252 MACHINERY IN CONNECTION "WITH AIR. 



sizes, which are fifty feet or more in diameter, possess 

 much power, and are used for grinding grain, and other 

 purposes. 



The work which a wind-mill is capable of doing de- 

 pends very much on the site. If placed where the wind 

 has a long, uniform, and steady SAveep, it will accomplish 

 much more, and to better satisfaction, than if among hills 

 or other obstructions, where the blasts are uncertain and 

 changing. 



Wind-mills of large size are peculiarly adapted to pump- 

 ing water into reservoirs, or from mines or quarries, where 

 a few days of calm weather will not result in inconven- 

 ience ; but they are not suited to manufactories where a 

 constant power is required to furnish employment to men, 

 but can be used for work which may be intermitted or 

 changed. 



Brown's wind-mill is sold at $75 dollars for the small 

 size, with increase of prices up to $1,200 for large ones. 



CAUSES OF WIND. 



The motion of air, in producing w^ind, is explained by 

 the action of heat, although there are many irregular cur- 

 rents whose cause is not well understood. The simplest 

 illustration of the effect of heat in causing currents is fur- 

 nished by the land and sea-breezes in warm latitudes. 

 The rays of the sun during the day heat the surface of 

 the land, and the air in contact with it, also becoming 

 heated, and thus rendered lighter, flows upward ; the air 

 from the sea rushes in to fill the vacancy, and causes the 

 sea-breeze. During the night, the radiation of heat from 

 the land into the clear sky above cools the surface to a 

 lower temperature than that of the sea ; consequently, 

 the air in contact with the sea becomes heated the most, 

 and rising, causes the wind from the land to flow in and 

 supply the place. Trade-winds are caused in a similar 



