1870.] 



Deep-sea Researches. 



187 



of the Air : at 4 p.m. it was 73°*5 ; at 6 p.m., when we were passing 

 Cape St. Vincent, it was 72°*5 ; and at 8 p.m., when we were fairly in the 

 Atlantic, it was at 69°. The average of the next two days, while we were 

 proceeding steadily Northwards, was maintained at nearly the same point ; 

 and the surface-temperature of the Sea was now pretty constantly lower 

 than that of the Air. 



76. The most probable explanation of this excess seems to lie in 

 the fact that, besides the mid-current almost invariably setting eastwards, 

 there are two lateral streams, of which the direction is sometimes reversed 

 under tidal influences (§ 106) ; and that warmer water from the Mediter- 

 ranean basin thus finds its way outwards, chiefly along the Spanish shore. 

 — That the surface mdraught is greatest on the African side of the Strait, 

 and that the surface outworn is greatest on the Spanish side, is, we under- 

 stand, a fact well known to those who are in the habit of navigating it, 

 though we do not find it mentioned in the * Sailing Directions ; ' and this is 

 just what our observations of surface-temperature in the embouchure of the 

 Strait would lead us to infer. 



77. It is a circumstance worthy of remark, that an abnormally low tem- 

 perature showed itself during the whole of our first stay in Gibraltar Bay, 

 from Aug. 7 to Aug. 14 ; the surface-temperature of the Water ranging 

 between 64°*1 and 65°'6, — being apparently that of the mid-strait to the 

 eastward, — whilst the temperature of the Air ranged between 72°* 9 and 

 77°' 2, the greatest difference between the two being 12°*5. During 

 most of this time the wind was easterly, and the wet-bulb thermometer 

 ranged from 4°'l to 8°*2 below the dry; but the increased evaporation 

 that would result from the atmospheric condition thus indicated, could 

 scarcely have produced the marked depression observable in the surface- 

 temperature of the water; more especially as our general experience was that 

 the surface-temperature in the comparatively shallow water of a Harbour 

 was rather higher than in the deeper sea outside. — No such depression 

 presented itself on our return voyage. On approaching Gibraltar from the 

 Mediterranean side, there was a gradual reduction from 74°, which had 

 been the average of several previous days, to 72°, apparently in conse- 

 quence of the influx of the colder Atlantic water ; the water in the Bay 

 itself had an average surface-temperature of nearly 71° ; and the surface- 

 temperature did not fall until we came out into the mid-channel, where we 

 encountered the colder indraught as already described (§ 61). Hence the 

 depression observed during our first visit must be occasional only; and may 

 perhaps be attributable to a deflexion of the mid-current into the Bay, by 

 the opposing influence of the easterly wind which then prevailed. — This is 

 one of the points as to which further inquiries, such as may be easily in- 

 stituted by the Government authorities at Gibraltar, would doubtless fur- 

 nish information of great interest. 



78. Bottom-Temperature. — The Temperature-soundings taken during 

 the Atlantic Cruise (see p. 220) may be arranged in three sets: — 



