HYDROGRAPHY. l2 n 



however, too few to give us a true picture of the boundary-plane, in consequence of which the iso- 

 therms from the warm Atlantic-water, which are reaching to the northward of station 4, have been 

 drawn rather arbitrarily. 



The sections XVIII, XIX, XX and XXI have it in common with one another that they are 

 lying in the longitudinal direction of the East Icelandic Polar Current. The o° isotherms in the under- 

 layer have all a minimum point in each of the sections, that is to say, a point at which the isotherm 

 removes to the greatest distance from the surface, in order to rise again in a southerly direction. 

 This state of matters may be accounted for by the fact of the minimum-point lying at the boundaries 

 of the current. 



Another assumption may, however, be made to account for this. It will be seen by a look at 

 the chart, that the o° isotherm is going upwards just where the bottom-curve is rising, and in conse- 

 quence of this, it is not unlikely to suppose that there is a co-incidence between these two circum- 

 stances. Taking for granted that the under-layer of the Polar Current is moving in a southerly 

 direction, there may be a possibility of an upward going motion taking place in the water, when it 

 is forced to slide upwards on the sloping bottom. This would of course have the effect of producing 

 a rising to the southward of the o° isotherm, but the consequence of such a motion would be, however, 

 that the ice-water also was lifted upwards, and on account of this made to flow out at the sides, by 

 which it only would come to consist of a thinner layer. It can be seen from section XXI, station 59, 

 that this it not the case. The ice-water has just at this station, such as shown by the 35*00 isohaline, 

 its greatest thickness. It cannot therefore be the motion of the water in the under-layer, or regular 

 vertical currents that give the o° isotherm its characteristical shape; it is, on the contrary, deter- 

 mined by the position of the section in the current, and there is no reason to suppose 

 that there should be any regular vertical currents here. 



All the stations of the expedition that are lying north and east of Iceland are, as it appears 

 from the foregoing, situated in the Polar Current, and we have pointed out above what is the influence 

 of this latter on the water that is lying underneath it, the under-layer. As this latter, throughout, has 

 the same salinity, it must, in order to be in equilibrium, have higher and higher temperatures from 

 the bottom towards it uppermost boundary plane. As the ice-water that is lying on top of the under- 

 layer is cooling this latter from above, a maximum of temperature will be brought about, 

 and the higher the temperature of the uppermost part of the under-layer has been, 

 and the less the cooling influence of the ice-water has manifested itself, the higher 

 the aforesaid maximum of temperature will be. 



We have therefore in the greatness of the maximum temperature a measure for the influence of 

 the combination of these two powers. The longer the ice-water has been in contact with the under- 

 layer, the deeper the cooling influence of the ice-water may be traced, and the deeper the maximum 

 temperature will come to lie. In concequence of this, the depth at which the maximum 



