112 ANNUAL KECORD OF SCIENCE AND INDUSTRY. 



ning to the westward, as the counterpart of the surface cur- 

 rent from the eastward through the straits. This had been 

 suspected for a long time, in view of the fact that a current 

 was continually entering the straits from the Atlantic; it be- 

 ing, of course, readily inferred that this surface current was 

 to restore the level of the Mediterranean, lowered by the im- 

 mense amount of evaporation. In the opinion of some, the 

 effect was simply to concentrate the salt of this inland sea 

 and cause it to saturate the lower strata, and perhaps even 

 to form solid beds of salt at the bottom. This supposition, 

 however, can easily be proved to be untenable. The method 

 adopted by the Porcupine party to show the existence of an 

 outward under-current consisted in the use of what was call- 

 ed the " current drag," an apparatus so constructed as to pre- 

 sent a resisting surface so much larger than that of the boat 

 from which it was suspended, that although the latter might 

 tend to move in the direction of the surface current, this 

 would be counteracted by the action of the under-current 

 upon the " drag." In some instances the effect was simply 

 to retard the velocity of the surface movement, but in oth- 

 ers the boat was actually carried against the surface current 

 by that bf a lower depth. 



A chemical examination of the water brought up from great 

 depths in the Mediterranean proved, as suspected, that the 

 deep-sea water was more salt than that at the surface, and 

 that, consequently, the tendency to saturation existed, but 

 nothing could be found to show the existence of a bed of salt 

 at the bottom; and strata of water of less density were met 

 with below those of greater density. It will, therefore, be 

 readily understood that the outward current in large part car- 

 ries with it the excess of salt produced by the surface evapo- 

 ration referred to. The cause of the circulation itself is due, 

 according to Dr. Carpenter, to purely hydrostatic action, which 

 he explains as follows : The water of the Mediterranean is 

 continually losing by evaporation a larger amount than is re- 

 turned to it by rain or rivers, and consequently the inflow 

 from the Atlantic must take place to keep up this level. If 

 this inflow consisted of fresh water, the total quantity of salt 

 in the Mediterranean would remain the same, and the densi- 

 ty would therefore undergo no increase. But as the upper 

 current of salt water brings in a certain quantity of salt, in 



