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



APPENDICES. 



APPENDIX A. On the Accurate Measurement of High Pressures. 

 (Mainly from Proc. R.S.E. 1879-80.) 



In the course of an examination of some of the Challenger deep-sea thermometers, I have recently 

 had occasion for measurements, accurate to one or two per cent., of pressures such as five or six tons 

 weight per square inch. The ordinary gauges showed themselves to be quite untrustworthy, and it 

 was necessary to devise some plan of whose accuracy the experimenter can feel assured. The follow- 

 ing process has proved completely successful, and is capable of any desired degree of accuracy. 



Simple methods based on the compression of gases, such as air or nitrogen, are of the highest 

 value wherever they can be adopted ; for the law of compression of these bodies is known with great 

 accuracy (at least for one definite temperature) from the measurements recently made by Amagat, in 

 which the pressures were directly reckoned in terms of a column of mercury. A simple form of 

 gauge, in which the column of mercury compressing the gas into a small bulb at the extremity is 

 made to break off at a constriction in the connecting tube, enabling us (by weighing the mercury 

 forced over into the bulb) to measure the compression very accurately, suffices amply for all pressures 

 up to a ton weight per square inch, or even farther. 



But this instrument becomes rapidly less and less sensitive at higher pressures ; so that, though 

 the law of compression for a considerably extended range is now known, for pressures above a ton 

 something else is required. Besides, this method is very laborious, and therefore is not to be employed 

 oftener than is absolutely necessary. 



Hooke's Law now comes to our assistance. An instrument resembling a thermometer in form 

 supplies the next step. Its bulb is all but filled by a glass tube closed at each end, and it is thus 

 practically unaffected by the changes of temperature produced in such experiments. Over the 

 mercury in the stem is a long column of alcohol in which the index moves, and the rest of the tube 

 contains alcohol vapour only. The bulb is made cylindrical for several reasons ; the chief being to 

 secure uniformity of thickness, which is practically unattainable (or at least unverifiable) in a sphere. 

 By properly choosing the thickness of the cylinder in proportion to its bore, and its volume as com- 

 pared with that of an inch of the fine tube, the sensitiveness of this gauge may be made as great or 

 as small as we please. And, by having two or more, with bulbs of nearly the same internal dimensions, 

 but differing considerably from one another in the thickness of the cylindrical walls, a very important 

 advantage is secured. For, under the same pressure, the maximum amounts of distortion of the glass 

 are greater in the thinner bulbs, and thus these begin to deviate from Hooke's Law at pressures under 

 which the thicker ones are still following it accurately. Thus, by comparison, we can easily find 

 through what portion of its range each instrument gives effects strictly proportional to the pressure. 

 The thinnest of these has the unit of its scale determined by comparison with the nitrogen gauge. 



