274 WELLS'S NATURAL PniLOSOPHY. 



the least depression of temperature will compel it to relinquish somo of its 



moisture. 



^ . , The moisture contained in the air we expel from the lungs 



"Why is the . , ^ ,,■•., ^ x • 



moisture of m the process of respiration, is visible m winter, but not in 



our breath vis- summer. The reason of this is, that in cold weather the vapor 

 able in winter ' . 



and not in is condensed by the external air, but in summer the tempera- 

 summer ? |^jj.Q pf ^j^g ^jj. jg jjQ^ sufficiently reduced to effect condensation, 

 J , During the daily process of evaporation from the surface of 

 Her are clouds the earth, warm, humid currents are continually ascending; 

 formed? ^j^^ higher they ascend, the colder is the atmosphere into 

 which they enter ; and as they continue to rise, a point will at length be 

 attained where, in union with the colder air, their original humidity can no 

 longer bo retained : a cloud will then appear, which increases in bulli with 

 the upward progress of the current into colder regions. 



To a person in the valley, the top of a mountain may seem enveloped in 

 clouds ; while, if he were at the summit, he would be surrounded by a mist, 

 or fog. 



„„ , , , The reason why clouds, which are condensed vapor, float 



Whydoclouds . , . , , . - . , , 



float ill the at- m the atmosphere is, that they consist of very minute glob- 



mosphere ? yj^g (called vesicles), which, although heavier than the sur- 



rounding air, have a great extent of surface in comparison with their weight. 

 On account of the resistance of the air, they sink very slowly, as a soap- 

 bubble, which greatly resembles these vesicles, sinliS but slowly in a calm 

 atmosphere. As these vesicles do, however, gradually sink, the question 

 arises, why do not the clouds fall to the ground ? The explanation of this is, 

 that the vesicles which sink in calm weather can not reach the ground, be- 

 cause in their descent they soon meet with warmer strata of air which arc not 

 saturated with moisture, where they again dissolve into vapor and are lost to 

 view : at the same time that the vesicles of vapor dissolve at the lower limits 

 of the clouds, new ones are formed above, and thus the cloud appears to float 

 immovably in the air. 



When the atmosphere is agitated, the vesicles of vapor constituting clouds 

 are driven in the direction of the currents of air. A wind moving in a hori- 

 zontal direction will carry the clouds in the same direction ; and an ascend- 

 ing current of air will lift them up, as soon as its velocity becomes greater 

 than the velocity with whicli the vesicles would fall to the ground in a calm 

 condition of the air. In like manner, soap-bubbles are elevated by the wind 

 and carried to considerable distances. i 



„ . . , Clouds frequently appear and disappear with a change in 



now do winds , ,. , , ^ , . , „,, .„ ,, . , 



Bffect the the du-ection and character of the wind. Thus, if a cold wind 



clouds? blows suddenly over any region, it condenses the invisible va. 



por of the air into cloud or rain ; but if a warm wind blows over any region, 



it disperses the clouds by absorbing their moisture. 



The average height at which clouds float above the surface 



Average height ^^ tlie earth in a calm day, is between one and two miles. 



of clouds ? Light, fleecy clouds, however, sometimes attain an elevation 



ef five or six miles. 



