On the general Causes of the Ocean-Currents. 7 



I:o) Streams caused bij chaiu/es of température. 



If we for a moment suppose the water of the whole ocean of the 

 same temperature, but that an extensive portion of the surface has just 

 been exposed to the influence of the sohir heat, the water in that loca- 

 Hty would expand and its level would be raised. The new level may- 

 be supposed such as that required by the specific gravity of the warmed stra- 

 tum, without any lateral motion of the warmer water having been necessary. 

 But as the level was raised and the differences of pressure in the upper 

 strata increased, the warm water would presently begin to stream outwards. 

 Its level would thus again sink and the other water's rise, whereby the 

 equilibrium would be disturbed down to the bottom, and the consequence 

 would be, that the water from below would flow to the warmed place 

 till the equilibrium should be restored. 



If the source of heat, that produced the change, ceased to act, 

 both the above described motions, the surface stream and imder-stream, 

 would continue, though with continually decreasing velocity on account 

 of the continually decreasing differences of pressure, untill all strata of 

 the same height attained the same temperature, and the surface accor- 

 dingly become everywhere normal. If on the other hand the source of 

 heat continued to operate with constant power, the water arriving from 

 below would in its turn be warmed and thus the motion be perpetuated. 

 In proportion as the warmed water receded from the warm locality in 

 the form of a superficial stream, it would part with its heat, -and at last 

 such an equilibrium would be established between all the circumstances, 

 that caused the two streams, that the quantity of water, brought in a 

 unit of time by the imder-stream, would be equal to what was carried 

 off by the surface-stream. What position would the warmer water take 

 with respect to the colder, when this state of things came to pass? 



It is clear, that, if the two water-surfaces are at the same level, 

 there can be no difterence of pressure to set the surface-stream in motion, 

 and again, if the warmer stratum assumed the position determined by 

 its specific gravity only, there would be no driving force to produce the 

 under-stream. The position of the warmer stratum must therefore lie between 

 these extremes: it must be such, that both strata are cut through by an 

 imaginary surface, above which the differences of pressure cause an out 

 ward-flowing stream of warm water and beneath it at under-stream of 

 cold. If the proportions were such, that the volumes of water, which 

 the two streams carried onward in the unit of time, had sections of equal 



