EROSION PERFORMED BY THE ATMOSPHERE. 321 



age velocity of the wind increases very fast and apparently not 

 according to any definite law upwards for the first fifteen feet 

 above the ground. Above this height it increases as the bisected 

 chords of parabolas having their vertices in a horizontal line J2 

 feet below the surface. The parameters of these parabolas 

 increase directly in the ratio of the squares of the velocities of 

 the different winds. With a velocity of ten miles per hour at an 

 elevation of fifty feet above the ground there will then be a veloc- 

 ity of about one hundred miles per hour one mile above the 

 o-round. but of less than one mile per hour near the surface. 

 Observations made on the movements of clouds verify these 

 calculations as to high velocities some distance up in the atmos- 

 phere. Whatever is to be transported any great distance must 

 be lifted up to some considerable height above the surface of the 

 earth, where the winds attain high velocities. 



Over level plains, under ordinary circumstances, the condi- 

 tions seem to be unfavorable for effecting any such upward 

 transference, and little or no removal of material is apt to take 

 place. But when a strong wind runs up against a vertical cliff, 

 such as are seen in the bad lands or in the country of the pla- 

 teaus and "mesas," eddies are no doubt set up which rise high 

 above these vertical reliefs. A short valley or a reentrant exca- 

 vation in such a cliff will gather the wind and start it with 

 increased force obliquely upwards, as it enters from the open end. 

 In such a mobile element as the atmosphere an eddy like this 

 may rise a considerable distance. No less effective in this 

 respect are the whirlwinds in arid regions, which have been 

 described by nearly every traveler in such countries.' During 

 the warm part of the day these can be seen, it is said, at almost 

 any time in some direction of the horizon. They often rise to a 

 great height, carrying with them the loose materials of the desic- 

 cated soil and giving them up to the incessant and steady run of 

 the winds above. 



The explosive outburst of a volcano similarly launches enor- 

 mous quantities of minute fragments of pumice on the currents 



' Geo. p. Merrill, Engineering Magazine, Vol. II., p. 599 et seq. 



