^94 Prof. BischofF on the Temperature of 



Notwithstanding that the heat which is continually rising in 

 the sea increases in proportion to the depth, yet no perceptible 

 (difference in the temperature at the surface can be caused by any 

 variations in the more considerable depths, for in proportion as 

 the t[uantity of heat which rises from the more considerable 

 depths is gretiter, so is the quantity of water greater through 

 which it is dispei-sed. But in shallows this difference must be 

 perceptible ; and this is a fact which, it is well known, was ob- 

 served by Franklin and J. Williams,* and confirmed by later 

 observers, by V. Humboldt+ ^nd J. Davy.t On approaching 

 land, Williams describes the decrease of temperature as so per- 

 ceptible, that coasts and shallows were indicated by the thermo- 

 meter, at distances in which they were not yet visible. He not 

 unfrequently found a decrease in the temperature of the sea of 

 6°.75 during a sail of three hours, and yet they were out of all 

 danger. In some cases he even observed differences of 11°.25. 

 The more rapidly the depth decreases on approaching a coasr, 

 the more rapid is the decrease of temperature. These great 

 differences very clearly shew the influence of the warmer waters 

 rising from below upon the temperature at the surface, and this 

 influence is independent of the seasons, as the experiments also 

 prove. If, on the other hand, Sir H. Davy's § explanation of this 

 phenomenon were right, namely, that the sea becomes cooled at 

 its surface by certain causes, and that in deep places the cooled 

 layers of water are carried to a distance below the surface, whilst 

 in shallows they remain near the surface, this decrease of tem" 

 perature could only take place when the air is cooler than the 

 surface of the sea. Neither is it to be expected, as Sir H. Davy 

 asserts, that this decrease of temperature should not take place 

 in very high latitudes, whei*e the temperature of the surface of 

 the air is near 27°. 50 ; for in deep places, where the bottom of 

 the sea would always possess a temperature higher than the 

 mean temperature of the air, currents of warmed water must 



" Transactions of the American Society, vol. iii. p. .32 ; and Williams 

 Thermometrical Navigation. Philad. 1700. 

 t Gilbert's Annal. vol. vii. p. 342. 

 * Philos. Transac. 1817, ii. 275. 

 § Journ. of Sc. vol. iii. 1817 ; and Gilbert's Annal. vol. Ix^i, p. 139. 



