164 Use of the Piezometer in Deep-Sea Sounding. [June 15^ 



of line which has so run out will give the depth approximately, but more 

 or less in excess of the truth according to circumstances. Allowing for 

 the contraction which would be produced by this depth in the case of 

 the mercury piezometer, a first approximation to the temperature of the 

 bottom-water is at once obtained ; and it is sufficiently accurate for the 

 purpose of correctly determining the contraction produced on the water 

 piezometer by the change of temperature, and consequently for deducing 

 the depth to which the instrument has been sunk. By now applying the 

 more correct depth to the reading of the mercury instrument, we obtain 

 the correct temperature, and if necessary the approximation might be 

 carried still closer. 



As an example of the use of the combined instruments, the observa- 

 tions made on the 29th February, 1876, may be taken. The position of 

 the sounding was lat. 36° 9' S., long. 48° 22' W., and the depth by line 

 was 2800 fathoms. The sea was quite calm, but there was a strong cur- 

 rent setting to the south-east, rendering it probable that the depth, as 

 determined by line, was considerably in excess of the true depth. The 

 mercury instrument (XYII. a) registered 166'2 millims. In order to 

 clear this reading for a depth of 2800 fathoms, we have to subtract 16 

 millims., and we obtain 150-2 millims. as the corrected reading, from which 

 we determine the temperature to be + 0°"2 C. The reading of the water 

 instrument was 283*8 millims. Assuming the temperature to have been 

 0°-2 C, this would indicate that the water had suffered an apparent con- 

 traction, owiug to pressure alone, of 0*1923 c. c, which w^ould be pro- 

 duced by a column of 2480 fathoms of sea-water. Assuming now 2480 

 fathoms to be the true depth, we find the corrected reading of the mer- 

 cury instrument (XVII. a) to be 152*1 millims., which indicates a tem- 

 perature of — 0°*5 C. The Millar-Casella thermometers gave the tempera- 

 ture as — 0°*4. Assuming this as the correct bottom temperature, and 

 reducing the reading of the water instrument (C. No. 1) accordingly, we 

 find the contraction produced by pressure to be 0*1924 c.c, which agrees 

 sensibly with that found on the assumption of the higher bottom-tem- 

 perature of +0°*2 C. 



It will thus be seen that the two instruments fulfil the conditions re- 

 quired of them; namely, that the one which is to indicate the temperature 

 of the M^ater shall be independent of great accuracy in the determination 

 of the depth, and the one which is to indicate the depth shall be equally 

 independent of accurate determination of the temperature; whilst by 

 combining the results obtained by the two, an accurate determination is 

 obtained both of the depth and of the temperature of the water at that 

 depth. 



