NARRATIVE OP THE CRUISE. 107 



volume of the stem. The capacity of F is equal to the contractions due to the fall of 

 temperature and the increase of pressure produced by the first 1000 or 1500 fathoms of 

 depth, so that the instrument would only register depths greater than 1000 or 1500 fathoms, 

 but it would do so with almost as much precision as can be obtained at less depths. 



The observations which have been taken as a basis for determinations of depth were 

 made in the latter part of the year 1875, in the South Pacific Ocean. They were twenty 

 in number, and were made at depths varying from 500 to 2300 fathoms, and at 

 temperatures varying from l°-4 to 4° -03 C. The mean compressibility of water 

 determined from these observations was 0*0008986 per 100 fathoms of sea water, the 

 extreme values being 0'000915 and - 000882. Observations made at greater depths in 

 the North Pacific, gave as a mean of six observations at depths varying from 2740 to 

 3125 fathoms the value - 000878, indicating a slight diminution in the coefficient of 

 compression at very high pressures. 



The change of volume of water with change of temperature at the low temperatures 

 found in the deep sea is very slight. The change of volume of mercury, however, for 

 all ordinary temperatures is very considerable. On the other hand, the compressibility 

 of water, or its sensibility to change of volume with change of pressure is very great, 

 whereas that of mercury is very small. Consequently, by sending a pair of these 

 instruments down on the sounding line, and reading them when they come up, two 

 independent values of the sum of the effects of change of temperature and of pressure are 

 obtained. Taking as the first approximation to the depth the length of the sounding line, 

 applying it to the reading of the mercury instrument, and so correcting it for pressure, 

 we have a first approximation to the temperature ; applying this temperature to the 

 reading of the water piezometer, we obtain a second approximation to the depth, indeed, 

 practically the true depth. The reading of the mercury piezometer now being corrected 

 for pressure by this value of the true depth, we have a second approximation to the 

 temperature. In fact we have now practically the true depth and the true temperature. 



Fig. 36« refers to the water piezometer, and fig. 366 to the mercury piezometer; the 

 thick lines represent the apparent changes of volume for changes of pressure, and the 

 dotted lines the apparent changes of volume for changes of temperature. Distances 

 measured along the horizontal line of abscissae represent depths on the scale of 0"01 

 inch to a fathom, and temperatures on the scale of 0'1 inch to a degree centigrade. 

 Distances measured along the line of ordinates represent scale divisions (millimetres) on 

 the scale of 0"1 inch to a division. For 100 fathoms of depth the apparent contraction 

 of the mercury instrument was 0*7 millimetre on the stem ; in the water instrument the 

 apparent contraction for 100 fathoms was somewhat over 7 '8 millimetres. Considering that 

 the effect of a change of temperature of 1° C. causes an apparent change of volume in the 

 mercury piezometer represented by about 2*5 millimetres, while in the water piezometer at 

 the low temperature always found in the deep sea the temperature may be anything 



