646 THE POPULAR SCIENCE MONTHLY. 



commonplace cloud formation fog which on earth is often ag- 

 gravating and trying to health and temper, becomes beautiful 

 as soon as it leaves the earth. 



A fog may be defined as a cloud viewed from within, and is 

 therefore the first distinct cloud type. The next low type is the 

 stratus or " raised fog," less than one thousand metres high. And 

 here it may be noticed that in summer the earth pushes her 

 cloud mantle away from her and draws it closer to her in winter. 

 In other words, clouds are lower in winter than in summer. The 

 highest cloud is the cirrus, with a mean elevation of nine thou- 

 sand metres. The cirrus is a fine, featherlike cloud, and its neigh- 

 bor, cirro-stratus, something like it, only more diffuse and lower. 

 When a veil of cirro-stratus is drawn before the sun or moon, 

 large halos forty-four and eighty degrees in diameter, with faint 

 red on the inside or nearest the sun, and blue on the outside, ap- 

 pear. These are caused by the refraction of light by ice crystals. 

 A lower cloud, alto-stratus, without causing halos may cause 

 coronas or smaller circles of prismatic colors, about one fourth 

 the diameter of halos. In coronas the red is on the outside. The 

 Brockenspecter is a particular kind of coronal cloud shadow. 

 Midway between high and low clouds are the cirro-cumuli and 

 alto-cumuli. These give perhaps the most beautiful of all cloud 

 effects. The fairest meadows of earth seldom show such flocks 

 grazing so leisurely and scattered so harmoniously. Cirro-cumuli 

 are small, white, fleecy clouds, often arranged in rows, while the 

 alto-cumuli are denser, larger, and less regular. Both types are 

 like tranquil fleets upon a serene sea. " Their very motion is rest," 

 as John Wilson said of them long ago. Trailing in lustrous 

 glory before the midnight moon, they turn into silver bars and 

 " streak the darkness radiantly." Of the low clouds, the strato- 

 cumuli and nimbi are most common : the former, large rolls of 

 dark cloud, often covering the whole sky and of somewhat dreary 

 aspect ; the latter, nondescripts without definite form and with 

 little gradation in color. The sky effects of both are as a rule 

 somber and depressing, though there are times, especially if the 

 sun be close to the horizon, when the nimbus gives the golden 

 rain of Greek mythology, a downpour inexpressibly beautiful. 

 The cumuli and cumulo-nimbi are the largest clouds in cloud- 

 land. The familiar " castles in air " are the turreted cumuli, 

 thick clouds with domes and summits. The cumulo-nimbus, or 

 towering thunder cloud, rises mountain high, and has peaks of 

 snowy whiteness with a flat and frowning base. Its monstrous 

 size can be better appreciated if we imagine Mont Blanc (14,i:J4 

 feet high) lifted into the air and set down on top of Mount Wash- 

 ington (6,279 feet). This would make a medium-sized cumulo- 

 nimbus. The thunder cloud is noteworthy in another respect, 



