1876.] Use of the Piezometer in Veep-Sea Sounding. 161 



XVII. ^' Preliminary Note on the Use of the Piezometer inDeep- 

 Sea Sounding/-* By J. Y. Buchanan, Chemist to the Chal- 

 lenger^ Expedition. Communicated by Professor Williamson, 

 Foreign Secretary B.S, Beceived June 14, 1876. 



In order to determine the depth o£ the sea independently o£ the leugth 

 of sounding-line used, piezometers filled with distilled water were fre- 

 quently attached to the line along with the deep-sea thermometers. The 

 combined effects of change of temperature and change of pressure were 

 registered by a steel index of ordinary form. The temperature of the 

 bottom-water being given by the deep-sea thermometer, the effect of 

 temperature on the apparent volume of water in the piezometer could 

 easily be calculated ; and from the residual effect, the pressure, and 

 therefore the height, of the column of water to which the instrument 

 had been subjected could be deduced. 



The piezometer did not differ materially from the ordinary ones used 

 for the determination of the compressibility of liquids. A minute descrip- 

 tion of the fittings necessary for their safe use on the sounding-line 

 cannot be given without reference to a drawing or model, and must there- 

 fore be postponed. 



It is manifest that if the apparent compressibility of water is accu- 

 rately known, we shall be in a position to determine, by means of our 

 instrument and a deep-sea thermometer, the depth to which it has been 

 sunk, independently of the lengths of sounding-line used ; for the indica- 

 tions of the instrument depend solely on the temperature of the water at 

 the depth in question, and on its vertical distance from the surface. 



The determination of the effect of change of temperature on such an 

 instrument does not demand explanation. It is, however, otherwise with 

 the effects of pressure. In submitting an instrument of the kind to high 

 pressures in an hydraulic machine, we encounter difficulty in accurately 

 determining the pressure to which it is exposed, and also, although in a 

 minor degree, in making our observations at the low temperature usually 

 obtaining in deep ocean waters. I have therefore taken as basis for the 

 determination of the apparent compressibility of water the results ob- 

 tained when the instrument has been sent down on the sounding-line, 

 either to the bottom or to intermediate depths, in positions where there 

 has been no apparent disturbance from currents, and where the amounts 

 of compression produced have been proportional to the depths recorded 

 by the sounding-lines. "Where currents are absent, and their presence is 

 at once detected by the behaviour of the sounding-line, the depth, as 

 determined according to the method of sounding adopted on board the 

 ' Challenger,' gives an excellent measure of the pressure exercised on the 

 instruments. As the variations in the temperature, the salinity, and the 

 compressibility of sea-water with the depth have been thoroughly inves- 



