194 



Messrs. Carpenter and Jeffreys on 



[Dec. 8, 



it, a progressive reduction was observable, from 74°, which had been the 

 average of several previous days, to 72°, with a further reduction to 71° 

 when we entered Gibraltar Harbour. The scorching power of the Sun's rays 

 was often very strongly felt ; and we much regretted that we were not provi- 

 ded with a Thermometer having a range sufficiently high to enable us to esti- 

 mate the influence of direct solar radiation * . There can be no question that 

 the effect of this radiation upon the surface must be to produce a rapid eva- 

 poration, especially when the air is dry. The difference between the Dry- 

 and "Wet-bulb thermometers averaged about 4°, but rose occasionally to 

 above 8° ; we could not, however, trace any relation between this difference 

 and the Surface-temperature of the Sea. 



87. Temperature of the Upper Stratum. — Finding that the reduction in 

 Temperature with depth was so extremely rapid as to show that the direct 

 influence of Solar radiation is limited to a comparatively thin stratum 

 of surface-water, we took Serial soundings at three Stations, at intervals 

 near enough to show the rate of diminution. The first of these Stations 

 (Stat. 40), although the nearest to the Strait, seems to have been out of the 

 direct influence of its cold in-current, which is shown very strongly in the 

 second (Stat. 47) ; the third (Stat. 53) may perhaps be taken as representing 

 most characteristically the thermal condition of the upper stratum of the 

 water of the Mediterranean during the season of greatest heat : — 



I. II. III. 



Fahr. Diff. ° Fahr. Diff. Fahr. Diff. 



Surface ....... 



74-5 







10 „ ... 



69-3 



20 „ ... 



65-0 



30 „ ... 



63-0 



40 „ ... 



. .. 61-7 



50 „ ... 



59-7 



100 „ .... 



... 55-1 



77-0) 

 76-0\6'0 



61-5f y 



60 '°|27 

 \ 0-6 



55-5' 1 Z 



Taking No. III., therefore, as the standard of comparison, we ob- 

 serve that while the Thermometer fell only 1° in the first five fathoms, it 

 fell 5° in the second five, and no less than 9°*5 between 10 and 20 fathoms, 

 below which depth the reduction was very slow. In No. I., with a lower 

 surface-temperature, the reduction in the first ten fathoms was nearly the 

 same ; but it was much less between 10 and 20, so that for depths between 

 20 and 50 it was between 3° and 4° higher than at corresponding depths 

 in No. III.; at 100 fathoms, however, the two were brought to an almost 

 precise accordance by the larger reduction which took place in No. I. 

 between 50 and 100 fathoms. In No. II., on the other hand, the great 



* We learned from Colonel Playfair, the Consul General at Algiers, that whilst he 

 was at Aden, a Thermometer with a blackened bulb having been laid upon a black 

 surface, and exposed to the full glare of the Sun, had risen to 215°. 



