302 HISTORY OF 



Upon narrowing the passage through which 

 the air is driven, both the density and the swift- 

 ness of the wind is increased. For, as currents 

 of water flow with greater force and rapidity by 

 narrowing their channels, so also will a current 

 of air, driven through a contracted space, grow 

 more violent and irresistible. Hence we find 

 those dreadful storms that prevail in the defiles 

 of mountains, where the wind, pushing from be- 

 hind through a narrow channel, at once increases 

 in speed and density, levelling, or tearing up, 

 every obstacle that rises to obstruct its passage. 



Winds reflected from the sides of mountains 

 and towers, are often found to be more forceful 

 than those in direct progression. This we fre- 

 quently perceive near lofty buildings, such as 

 churches or steeples, where winds are generally 

 known to prevail, and that much more powerful 

 than at some distance. The air, in this case, by 

 striking against the side of the building, acquires 

 additional density, and therefore blows with 

 more force. 



These differing degrees of density which the 

 air is found to possess, sufficiently show that the 

 force of the winds do not depend upon their ve- 

 locity alone; so that those instruments called 

 anemometers, which are made to measure the ve- 

 locity of the wind, will by no means give us cer- 

 tain information of the force of the storm. In 

 order to estimate this with exactness, we ought 

 to know its density ; which, also, these are not 

 calculated to discover. For this reason, we often 

 see storms with very powerful effects, that do not 



