Meteorological Sketches. 65 
Land and Sea Breezes. 
Near the shores of an island or country it is often found that the 
wind, during different hours of the day and night, blows alternately to 
and from the land. Or in the case of a general or trade wind which 
is parallel to the coast, its course becomes alternately modified by 
an approximation to the above result. This effect. has been justly 
ascribed to the influence of diurnal heat and cold. Not that any 
vacuum is created by the heat into which the surrounding air rushes, 
as has sometimes been supposed, nor that a warmed stratum of air 
necessarily rises from the surface and ascends to the higher regions ; 
for, aside from the general error of these notions, a flat, low, and 
strongly heated island or coast, is found to have less effect in pro- 
ducing these breezes than a high and sloping country of more even 
temperature. 
The truth appears to be, that when the stratum which lies upon 
the inclined surface of a coast becomes warmed and rarefied by the 
daily heat, it is forced by the increment of pressure at its lowest mar- 
gin to move along the inclined surface of the country in the direction 
of the greatest elevation, or as near that direction as the prevailing 
tendency of the lower current will allow. Owing to the cooling 
process which goes on during the night, the specific gravity of this 
inclined stratum becomes predominant, and the reverse movement 
then commences and continues into the following morning. We 
find, too, that on the slopes of certain coasts and islands where there 
is sufficient elevation, the higher margin of this stratum, at certain 
seasons, will daily reach an altitude at which it is brought in contact 
with a higher stratum sufficiently cold to set in operation a squall or 
thunder storm, at a certain hour; after which the equilibrium is res- 
tored, and the usual counter movement again follows in its turn. 
Some diurnal effect of this kind upon the wind may be observed at 
times in almost every region; and, taken altogether, it is probably the 
most extensive agency which is exercised by heat in the — 
of winds, 
Vou. XXXIH.=—No. 1. 9 
