I(> 4 HYDROGRAPHY. 



well as in the shape of rain and snowfall — from being mixed with the salt water below, the Polar- 

 currents at the surface lead constantly fresh water to the regions situated more southerly. The 

 lower stratum, which is salter and warmer, receives an addition of water from the Atlantic. 

 In case of such a transmission of water from warmer sections of the sea not taking place, the aforesaid 

 lower stratum would successively be as cold and get as small a salinity as the upper stratum, which, 

 during its constant motion to the southward, gradually would carry away with it its heat as well 

 as its salt. It will be seen that the temperature of the lower stratum is positive as far as to a depth 

 of 200 (377) to 300 fathoms (565 m.), and from there is decreasing gradually towards the bottom, where 

 it can go down as far as about — 1°. Nothwithstanding the fact that the salinity as a rule is a little 

 lower at the bottom, it varies but little in the lower stratum. We have therefore every reason 

 to believe that the whole of this mighty lower stratum everywhere is due to the 

 same origin, and originates from Ocean-water which has been cooled in the northern 

 latitudes and made its way in under the lighter, cold, and fresh upper stratum. The 

 parts of the Ocean-water, which are subject to the greatest cooling, will have the greatest specific 

 gravity, and in consequence of this make their way quite down to the bottom, where they constitute 

 the lowermost and coldest layers. As furthermore there is every reason to believe that an ample 

 downpour is the consequence of a great cooling of the surface of the sea, it can easily be accounted 

 for, that in consequence of this, the salinity in the lowermost and coldest strata, as a rule, is a little 

 smaller than in the more elevated and warmer strata. 



Speaking about Polar-water, we must distinguish between two different kinds, the upper stratum 

 and the lower stratum; the upper stratum whose distinctive feature is due to the forma- 

 tion and melting of ice, and the lower stratum which hardly can have been in con- 

 tact with or under the direct influence of the ice. There is still a third layer between these 

 two strata, which, as a transitional link between the two others, has a temperature and a salinity which 

 is increasing rapidly with the depth, and evidently has been formed by reciprocity of action between 

 the two other layers, that is to say, by the conduction of heat and diffusion of salt, and perhaps by 

 a process of mixture, when the physical state of matters allowed it. Where no process of mixture is 

 going on, the exchange between the two layers of water must take place very slowly, it can also be 

 seen from the curve of temperature, that frequently the intermediate layer is not very thick. 



The above mentioned qualities, which characterise the water in each of the two principal types, 

 are such reliable and simple marks of distinction, that we never can make a mistake, when at a sta- 

 tion the question is to determine whether the water is to be reckoned as belonging to one or the 

 other of the two main types. On the other hand, the differences within each of the two principal 

 types will often be a mere trifle, so that we are frequently at a loss to decide, whether the differences 

 we have before us are real differences in the temperature and salinity of the water, or whether they 

 are due to observational errors. 



