20 SUBMARINE CABLE LAYING AND REPAIBING. 



used together, and the true bottom temperature aa well as the 

 true depth are thereby obtained. From the illustrations of 

 these instruments (kindly lent the author by the manufacturer, 

 Mr. Louis P. Casella) it will be seen that both are of the same 

 construction, except that the mercury piezometer has a bent 

 tube to prevent the mercury flowing down the stem. The 

 stem of each instrument dips into a bulb partially filled with 

 mercury and is rather a loose fit in the neck of the bulb. A 

 wide rubber band is used to bind the bulb and stem together, 

 but in order that water may gain access to the mercury in the 

 bulb (to exert its pressure in the same way as air upon the 

 mercury in an ordinary barometer), a glass pin is pushed in 

 the band between the stem and the bulb, as shown in the 

 figures. The apertures left on either side of the pin sufl&ce 

 for allowing the pressure to act, but are so small that it is 

 practically impossible for the mercury to escape. In the water 

 piezometer the cylindrical bulb at the top, and the stem, are 

 nearly filled with distilled water, the junction between the 

 water and mercury in the stem being arranged at a convenient 

 height at ordinary temperature. At the junction of the two 

 liquids there is a magnetic index, as in Six's thermometer, held 

 in the tube by a small piece of human hair, acting as a spring. 

 This registers the maximum range of pressure and temperature 

 to which the instrument has been subjected when let down to 

 the bottom of the sea and drawn up again on the sounding line. 

 As the tube descends the pressure of sea-water on the 

 mercury in the lower bulb increases at the rate of 2681b. per 

 square inch (or about 18 -2 atmospheres) for every 100 fathoms 

 depth. This forces up the water in the stem, and contracts it 

 in volume. The mass of water in the upper bulb is also con- 

 tracted by the decrease in temperature, so that the position of 

 the index registers the sum of these two eflfects. How much 

 of the reading is due to pressure, and how much to tem- 

 perature, can be found by sinking a thermometer at the 

 same time as the piezometer. Then, deducting the contraction 

 of the piezometer column due to the temperature shown on the 

 thermometer, we have that due to depth alone. The apparent 

 contraction of the water (that is in the glass, the glass also 

 being contracted in volume) per degree of temperature and per 

 100 fathoms depth was determined by Mr. Buchanan for the 



