THE EARTH. 181 



previously marked it by a ring ot rron. Some time after a dol- 

 phin was caught in the Red sea, and quickly known by the ring 

 to be the same that had been taken in the Mediterranean be- 

 fore. Such, however, as have not been willing to found their 

 opinions upon a story, have attempted to account for the dispo- 

 sal of the waters of the Mediterranean by evaporation. For 

 this purpose they have entered into long calculations upon the 

 extent of its surface, and the quantity of Mater that would be 

 raised from such a surface in a year. 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 Eiixine, 

 evaporate as well as the Mediterranean : and as these are sub- 

 ject 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 unperceived 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 the 

 South Foreland, the tide is known to rini one way at top, and 

 the ebb another way at bottom. This double current he also 

 confirms by an experiment communicated to him by an able sea- 

 man, who being with one of the king's frigates in the Baltic, 

 found he went with his boat into the mid- stream, and was car- 

 ried violently by the current ; upon which a basket was sunk, 

 with a large cannon-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 dowoi, 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 hence we may readily infer, 

 that the same cause may operate at the straits of Gibraltar ; and 

 that while the Mediterranean seems replenishing at top, it may 

 be emptying at bottom. 



The number of the currents at sea are impossible to be recount- 

 ed, nor mdeed are they always known ; new ones are daily pro- 



Q 



