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Our fenfes accordingly convince us that the fea-air is al- 
ways replete with moift vapors, even when its natural 
tranfparency is not in the leaft interrupted by them. 
Hence in a hot fummer’s day, when the wind fuddenly 
fhifts from weft to eaft, we immediately perceive a chill 
from the fea-breeze; and fometimes long before the ther- 
mometer indicates a change in the temperature of the 
atmofphere. Thefe vapors, when they firft arife from the 
fea, are generally fo nearly of the fame denfity with the 
furrounding and contiguous air, that the rays of light in 
paffing through them, undergo no fenfible change in their 
refraction; they are therefore at firft generally invifible, 
but when the weather is extreamly cold, and the air of 
confequence uncommonly denfe, they are always vifible, 
and appear like a fteam arifing from boiling water*. Not 
that vapors afcend moft copioufly in the coldeft feafons, 
which feems contrary both to reafon and experience; but 
that the different denfities of the air next the furface of the 
water, and of the vapors which afcend in it, render the 
latter vifible, by the irregular refractions of the rays of 
light in pafling through them. For the fame reafon our 
breath is vifible in the winter, but not in warm weather. 
Let us now fuppofe the atmofphere, on a fummer’s 
morning, to be all around in a homogenous {ftate, as in-= 
dicated by a cloudlefs {ky anda dead calm. As the fun 
rifes on the eaftern coafts of America, and warms and ra= 
refies the atmofphere eaftward, the rarefied air naturally 
afcends, and a current of air as naturally flows thither from 
the oppofite quarter, which is but juft emerging from the 
cool fhades of night, to fupply its place. The confequence 
of which is a light wefterly breeze. As the fun afcends 
higher, the air over the land becomes heated and rarefied, 
both by the paflage of the fun’s dire& and reflected rays 
through it, and by the reverberation of the heat acquired 
from 
\ 
* This is always the appearance in a clear, ftill morning, when the mercury in Farenheit’s 
thermometer is at 0, or below it. 
