NARRATIVE OF THE CRUISE. 



105 



Sprengel pump, which acted quite satisfactorily, even when the ship was rolling 

 steadily through 20° to 30°. 



The apparent compressibility of the mixture of mercury and water in glass represented 

 by this instrument was determined in a number of satisfactory soundings between 

 Tahiti and Valparaiso, the temperature being determined by one or 

 more protected thermometers attached to the line close to the piezo- 

 meter. Of course no "pressure correction" was applied to the 

 readings of these thermometers. The result was, that the apparent com- 

 pressibility of mercury for 100 fathoms was 0'0000271, being per atmo- 

 sphere 0"0000015. After the return of the Expedition, the absolute 

 compressibility of glass was directly measured by Mr. Buchanan 1 

 in a specially designed apparatus, and found to be "0000029 2 per 

 atmosphere up to 240 atmospheres, at a temperature of 12° to 13° C. 

 The absolute compressibility of mercury would therefore be "00000442 

 per atmosphere. 



The water piezometer has already been referred to. It is shown 

 in fig. 35. It consists essentially of a thermometer-shaped instrument 

 open at the end. A cylindrical bulb A contained, in the one that was 

 chiefly used, about 9 c.c. The stem, which was rather more than a 

 foot long, had a diameter of almost exactly one millimetre The 

 end of the stem dips into the bulb D, which was filled so far with 

 mercury, and the instrument was set by heating it to such a temperature, 

 that when it cooled down to the atmospheric temperature the mercury 

 would rise to a convenient height so as to be visible and able 

 to be read at any moment at a given temperature by plunging it into 

 water. The arrangement for protecting the open end of the instrument 

 is somewhat peculiar. It is necessary to allow the water on the outside 

 to have access to the mercury in the bulb in order that the pressure may 

 exert itself in the interior of the instrument, in the same way as air 

 must have access to the mercury in the reservoir of the barometer. At 

 the same time it is of importance that the mercury should not be able 

 to come out of the bulb. For this purpose care was taken to have 

 a bulb D blown, into the neck of which the stem of the instrument 

 fitted with some accuracy. This was connected with the stem by 

 means of a piece of india-rubber, which was prevented from fastening hermetically 

 on the stem by having a small piece of glass rod H pushed in between the india- 

 rubber and the stem. In this way communication was constantly kept open between 

 the outer water and the mercury in the bulb. 



1 Trans. Roy. Soc. Edin., vol. xxix. p. 589, 1880. 

 (nakb. chall. exp. — vol i. — 1884.) 14 



Fig. 35.— Water 

 Piezometer. 



