404 THE DEPTHS OF THE SEA. [CHAP. VILL 
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.’ 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. 
* 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. (ature, 
July 28th, 1870.) 
