1870.] 



Deep-sea Researches. 



193 



from the Polar stream, that of the Surface at 52°*5 being 20*17. Pos- 

 sibly, as suggested by Dr. Forchhammer (Joe. cit.), the several parts of the 

 Polar stream may vary in density, according to the amount of nearly fresh 

 water which each may have derived from the icebergs that have liquefied 

 in it. 



85. Specific Gravity. — As the determination of Specific Gravity by obser- 

 vations taken with the Hydrometer on board ship is open to two sources of 

 error, — that of the instrument, and that of the reading (which, when the 

 vessel is unsteady, cannot be precise), — we deem it safer to depend upon 

 the more exact determination of the Specific Gravity of a smaller number 

 of samples by means of the Balance, and to estimate that of others by the 

 Chlorine-determinations. In this manner we arrive at a range of from 

 1'02G1, the Specific Gravity of the sample of Bottom-water of minimum 

 density, to 1*0269, the Specific Gravity of the sample of Surface-water of 

 maximum density; the average of all being 1*0265. This agrees very well 

 with the results obtained by Forchhammer. 



Temperature and Composition of Mediterranean Water. 



86. Surface -Temperature. — With only two days' exception, the range of 

 the daily average Surface-temperature of the Mediterranean, between the 

 16th August on which we entered it, and the 28th September on which we 

 quitted it, was between 73° and 79°. The increase at once experienced 

 as we passed into it from Gibraltar Strait was extremely marked (§ 45); 

 and this was maintained for the next two days. On the 19th, however, 

 the average of the day fell from 72°* 2 to 66 0, 9, the average of the Air 

 being 69°'8 ; and on the 20th it was 68°*9, the average of the Air being 

 74°*3. On the first of these days we were crossing from the African 

 towards the Spanish coast, and experienced very strongly the effect of the 

 in-current on the movement of our vessel (§ 47); and it can scarcely 

 be doubted that the low surface-temperature was due to the colder 

 stratum introduced from the Atlantic. On the following day we were 

 between Cape de Gat and Cartagena ; and were still within the influence 

 of the cold in-current, which is known to flow past Cape de Gat at 

 the rate of about a mile per hour. On leaving Cartagena, we 

 came into a surface-stratum of true Mediterranean water, as indicated 

 by its temperature of 73° ; and the daily average never afterwards fell 

 below this. The greatest heat was experienced in the neighbourhood of 

 the Tunisian coast, when for several days the average Surface-temperature 

 was 78°. The average Temperature of the Air during the greater part of 

 our Mediterranean Cruise was from 1° to 2° above that of the Sea ; but 

 during our return from Malta towards Gibraltar, between the 20th and 26th 

 September, the temperature of the Air averaged about 3°*5 below that of 

 the surface of the Sea, the former having fallen, while the latter remained 

 nearly stationary until we neared the Strait of Gibraltar. As we approached 



VOL. XIX. R 



