148 CHANGE OF THE SEASONS. 



ky 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 diiference 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 26 of the centigrade 

 thermometer less than the equatorial heat, the diiference in 

 summer is scarcely from 4 to 6. The sun being in the 

 zenith, and the breeze 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 

 ?apour 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. 





