WIND EROSION 



281 



it, yet in the moist and changeable climate of New York 

 it was found necessary within a year to cover its surface with 

 a preservative substance. 



Not only do different climates affect differently the 

 wearing away of rocks, but different kinds of rocks them- 

 selves vary much in 

 the rate at which 

 they crumble. It 

 has been found that 

 while marble in- 

 scriptions, in a large 

 town where there is 

 much coal smoke 

 and considerable 

 rain, will become 

 illegible in fifty 

 years, that after a 

 hundred years in- 

 scriptions cut in 

 slate are sharp and 

 distinct. 



Where the tem- 

 perature varies 

 greatly during the 

 day the expansion 

 and contraction due to the heating and cooling sometimes 

 cause a chipping off of the rock surfaces. 



Wind Erosion. The artificial sand blast is in common 

 use. In it a stream of sand is driven with great velocity 

 upon an object which it is desired to etch. In nature the 

 same kind of etching is done by the wind-blown sand. 



CLEOPATRA'S NEEDLE, CENTRAL PARK, 

 NEW YORK 



