 for, the change of elasticity with temperature — in this, too, being 

 analogous to the typical form. It was sought to meet this very 

 serious difficulty by keeping the air at the temperature of melting ice. 

 There is another method — perhaps not less difficult to practise 

 successfully; that, namely, of selecting three (or preferably four) 

 more accessible temperatures, and so experimenting at these that in 

 the long run the actual observations shall supply a constantly accu- 

 mulating body of evidence as to the requisite factor, as well as two or 

 more actual data in each case from which to infer what is required. 

 But it might turn out after all that the uncertainty must remain 

 greater than the variation to be measured. The possibility of such a 

 result militates strongly against methods involving large corrections. 

 But, perhaps, least of all in cases such as this where the whole 

 instrument may be immersed without injury. It then becomes 

 chiefly a thermometric question. 



In the bathometer, on the contrary, which superseded the one just 

 mentioned, "changes of temperature are entirely eliminated from the 

 result," at least in theory. In practice such a thing would perhaps 

 be hardly possible ; but in any case residual effects may be regarded 

 as subjects for ultimate consideration. I will now endeavour to con- 

 dense a description of this instrument, so as to assign it a place in the 

 present category. 



There are two spiral steel springs which, in the drawing of the 

 smaller of two instruments, are about 16 inches long and rather more 

 than one in diameter. The lower ends are attached to a cross-bar 

 which is to bear the intended weight. This weight is that of a 

 column of mercury resting upon a certain surface to be presently 

 described. There is an arrangement for maintaining the height of 

 the column constant. I am not sure that I understand how this 

 works ; but supposing it effective, the weight upon the cross-bar must 

 depend upon the area pressed by the mercury; and if this also is 

 constant we have in effect the same principle as in the gravimeter 

 depicted in the " Outlines." 



The mercury is contained in a vertical steel tube having cup- shaped 



