164 



Messrs. Carpenter and Jeffreys on 



[Dec. 8, 



of the water of the Atlantic ; whilst the water at 250 fathoms agreed equally 

 well with what proved to be the condition of the bottom water in the adja- 

 cent part of the Mediterranean (§ 43). We were not a little surprised, 

 however, to find that the water here taken from the bottom (517 fathoms) 

 was of much less density, as indicated both by Specific Gravity and by 

 Chlorine percentage, than that of the intermediate stratum ; its Specific 

 Gravity being 1*0281, and its proportion of Chlorine 21*465. This ap- 

 parent anomaly (the existence of which was confirmed by observations 

 made on our return voyage, § 61) pointed to the existence of an out-cm- 

 rent in the intermediate stratum as the probable explanation of the over- 

 laying of the lighter by the heavier water. The Specific Gravity of the 

 bottom-stmtvLva closely corresponded, as we subsequently found, with that 

 of the bottom-water over the deepest part of the area of the Western basin 

 of the Mediterranean (§93). 



35. These data having been obtained by the examination of the several 

 parts of the vertical column at one and the same point, and this point 

 being in the centre of nearly the narrowest part of the Strait, and at the 

 deepest part of the channel, we proceeded to test the actual movement of 

 water on the surface and at different depths beneath it. 



36. The rate of surface- movement was easily determined. The precise 

 position of the Ship having been ascertained in the manner already stated, a 

 small flat basket, presenting no such elevation above the water as would 

 cause it to be influenced in any considerable degree by a moderate wind*, 

 was sent adrift, so as to be freely carried along by the current ; it was 

 allowed to float for a determinate time, throughout which it was followed 

 by the ship ; and when it was taken up at the expiration of that time, the 

 place of the ship was again ascertained as before. The space between the 

 two points being then determined trigonometrically, the rate of the flow 

 per hour, and its precise direction, could be readily calculated. Thus on 

 the morning of August 15th the float was followed by the ship for fifteen 

 minutes, during which it was found to have moved 4377 feet in the direc- 

 tion E. by S. | S., or at the rate of 2-88 miles per hour (§ 40). 



37. For the determination of the movement of the water at different 

 depths below the surface, a " current- drag " (see figure, p. 165) had been 

 constructed by Capt. Calver on a plan suggested by his previous experience, 

 which had led him to the conclusion that a submerged basket lined with 

 sailcloth, which of course fills itself with water, presents a better resisting 

 surface than any vessel of wood or metal. Such a basket being made 

 the basis (so to speak) of the apparatus, its resisting surface was aug- 

 mented by fixing two pairs of arms at right angles to one another across 



* It is obvious that the movement of the Ship itself would be Liable to be consider- 

 ably affected by even a slight breeze, on account of the large surface of resistance pre- 

 sented by its transverse section (especially by its paddle-boxes) above the water. This 

 would cause its drift to be more rapid than the current, if the direction of the wind 

 should be with that of the current, and less rapid if the wind should be opposed to it. 



