468 PHYSICAL GEOGRAPHY. 



snow, and a thunder-storm. It is the common herald of the latter, and may be seen 

 rapidly forming during the calm which precedes a discharge of electricity, swelling to a 

 stupendous magnitude, its protuberances, like the domes of an aerial city, shining with a 

 strong silvery or golden light, finely contrasting with the darkness and density of its cen- 

 tral regions. Borne by the currents of air, the cirrostratus is often conducted towards 

 the summit of cumulostratus, and appears cutting through its whole extent. 



7. Nimbus Raincloud. Any of the preceding modifications of cloud may so increase 

 as to veil the sky completely, and put on an appearance of density, from which an expe- 

 rienced observer will augur rain. But they frequently dissolve without any shower, and 

 no rain falls till another modification has been experienced, which commonly occurs in 

 the case of cumulostratus. After exhibiting a great increase of density, and assuming a 

 louring aspect, the blackness of darkness is followed by a lighter shade, evidencing a 

 fresh disposition of the aqueous particles in the cloud, or the formation of nimbus, from 

 which rain falls. This change may frequently be very distinctly observed when the 

 cloud is over a distant spot ; and the transition from considerable blackness to a gray 

 obscurity is sure evidence that the shower has commenced, and may be expected to 

 reach the locality of the spectator, should the wind be blowing in his direction, and the 

 nimbus not be previously extinguished. Hence Virgil's reference to the husbandmen 

 anxious to gather in the harvest : 



" So while far off at sea the storm-cloud lours, 

 And on the darken'd wave its fury pours, 

 Mid crops unreap'd the hapless peasants stand, 

 And shuddering view its rapid course to land." 



The nimbus the least interesting modification of the clouds to the eye is first in point 

 of attraction when the rainbow appears upon its front. The precipitation of the aqueous 

 vapours to the earth in the form of rain, is caused by contending aerial currents com- 

 mingling saturated strata of different temperatures, promoting a condensation of the 

 particles beyond what the air is capable of supporting, when the resulting mass gives out 

 a portion of its moisture, which descends by its own gravity in rain, snow, or hail, 

 according to the temperature of those regions of the atmosphere which it has to traverse. 

 This is the last stage of an extensive pilgrimage which the evaporating forces in action 

 constrain the waters of the globe to undertake through localities apart from its surface 

 a pilgrimage in which there is no halt, and which some portions of the element are per- 

 petually completing, commencing, and pursuing. How different and far apart the sites 

 which are the starting and terminating points of the journey ! Exhaled from the surface 

 of a ruffled ocean or tranquil lake, the aqueous particles ascend invisibly into the upper 

 air, where they are called into sensible existence by a change of temperature, and are 

 built up into the variously-formed beautiful and majestic clouds. These are wafted by 

 the atmospheric currents far apart from the scene of the ascension, change their shape 

 and direction at the will of the winds, pass into a state of invisibility, and again emerge 

 from it as warmer or cooler strata are encountered in their aerial flight, till, perhaps, a 

 thousand leagues away from the spot where the liquid element assumed its vaporous 

 form, that combination of circumstances occurs, which reduces it to its original condition, 

 and deposition ensues upon some thirsty prairie or parching field. 



The copiousness and energy of rain depend upon the amount of vapour in the atmo- 

 sphere, and the gradual or rapid manner in which its particles are brought into mutual 

 contact. We have a slow drizzle in the one case, and a violent shower in the other. The 

 drops of rain vary in size, according to Leslie, from the twenty-fifth to the fourth of an 

 inch in diameter. He remarks, that in parting from the clouds, their descent accelerates, 



