102 REPORT—1840. 
Ey = 0°1370 + 8°9004 cos? ¢ in French lines, where Eg is 
the elasticity of vapour in lat. ¢. 
IV.— Winp*. 
192. The immediate cause of wind is the inequality of pneu- 
matic pressure in the atmosphere, occasioned by differences 
(permanent or variable) of temperature. 
193. Of Permanent Differences, the most important is the 
warmth of the equatorial regions compared to the polar. The 
combination of this cause with the rotation of the earth is the 
well-known cause of the trade winds ; and, we may add, of the 
prevalent west winds in northern latitudes, as well as of the 
counter currents observed at certain elevations. 
194. Of Variable Differences of temperature there are very 
many ;—as (1) the variable temperature of any spot, occasioned 
by the annual change of position of the sun respecting it ; (2) a 
similar variation for different hours of the day or night; (3) a 
variation due to the continental and insular character of climate 
affecting the annual temperature curve; (4) a similar influ- 
ence of the solid or fluid, and more or less heat-absorbing, cha- 
racter of the surface, in varying the distribution of temperature 
during twenty-four hours ; (5.) the variable nature of surface 
depending on elevation, such as the presence of mountains, 
which receive heat from and part with it to the adjacent plains, 
according to different laws. 
195. All these causes produce their peculiar and local effects, 
which it may be sufficient to advert to in the most general way. 
To the first cause is due the variation in the position of the 
limit of the trade winds at different seasons. ‘To the second, 
the probable tendency of the warmth at the part of the earth’s 
surface, on which, in his diurnal course, the sun has just exer- 
cised its greatest energy, to attract the colder air from the parts 
of the earth on whose horizon he is just appearing. To the 
third, are attributable the very important effects of the mon- 
svons, the local variations of the trades (as on the coast of 
Africa and Mexicoy), the prevalence of east winds in Europe 
in spring, and many similar phenomena. To the fourth, the 
recurrence of land and sea breezes in all climates, especially 
between the tropics. To the fifth, the very remarkable but 
little-noticed diurnal phenomena of hill and valley breezes, 
occurring with great regularity in mountainous countries having 
a pretty uniform climate (as in the South of Europe), and 
* See former Report, p. 246. Mahlmann, p. 155. 
+ See Capt. Hall’s Fragments of Voyages and Travels, 2nd Series, ii., and 
Daniell’s Meteorological Essays, 2nd Edition. 
