- 67 — 
We have frequent opportunities in Mauritius of observing 
the operation of some of these processes, and especially of the 
last mentioned. Daring the heat of the day, the Trade wind 
is often met, and partly overpowered by a sea breeze, drawn 
towards the island from the Westward, by the upward current 
of air, caused by the great heat of the mountain masses. The 
transparent air allows the sun’s rays to pass through it with- 
out intercepting much of their heat ; but the earth on which 
they fall is quickly raised in temperature ; and it then imparts 
heat, by actual contact, to the air brought near to it, which at 
once expands and rushes upwards. From this cause we may 
often see the phenomenon of a Westerly wind in the Harbour 
and neighbourhood of Port Louis, at the same time that the 
Trade is blowing over the flat and windward parts of the 
Island. At such times the mixing of the two winds is accom- 
panied by clouds and mist ; sometimes presenting the ap- 
pearance of heavy banks of clouds, driven back by the wind 
from the sea, and heaped up above the Island, until their ele- 
vation brings them again within the influence of the Trade 
wind. The clouds are more or less dense, according to the 
degree of humidity of the air, and in general they deposit 
rain ; the horizon to the Westward being all the time clear. 
In the evening the mountains cool, by radiation, the up- 
ward draught of air ceases, and the clouds disappear, or seem 
to be swept away by the trade. 
The exciting cause in this operation is evidently the variable 
temperature of the earth, in relation to the surrounding air. 
The transparent air cannot receive heat from the sun’s rays, 
nor part with it by radiation ; the air can only be affected by 
actual contact with bodies of a different temperature, whereas 
the earth is soon treated by the sun, and as readily cooled by 
radiation, when the sun is not in a position to act upon it. 
There is only one way in which forests can affect the cau- 
ses which bring together different bodies of air, that is through 
the influence they exercise upon the temperature of the soil ; 
but from what has been said upon this subject, it is clear that 
the forests will in no way assist the process, but will rather 
tend to impede it, by preventing those changes of tempera- 
ture which give rise to the opposing currents of air. 
