On the general Causes of the Ocean-Cukrents. 41 



l.ii"™ According to observations, wliich I have lately caused to be made 

 on a little island in the Skagerack, in which the evaporation is measu- 

 red in evaporiraeters of portland-cement with surfaces of between 4 and 

 5 square mètres, it amounts there (in 60" N. Tjat.) on an average to 

 5'"™- a day during July and August. *). Hence, even if we do not lay 

 to much weight upon the statements of daily evaporation of 13"™- within 

 tlie tropics, we may nevertheless rest assured, that the evaporation in 

 that extensive and with rain ill supplied region of the ocean, which is 

 swept over by the trade-winds, must produce a far deeper hollow^ than 

 can be filled up bi/ the dilatation of the icater there occasioned during the 

 same time by the solar heat. 



While the amount of evaporation decreases in higher latitudes, 

 that of rainfall increases. Within the ice-covered regions of the Arctic 

 zone the evaporation is ver}- nearly = 0. The precipitation of atmos- 

 pherical moisture however continues to display its activity by depressing 

 the masses of ice beneath the weight of new layers formed upon them, 

 and by the ice-blocks detached from the glaciers. But of the two kinds 

 of motion, which, as I have shown above, rainfall occasions in the ocean, 

 the one is exactly similar to that produced by evaporation, so that rain- 

 fall in hit/her latitudes has just the same effect on the ocean's mollement as 

 evaporation in lou-er. Evaporation in fact disturbs the equilibrium down 

 to the bottom, and thus causes throughout the whole area of the section 

 an afflux to the evaporating region. Rainfall, as long as the fall conti- 

 nues, also disturbs the equilibrium all the way down to the bottom, and 

 causes an efflux from the region, where it falls. The other effect, pro- 

 duced by rainfall, consists in a perturbation of the equilibrium of the 

 superficial strata, which continues long after the rain has ceased to fall, 

 whereby, unless^ the motion be prevented by other causes, a surface- 

 stream of more durable character must be produced, followed by an in- 

 ward flowing current beneath it. 



Already the difference of level between- the equatorial and polar 

 tracts of the ocean, which was caused by evaporation, was much greater 

 than the opposite difference caused by heat. By the effect of rainfall, 

 which may be considered as a complement to evaporation, this level- 



') Of those observations, wliich have originated in the interest t;iken in the 

 question by the Göteborgs Hnshâllnings Society and the Lord-Lieutenant of the di- 

 strict, Count Eheensvârd, I hope hereafter, when they have been continued some 

 time longer, to give a fuller account. 



Nova Acta Reg Soc. So. Ups. Ser. lU. 6 



