ACTION OF THE ATMOSPHBEE 163 



(5) Wind. — But it is to the action of tlie air when in motion 

 — to the wind — that is due a very considerable part of atmos- 

 pheric work. Particles of sand drifting along before the wind 

 become themselves agents of abrasion, filing away on every hard 

 object with which they come in contact. As a matter of course, 

 this phenomenon is most strikingly active in the arid regions, 

 though the results, when looked for, are by no means wanting 

 in the humid east It is thought by Professor Egleston that 

 many of the tombstones in the older churchyards of New York 

 City have become illegible by the wearing action of the dust 

 and sand blown against them from the street. There is among 

 the heterogeneous collections of the National Museum at Wash- 

 ington a large sheet of plate glass, once a window in a light- 

 house on Cape Cod. During a severe storm, of not above forty- 

 eight hours' duration, this became on its exposed surface so 

 ground from the impact of grains of sand blown against it as to 

 be no longer transparent, and to necessitate its removal. Win- 

 dow panes in the dwelling-houses of the vicinity are, it is stated, 

 not infrequently drilled quite through by the same means. 



Apply now this agency to a geological field in a dry region. 

 The wind, sweeping across a country bare of verdure and 

 parched by drought, catches up the loose particles of dust and 

 sand and drives them violently into the air in clouds, or sweeps 

 them along more quietly close to the surface, where they are 

 at first scarcely noticeable. The impact of a single one of these 

 moving grains on any object with which it may come in con- 

 tact is far too small to be appreciable; but the impact of 

 millions, acting through days, weeks, and years, produces re- 

 sults not merely noticeable, but strikingly conspicuous. We 

 have here, in fact, a natural sand blast, an illustration on a 

 grand scale of a principle in common use in glass-cutting, and 

 to a small extent in stone-cutting also. Constantly filing away 

 on every object with which they come in contact, the grains 

 go sweeping on, undermining cliffs, scouring down mountain 

 passes, wearing away the loose boulders, and smoothing out all 

 inequalities. Naturally the abrading action on exposed blocks 

 of stone is most rapid near the ground, as here the flying sand 

 grains are thickest. First the sharp angles and corners are 

 worn away, and the masses gradually become pear-shaped, 

 standing on their smaller ends. Finally the base becomes 

 too small for support, the stone topples over, and the process 



