288 THE DEPTHS OF THE SEA. [chap. vii. 



of curves which has been used with some slight 

 modijB.cation 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, 

 a field 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 registering thermometer (I'ig. 53) 

 consists of a glass tube bent in the form of a T, 

 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 



