On the general Causes of the Ocean-Currents. 39 



is there set free when the evaporated vapour is condensed, is again in 

 part communicated to the sea by the winds, or, if one prefer to say so, 

 serves to reduce the loss of heat caused by evaporation. In conse- 

 quence of the rapid changes of the atmospheric strata over the ocean- 

 surface the quantity of warmth, thus afforded by the winds, ought not 

 to be destitute of effect. In other regions again the surface of the sea 

 may from an analogous cause suffer a loss of heat. Lastly the conver- 

 sion of .notion into heat has in various ways an effect on the sea. The 

 streams, that circulate there, are continually resisted in their motion by 

 resistance of many kinds. Thus the whole of the very considerable 

 force, required to over-come this resistance, is converted into warmth, 

 as also on the other hand every raindrop, that falls upon the surface, 

 must there set free a certain amount of heat, when its falling velocity 

 is stopped. The vis viva, which the winds contain, sets the ocean into 

 an undulatory motion and on their cessation there remains, as a conse- 

 quence of this motion, a certain amount of heat, which the ocean has 

 acquired, unless it should merely have served to compensate the loss of 

 heat through evaporation. 



Again to determine the depressum of level caused by ei^npnration _ 

 two methods are available: either to endeavour to determine it bi/ direct 

 observation, or to calculate it from meteorolugical observations of the locality, 

 havhig first fully ascertained the lain of evaporation. The first process has 

 been tried, but the observations are, it is known, so few and conducted 

 in such a manner, that the important question of evaporation from the 

 ocean's surface must as yet be considered as unanswered. 



Tlie clianye of level^ resulting from rainfall, would at first sight 

 appear relatively eas}^ to be determined. But as in this case the water 

 received from rivers and from the melting of ice, produced by the snow- 

 fall in the polar regions, must be taken into account, this question also 

 becomes difficult. We thus find, that no one of these three principal 

 questions can in the present state of science be answered, without leavhig 

 a very large margin of uncertainty. An especial hindrance in this case 

 is our want of accurate knowledge as to the amount of evaporation, a 

 quantity, which would be so necessary to know both for its own sake 

 and for the calculation of the w^arming and cooling of the ocean sur- 

 face. We have accordingly no other alternative, than to attempt an 

 approximate estimation of the said quantities' greatest or least value and 

 to see what conclusions may be drawn from them. 



