404 THE DEPTHS OF THE SEA. [CHAP. vm. 



First, the surface of the sea that is to say, the 

 upper surface of the Gulf-stream layer is losing 

 heat rapidly by radiation, by contact with a layer of 

 air which is in constant motion and being per- 

 petually cooled by convection, and by the con- 

 version of water into vapour. 1 As this cooling of 

 the Gulf-stream layer takes place principally at the 

 surface, the temperature of the mass is kept pretty 

 uniform by convection. Secondly, the band of con- 

 tact of the lower surface of the Gulf-stream water 

 with the upper surface of the cold indraught. Here 

 the interchange of temperature must be very slow, 

 though that it does take place is shown by the 

 slight depression of the surface isotherms over the 

 principal paths of the indraught. But there is a 

 good deal of intermixture extending through a con- 

 siderable layer. The cold water being beneath, 

 convection in the ordinary sense cannot occur, and 

 interchange of temperature must depend mainly 

 upon conduction and diffusion, causes which in the 

 case of masses of water must be almost secular in 

 their action, and probably to a much greater extent 

 upon mixture produced by local currents and by 

 the tides. The third surface is that of contact be- 

 tween the cold indraught and the bottom of the 

 sea. The temperature of the crust of the earth 

 has been variously calculated at from 4 to 11 C., 

 but it must be completely cooled down by anything 

 like a movement and constant renewal of cold water. 



1 On Deep-sea Climates. The Substance of a Lecture delivered to 

 the Natural Science Class in Queen's College, Belfast, at the close of 

 the Summer Session 1870, by Professor Wyville Thomson. (Nature, \ 

 July 28th, 1870.) 



