104 



THE VOYAGE OF H.M.S. CHALLENGER. 



I 



1 



of water as can be contained in the graduated tube increases very materially the 

 amount of contraction produced by pressure. The instrument which was chiefly 

 used contained 256 - 61 grammes of mercury in the bulb and stem immediately above 

 it; the volume of the part of the stem filled with water was 0'1935 c.c. The 

 apparent contraction of this mass of mercury and water was 0'000581 c.c. per 100 

 fathoms, and 0^0025 c.c. per degree (C.) respectively. A fall therefore 

 of one degree (C.) in temperature produced the same effect as an increase 

 of pressure equal to 430 fathoms of sea water. Hence (and this forms 

 the important peculiarity of the instrument) as long as the temperature 

 of the sea does not increase with the depth at a greater rate than 1° C. 

 per 430 fathoms, the instrument will record the temperature correctly. 

 The ratio subsisting between the rise or fall of temperature and the 

 column of water, which produced the same effect on the apparent 

 volume, is a constant for each instrument ; in this it is -^jj. By 

 altering only very slightly the amount of water, the sensibility to 

 pressure is greatly increased or diminished, while that to temperature 

 remains practically unchanged. As the instrument described was 

 intended principally for bottom waters, the above 

 ratio (flu) was considered sufficient, and it proved 

 practically useful. It must be remembered that 

 the greater the value of this ratio is made, the 

 greater is the error introduced into the determina- 

 tion of the temperature by any inaccuracy in the 

 measurement of the depth. 



This instrument was constructed entirely at 

 sea, and though the chemical laboratory was 

 specially lucky in its freedom from breakage, 

 there seemed to be an exception in the case 

 of the mercury piezometer. Four of them had 

 to be made before one stood. The first was 

 broken by accident, the second by a foul on 

 the sounding line, the third in the receiver 

 of the hydraulic apparatus by the collapse of 

 a protected thermometer, which was being ex- 

 and the fourth stood, being used as often as 

 with other instruments until the ship 



Fig. 33. — Mercury 

 Piezometer. 



Fig. 34.— Enlarged view, 

 showing attachment of 

 ljulb to end of Piezo- 

 meter. 



along 



sounding 



with it, 

 line 



along 



posed to pressure 

 possible on the 



returned home, -when it was broken in its turn by an over-curious instru- 

 ment-maker who was employed to copy it. The filling of the instrument 

 with so large a quantity of mercury was effected by means of an improvised 



