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



penter in a lecture to the Royal Geographical 

 Society, in an illustration drawn from two supposed 

 basins, one under equatorial conditions and the other 

 under polar, connected by a strait, 1 says : "The effect 

 of surface-^ upon the water of the tropical basin 

 will be for the most part limited to its uppermost 

 stratum, and may here be practically disregarded. 

 But the effect of surface-cold upon the water of the 

 polar basin will be to reduce the temperature of its 

 whole mass below the freezing-point of fresh water, 

 the surface stratum sinking as it is cooled, by virtue 

 of its diminished bulk and increased density, and being 

 replaced by water not yet cooled to the same degree. 

 The warmer water w r ill not come up from below, but 

 will be drawn into the basin from the surface of the 

 surrounding area ; and since what is thus drawn 

 away must be supplied from a yet greater distance, 

 the continual cooling of the surface stratum in the 

 polar basin will cause a 'set' of water towards it to 

 be propagated backwards through the whole inter- 

 vening ocean in connection with it, until it reaches 

 the tropical area." And further on in the same 

 address : " It is seen that the application of cold at 

 the surface is precisely equivalent as a moving power 

 to that application of heat at the bottom by which 

 the circulation of water is sustained in every heating 

 apparatus that makes use of it." No doubt the 

 application of cold to the surface of a mass of water 

 previously at the same temperature throughout, would 



i On the Gibraltar Current, the Gulf-stream, and the general 

 Oceanic Circulation. By Dr. W. B. Carpenter, F.R.S. Reprinted 

 from the Proceedings of the Royal Geographical Society of London, 

 1870. 



