142 A HISTORY OF 



Winds are generally more powerful on elevated situations than oc 

 the plain, because their progress is interrupted by fewer obstacles. 

 In proportion as we ascend the heights of a mountain, the violence 

 of the weather seems to increase until we have got above the region 

 of storms, where all is usually calm and serene. Sometimes, however, 

 the storms rise even to the tops of the highest mountains ; as we learn 

 from those who have been on the Andes, and as we are convinced by 

 the deep snows that crown even the highest. 



Winds b owing from the sea are generally moister, and more at- 

 tended with rains, than those which blow over extensive tracts of 

 land ; for the sea gives off more vapours to the air, and these are roll- 

 ed forward upon land, by the wind's blowing from thence.* For this 

 reason our easterly winds, that blow from the continent, are dry, in 

 comparison of those that blow from the surface of the ocean, with 

 which we are surrounded on every other quarter. 



In general the winds are more boisterous in spring and autumn than 

 at other seasons : for that being the time of high tides, the sea may com- 

 municate a part of its motions to the winds. The sun and moon, also, 

 which then have a greater effect upon the waters, may also have some 

 influence upon the winds : for, there being a great body of air surround- 

 ing the globe, which, if condensed into water, would cover it to the 

 depth of thirty-two feet, it is evident that the sun and moon will, to a 

 proportionable degree, affect the atmosphere, and make a tide of air. 

 This tide will be scarcely perceivable, indeed ; but, without doubt, it 

 actually exists ; and may contribute to increase the vernal and autum- 

 nal storms, which are then known to prevail. 



Upon narrowing the passage through which the air is driven, both 

 ihe density and the swiftness of the wind is increased. For, as cur- 

 rents 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 dread- 

 ful storms that prevail in the defiles of mountains, where the wind, 

 pushing from behind 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 

 frequently perceive near lofty buildings, such as churches or steeples, 

 where winds are generally known to prevail, and are much more pow 

 erful 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 different degrees of density, which the air is found to pos 

 sess, sufficiently shew that the force of the winds do not depend upoi. 

 their velocity alone ; so that those instruments called anemometers, 

 which are made to measure the velocity of the wind, will by no means 

 give us certain information of the force of the storm. In order 10 es- 

 timate this with exactness, we ought to know its density ; which also 

 these are not calculated to discover. For this reason we often 



Derham's Physico-Theol. 



