148 
CHANGE OF THE SEASONS. 
sky in the northern equinoctial zone is constantly serene. 
The vesicular vapours are not condensed, because the air, 
unceasingly renewed, is far from the point of saturation. 
In proportion as the sun, entering the northern signs, rises 
towards the zenith, the breeze from the north-east moderates, 
and by degrees entirely ceases. The difference of tempera- 
ture between the tropics and the temperate northern zone is 
then the least possible. It is the summer of the boreal pole ; 
and, if the mean temperature of the winter, between 42° and 
52° of north latitude, be from 20° to 2G° of the centigrade 
thermometer less than the equatorial heat, the difference in 
summer is scarcely from 4° to 6°. The sun being in the 
zenith, uud the breezo having ceased, the causes which pro- 
duce humidity, and accumulate it in the northern equinoc- 
tial zone, become at once more active. The column of 
air reposing on this zone, is saturated with vapours, 
because it is no longer renewed by the polar current. 
Clouds form in this air saturated and cooled by the com- 
bined effects of radiation and the dilatation of the ascending 
air. This air augments its capacity for heat in proportion 
as it rarefies. With the formation and collection of the 
vesicular vapours, electricity accumulates in the higher re- 
gions of the atmosphere. The precipitation of the vapours is 
continual during the day; but it generally ceases at night, 
and frequently even before sunset. The showers are regu- 
larly more violent, and accompanied with electric explosions, 
a short, time after the maximum of the diurnal heat. This 
state of things remains unchanged, till the sun enters into the 
southern signs. This is the commencement of cold in the 
northern temperate zone. The current from the north-pole 
is then re-established, because the difference between the 
heat of the equinoctial and temperate regions augments 
daily. The north-east breeze blows with violence, the air of 
the tropics is renewed, and can no longer attain the degree 
of saturation. The rains consequently cease, the vesicular 
vapour is dissolved, and the sky resumes its clearness and 
its azure tint. Electrical explosions are no longer heard, 
doubtless because electricity no longer comes in contact with 
the groups of vesicular vapours in the high regions of the 
air, I had almost said the coating of clouds, on which the 
fluid can accumulate. 
