THE EARTH. 227 



They then compute how much water runs in by 

 its rivers and straits in that time ; and find, that the 

 quantity exhausted by evaporation greatly exceeds 

 the quantity supplied by rivers and seas. This 

 solution, no' doubt, would be satisfactory, did not 

 the ocean and the Euxine evaporate as well as 

 the Mediterranean : and as these are subject to 

 the same drain, it must follow, that all the seas 

 will in this respect be upon a par ; and therefore 

 there must be some other cause for this unper- 

 ceived drain, and continual supply. This seems 

 to be satisfactorily enough accounted for by Dr 

 Smith, who supposes an under current, running 

 through the straits of Gibraltar, to carry out as 

 much water into the ocean, as the upper current 

 continually carries in from it. To confirm this, 

 he observes, that nearer home, between the north 

 and south foreland, the tide is known to run one 

 way at top, and the ebb another way at bottom. 

 This double current he also confirms by an expe- 

 riment communicated to him by an able seaman, 

 who being with one of the king's frigates in the 

 Baltic, found he went with his boat into the mid- 

 stream, and was carried violently by the current ; 

 upon which a basket was sunk, with a large can- 

 non-ball, to a certain depth of water, which gave 

 a check to the boat's motion ; as the basket sunk 

 still lower, the boat was driven, by the force of the 

 water below, against the upper current ; and the 

 lower the basket was let down, the stronger the 

 under current was found, and the quicker was 

 the boat's motion against the upper stream, which 

 seemed not to be above four fathom deep. From 



