1904] 



On the Compressibility of Solids. 



299 



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 " was in the receiver. This 

 piezometer, known as C No. 1, 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 3 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 G No. 1, 

 the standardisation of which was effected under an open-air water 

 column. The observations made with C No. 1 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 

 compressibility of distilled water deduced from them. 



In the table the vertical lines represent apparent compressibility in 

 volumes per million per atmosphere, rising by steps of 1 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 compressibility will be found the depth in 

 metres to which the instrument was sent, and 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 this part of the 

 South Pacific, allowance being made for the vertical distribution of 

 temperature, compression, and salinity, is such that a vertical column 

 of it 10 metres high exercises very exactly the same pressure as 

 760 mm. of mercury. So that the depth in metres, divided by 10, 

 gives the pressure in ordinary atmospheres. At great depths a very 

 slight correction has to be made ; the nature of this will be apparent 

 from the following table, in which, for different depths in metres, D, 

 the pressure P in atmospheres is given : — 



D 1400 2000 3000 4000 5000 6000 



P 139-96 200-14 300-82 401-98 503-62 605-75 



Owing to the preponderance of water of low temperature and of 

 very uniform salinity in a vertical column of water in any part of the 

 open ocean, the pressure exercised by it per thousand metres does not 

 differ appreciably from 100 atmospheres. 



Inspection of Table I shows at once in which column the true 

 value of the apparent compressibility is most likely to be found. It is 

 the one which includes values between 49 and 50. Outside of this 



