EVAPORATION. 263 



the atmosphere ? The density of the atmosphere rapidly 

 decreases upwards. One half of the whole quantity of air 

 is within about three miles from the earth. Above this 

 height, the air is unable to support any considerable quan- 

 tities of vapor. Hence we see the reason why clouds 

 rise no higher, and why the thinnest and lightest rise 

 highest. 



Form and Color of Clouds. 



To an attentive observer the clouds present many in- 

 teresting subjects of contemplation. Their ever varying 

 forms, their bea*utiful and richly variegated colors, and 

 their silent motion varying often in velocity and direction, 

 while they furnish the poet with a field in which his fancy 

 may rove delighted, also afford to the student of nature, 

 many an interesting theme for reflection. At one time, 

 dark and portentous, fancy might easily imagine them 

 the ruins of some ancient castle, or time-worn tower. 

 At another, they gather in beautiful and glorious forms 

 around the path of the descending sun, and seem to vie 

 with that luminary itself in splendor. Sometimes they 

 move swiftly over the face of heaven, and soon recede 

 from our view ; sometimes they seem to meet each other, 

 and soon, like hasty travellers, pass each other by, 

 without a sign of recognition. At one time while we 

 gaze upon them, they vanish ; at another they gather into 

 darker and heavier masses of collected gloom. Now 

 they collect, now they disperse, and now they change 

 form and color with surprising rapidity. To the inquir- 

 ing mind the question naturally occurs, what is the cause 

 of- all these varied appearances ? The inquiry leads to 

 careful observation, and though in many instances, that 

 cause clouds our search, yet, in many others, we are ena- 

 bled to arrive at general principles and uniform relations, 

 which enable us to anticipate the storm, and predict the 

 time of its termination. The principal circumstances 

 which influence the form of clouds are the motion of the 

 air and the formation and condensation of vapor. Sub- 

 stances so light as clouds, readily change form, when sub- 

 jected to greater atmospheric pressure on one side, than 

 on the another. Different portions of the air move with 



