288 THE DEPTHS OF THE SEA. [CHAP. VII. 
of curves which has been used with some slight 
modification in the construction of this chart. 
Until very recently little or nothing has been 
_ known with any certainty about the temperature of 
the sea at depths below the surface. This is, however, 
afield of inquiry of very great importance in Physical 
Geography, as an accurate determination of the tem- 
perature at different depths is certainly the_best, 
frequently the only available means of determining 
the depth, width, direction, and generally the path of 
the warm ocean currents, which are the chief agents 
in the diffusion of equatorial heat; and more espe- 
cially of those deeper indraughts of frigid water 
which return to supply their place and to com- 
plete the general cycle of oceanic circulation. The 
main cause of this want of accurate knowledge of 
deep-sea temperatures is undoubtedly the defective- 
ness of the instruments which have been hitherto 
employed. 
The thermometer which has been almost universally 
used for this purpose is the ordinary self-registering 
thermometer on Six’s construction, enclosed in a 
strong copper case, with valves or apertures below 
and above to allow a free current of water to pass 
through the case and over the surface of the 
instrument. Six’s registermg thermometer (Fig. 53) 
consists of a glass tube bent in the form of a V, 
one limb terminating in a large cylindrical bulb, 
entirely filled with a mixture of creosote and water. 
The bend of the tube contains a column of mercury, 
and the other limb ends in a small bulb partially 
filled with creosote and water, but with a large 
space empty, or rather containing the vapour of the 
