io the Antarctic Regions. 1 93 



6. Currents of the Ocean. 



These are either subaqueous or superficial, and, like those in the 

 atmosphere, both may coexist at the same place, with different direc- 

 tions and velocities. Of the former 'we know almost nothing, and 

 of the latter but little compared with what would be desirable and 

 most useful. The practice of daily throwing overboard a bottle 

 ■corked and sealed with the latitude and longitude of the ship at noon 

 •ought not to be neglected. A single instance of such a record being 

 found may suffice to aiFord indications of the utmost value, while the 

 trouble and cost are too trifling to mention. 



As no sea can be supposed absolutely motionless, the presence of 

 -a shoal, by casting up at the surface water which, but for it, would 

 have continued to sweep along at a greatly lower level with the ge- 

 neral body of the current, must bring the temperature of the surface 

 water into nearer correspondence with that below. In low latitudes 

 the surface water is hotter than that below ; and accordingly it is a 

 general remark, that the temperature sinks as the water shoals, or 

 even in passing over banks whose depth is very considerable. If 

 this theory of the phenomenon be correct, the contrary ought to be 

 observed in situations where the surface water is colder than that 

 below, as it is known to be under particular circumstances in the Polar 

 Seas. In still larger tracts in high latitudes the seas have nearly a 

 uniform temperature throughout their whole depth. In such cir- 

 cumstances should any superficial variation of temperature be ob- 

 served in passing over a shoal or bank, it could only be ascribed to 

 radiation. The subject is one of considerable interest to the navi- 

 gator, as the approach to land or to shoal water is indicated by the 

 thermometer with a high degree of sensibility. We have before us 

 recent observations of this kind, the one at entering Table Bay in 

 1834, the other at quitting it in the present year. In the former 

 case the temperature fell 9° Fahr. in passing from deep water into 

 the Bay ; in the latter under reverse circumstances a rise of no less 

 than 13° Fahr. was experienced, the temperature of the air remain- 

 ing unaltered. The last- mentioned observations being very remark- 

 able, the particulars are annexed. 



Memorandum of observations made on board the Earl of Hardwicke, 

 H.E.I.C.S. by Captain Henning. 



March 17th, 1839. Temperature of air at 5^ p.m., four miles from 



Cape Town 64°-0 



Of Sea 52°0 



5^ 30'» One mile north of Robben Island. Air 64°-0 



Sea 5700 



March 18th at sea. Air 64°0 



Sea 65° 



The opportunity of re-examining this point, and in general, of in- 

 vestigating more closely the phenomena of temperature in the neigh- 

 Phil. Mag. S. 3. Vol. 15. No. 95. Sept. 1839. O 



