On the Compressibility of Solids 375 



meters, which measure the expansion or contraction of the body 

 nnd -'.tilion. arc independent of the instrument which holds it. 



The manometer, which indicates the pressure in the instru- 

 ment, is seen under the steel block which carries the tubes. 

 It is simply a mercurial thermometer with a very thick bulb. 

 The scale on it is an arbitrary one, and its value as a measure 

 of pressure is fixed by observing its reading when the principal 

 piezometer which I used during the voyage of the "Challenger" 

 in the receiver. This piezometer, known as C. No. I, 

 contained distilled water, and from very many carefully 

 executed experiments at depths from 800 fathoms (1440 metres) 

 up to 2500 fathoms (4500 metres), made in the South Pacific 

 where the oceanic conditions were most favourable, the apparent 

 compression of distilled water in this particular instrument at 

 the temperature ruling in these depths, which averages in 

 round figures 2 C., and when exposed to measured columns of 

 sea-water, of known quality as regards density, was accurately 

 known. The indications of the manometer are, therefore, 

 equivalent to those of piezometer C. No. i, the standardisation 

 of which was effected under an open-air water column. The 

 observations made with C. No. i on board the "Challenger," 

 which form the basis of the scale of pressures, are collected in 

 Table I. They are expressed in terms of the apparent com- 

 pressibility of distilled water deduced from them. 



In the table the vertical lines represent apparent com- 

 pressibility in volumes per million per atmosphere, rising by 

 steps of i per million from 45 55, so that all the values of the 

 compressibility falling between say 45 and 46, or 49 and 50 are 

 arranged in one column. Above each entry of apparent com- 

 pressibility will be found the depth in metres to which the 

 in-trumrnt was sent, ami the temperature ( C.) of the sea- 

 water at that depth. The depth is expressed in metres because 

 it so happens that the average density of the water in thU part 

 of the South Pacific, allowance being made for the 

 distribution of tempi : <>mpression, and -aimitv. i- MK h 



that a vertical column of it 10 HMtra hi.d 

 exactly the same pressure as 760 mm. of mercury. So that 



i in ITU tit-, divided by 10, gives the pressure in ordinary 



