their Prodvclion, Suspension, and DestrucHon. 101 



barometer, thermometer, hygrometer, and electrometer, 

 have not yet beea attended to. 



Of the Cumulus^ 



Clouds in this modification are commonly of the most 

 dense structure : they are formed in the lower atmosphere, 

 and move along with the current which is next the earth. 



A small irregular spot first appears, and is, as it were, the 

 micleus on which they increase. The lower surface conti- 

 nues irregularly plane, while the upper rises into conical or 

 hemispherical heaps ; which may afterwards continue long 

 nearly of the same bulk, or rapidly rise to mountains. 



In the former case they are usually numerous and near 

 together, in the latter few and distant ; but whether there 

 are few or many, their bases always lie nearly in one hori- 

 zontal plane, and their increase xipward is somewhat pro- 

 portionate to the extent of base, and nearly alike in many 

 that appear at once. 



Their appearance, increase, and disappearance, in fair 

 weather, are often periodical, and keep pace with the tem- 

 perature of the day. Thus they will begin to fonn some 

 hours after sun-rise, arrive at their maximum in the hottest 

 part of tiie afternoon, then go on diminishing and totally 

 disperse about sun-set. 



But in changeable weather thej' partake of the vicissitudes 

 of the atmosphere ; sometimes evaporating almost as soon 

 as formed, at others suddenly forming and as quickly pass- 

 ing to the compound modifications. 



The cumulus of fair weather has a moderate elevation 

 and extent, and a well defined rounded surface. Previous 

 to rain it increases more rapidly, appears lower in the at- 

 mosphere, and with its surface full of loose fleeces or pro- 

 tuberances. 



The formation of large cumuli to leeward in a strong; 

 wind, indicates the approach of a calm witli rain. VVheii 

 they do not disappear or subside about sun-set, but conti- 

 nue to rise, thunder is to be expected in the night. 



Independently of the beauty and magnificence it adds to 

 the face of nature, the cumulus serves to skrccn the earth 

 from the direct rays of the sun, hy its multiplied reflections 

 to diffuse, and, as it were, economise the light, and also to 

 convey the product of cv;iporati(jn to a distance from the 

 place of its origin. The relations of the cumulus with the 

 state of the barometer, 84c. lia\e not yet been enough at- 

 tended to. 



G 3 qf 



