in a Voyage across the Atlantic. TSE 
It sometimes amounted to 2° or 3°; and not unfrequently was 
null; and was so for air both in shade and sunshine. 
The diurnal changes of temperature of the sea were less dili- 
gently watched. In ordinary instances, so far as observed, the 
fluctuation scarcely exceeded 2°, and was often null. 
In a brisk current of air, such as usually prevails on the open 
sea, the temperature of an object, exposed to sunshine and 
wind, is very little raised by the sun’s direct influence; because 
the quick passage of a current of air diffuses an equable 
heat, taking away and scattering particular superabundant 
warmth. On ship-board, therefore, in a fresh breeze, the tem- 
perature is nearly or precisely the same in the shade and in 
ventilated sunshine, even at noon; excepting of course such 
surfaces as from colour or texture are peculiarly fitted to absorb 
heat; and excepting likewise objects skreened from the wind 
and exposed to reflected heat. 
The remark has been very frequently verified in course of 
the voyage; and the same observation may be made on shore, 
if care be taken to exclude the influence of reflected heat, and 
to admit free ventilation. . 
But, in calms and light breezes, a sensible difference of tem- 
perature (from 1° to 4°) occurs in sunshine and in shade, how- 
ever open the exposure may be. That difference is attributable 
to the direct effect of the sun’s rays on the instrument with 
which the observation is taken. 
The sea in few instances was at the pointof dew; in fewer, 
below it. The weather was in such case hazy; or mist attended 
the deposition of dew, which of course was then going on at 
the surface of the water. 
Amidst fogs and mists, this is frequent; but such is not the 
climate of the middle Atlantic; and the temperature of the sea’s 
surface rarely descends to that point during either the night or 
the day. 
On ship-board, dew is not uncommon ; especially in clear 
nights, upon black surfaces and other cooling substances. But 
the sea does not so readily arrive at the same reduced tempera- 
