THE DEEP SEA AND ITS CONTENTS. 339 



tenance of the glacial temperature, not only of the sea-bottom, 

 but of the great mass of the water contained in the vast oceanic 

 basin, cannot be maintained without a continual indraught of the 

 upper stratum towards the Poles ; this, as its temperature is pro- 

 gressively lowered, decreases in volume and increases in specific 

 gravity ; and as the lower stratum flows away under the excess of 

 pressure, the upper stratum, now cooled down nearly to the 

 freezing-point of salt water, will sink into its place, making way 

 for a new indraught above. The two Polar underflows, on the 

 other hand, meeting at or near the Equator, will there tend to rise 

 towards the surface, replacing the water which has been draughted 

 away towards either Pole; and thus a constant "vertical circulation " 

 must be kept up by opposition of temperature alone, analogous to 

 that which takes place in the pipes of the hot-water apparatus by 

 which large buildings are now commonly warmed. The only 

 essential difference between the two cases is, that whilst the 

 primum inobite in the latter is the heat applied to the bottom of 

 the boiler, making the warmed water ascend by the reduction of its 

 specific gravity due to its expansion, the moving power in the 

 former is the cold applied to the surface of the Polar water, making 

 it descend by the increase of specific gravity due to the diminution 

 in its bulk as its temperature is lowered. 



This doctrine was first distinctly promulgated nearly forty years 

 ago by the eminent physicist Lenz, on the basis of the temperature- 

 observations he had made in Kotzebue's second voyage more than 

 ten years previously ; these having satisfied him of two facts — first, 

 the general diffusion of a glacial temperature over the ocean-bottom, 

 which he rightly interpreted as dependent on an underflow of 

 Polar water ; and, second, the near approach of cold water to the 

 surface under the Equator than either on the north or on the 

 south of it, which he considered to indicate an uprising of that 

 Polar water from below, where the two underflows meet. But, 

 though accepted by Pouillet and other distinguished physicists, 

 this doctrine, with the observations by which it was supported 

 was entirely lost sight of until independently advanced by myself 

 as the only feasible explanation of the Poleward movement of 

 the whole upper stratum of North Atlantic water, and of the 

 southward outflow of glacial water from the Arctic basin, of which 



