THE ATMOSPHERE. 631 



miles, 50 pounds. But the actual force exerted depends largely on 

 the form of the surface struck. This is well shown in the anemometer 

 made of hemispherical cups : the difference between the pressure on 

 the concave and convex sides being such that the cups move one third 

 as fast as the wind, whereas with flat disks there would be no motion. 

 A velocity of 186 miles an hour (or 170 pounds to the square foot) 

 has been registered by the anemometer. 



1. Rending and Abrasion. 



1. By direct impulse. — The rending effects of hurricanes need no 

 special illustration. The effects are a consequence of the direct im- 

 pulse of the air, and are especially destructive when the moving air is 

 brought to bear in a large volume through converging surfaces in sur- 

 rounding objects, or in the object itself. The adhesion of ordinary 

 hardened mud may not be overcome by a gale that prostrates a forest, 

 destroys houses, or throws over lofty piles or columns of rock which 

 degradation from any cause had left standing. 



2. Through the Material transported. — By means of the sand which 

 winds often carry, a large amount of wear is accomplished in arid re- 

 gions. Attention was first called to this point by W. P. Blake, who 

 described the granite of the Pass of San Bernardino, California, as 

 scratched like rocks of glacier regions, even the quartz being polished 

 and the garnets left projecting on pedicels of feldspar ; and the lime- 

 stone as eroded and channeled as if by dissolving waters. Many of 

 the bluffs, needles, and towers of soft sandstone characterizing the 

 scenery in different parts of the Rocky Mountain region have been 

 more or less shaped by this means. It was observed long ago that 

 the glass of windows on Cape Cod had been ground and bored through, 

 by the wind-driven sands. A blast of sand propelled by steam is now 

 employed (after Nature's suggestion) in grinding and carving glass, 

 gems, and even granite. Glass covered by lace-work, or by paper hav- 

 ing open patterns cut in it, is rapidly worn where its surface is exposed, 

 while the lace or paper, owing to its yielding before the sand, shows 

 scarcely any effect of the blast. Large cornices and mouldings of 

 granite are shaped by a blast of steam and sand. 



2. Transportation. 



The streets of most cities, and the roads of the country, in a dry 

 summer day, as well as deserts and sea-coasts, afford examples of the 

 drift of dust or sand by the winds ; and almost all regions, of the 

 drifting to long distances of leaves and forest debris, or of insects and 

 other lighter kinds of life within reach of the swiftly moving air. 



