derive their velocity, has been ascribed to heat : but a close study of the agents concerned has suggested that an 

 imporlanl — nay, a powerful and active agency in the system of oceanic circulation is derived, through the 

 instrumentality of the winds, of marine plants and animals, from the salts of the sea water. They give the 

 ocean great dynamical force. 



Let us, for the sake of illustration and explanation, suppose the sea in all its parts — in its depths, and at the 

 surface, at the equator and about the poles — to be of one uniform temperature, and to be all of fresh water. In 

 this case, there would be nothing of heat to disturb its equilibrium, and there would be no motive power to beget 

 currents, or to set the water in motion by reason of the difference of specific gravity due to water at different 

 densilies and temperatures. 



As yet we have not taken inio account in this siii)position, the effects of the winds and of evaporation in 

 hejjetlinn; currents. 



Let us thcrefoie take them into account : but first the winds. The winds by their force create partial sur- 

 face currents and agitate the waters to a certain depth, and thus would give rise to a feeble and partial aqueous 

 circulation in the supposed seas of fresh water with a supposed uniform temperature. 



This then is one of the sources w-hence power is given to the system of oceanic circulation ; but though 

 a feeble one, it is one which exists in reality, and, therefore, need not be regarded as hypothetical. 



Let us next call in evaporation and precipitation, that we may examine the effects of another and 

 a more powerful agent. Suppose the evaporation to commence from this fresh water ocean, and to go on 

 MS it does from the seas as they are. In those region'^, as in the trade wind regions, where evaporation is in 

 excess of precipitation, the general level of this supposed sea would be altered, and immediately as much water 

 as is carried off by evaporation would comruence to flow in from North and South towards the trade wind or 

 evaporating region, to restore the level. 



On the other hand, the winds have taken this vapor, borne it off to the extra-tropical regions and 

 precipitated it, we will suppose, where precipitation is in excess of evaporation. Here is another alteration of 

 sea level by elevation instead of by depression ; and hence we have the motive power for a surface current from 

 each pole towards the equator, the object of which is only to supply the demand for evaporation in the trade wind 

 regions — demand for evaporation being taken here to mean the difference between evaporation and precipita- 

 tion, or ihequaniity of water that is taken up into the air and carried, in the form of vapor, to other parts as before, 

 from tiie trade wind region. 



So far, we only derive from evaporation and precipitation over the supposed fresh water sea, a slin^ht 

 Muficc current towards the equator, and, of course, we have the forces for but a partial oceanic circulation.. 

 Till- ni iiive power of such a current would be gravitation, acting upon an inclined plane. 



So lar in the progress of illustration we have apparent counteraction ; for we have, on one hand, the sea- 

 level lowered in the equatorial regions by evaporation, antl raised by the expansive force of heat on the other; 

 we i;ave also the sea-level of the Polar regions raised on one hand by precipitation, and lowered on the other, 

 by the contraction due the diminution of temperature there. But this counteraction is only apparent, for the: 



