45$ Theory of 'Winds. [BookV. 



c The winds are more violent at certain heights than 

 upon the plain, and the higher we afcend Jolty moun- 

 tains, the greater is the force of the wind, till we get 

 above the ordinary heights of the clouds. Above this 

 the fky is ufually fcrene and 'clear. The reafon is, 

 that the wind, at the lurface of the earth, is continually 

 interrupted by hiils and rifings : fo that, on the plnin, 

 between any two of thele, the inhabitants are in a kind 

 of fhelter; but when once the interpofition of fmall 

 hills no longer ftops the wind's courfe, it then .becomes 

 itrbnger, as the interruptions it meets with are fewer. 

 At the tops of the higher mountains its interruptions 

 are leaft of all ; but it does not blow with violence 

 there ; for its denfity is fo much diminiihed by the 

 height, that its force is fcarcely perceptible, and the 

 fform falls mid;vay below. What is commonly called 

 a high vvind moves at the rate of about thirty-five 

 miles an hour. 



' A current of air al-ways augments in force in pro- 

 portion as the pafihge through which it runs is dimi- 

 nilhed. The law of this augmentation is, that the air's 

 force is compounded of , its fwiftnefs and denfity, and 

 as thefe are increafed, fo will the force-of the wind. If 

 any quantity of wind moves with twice the fwiftnefs of 

 a fimilar quantity, it will have twice its force ; but if, 

 at the fame time that it is twice as fwift, it moves 

 through twice a fmaller tube, and the fides of the canal 

 give no refiftanceto its motion, it will have four times 

 the force. This, however, is not entirely the cafe ; 

 for the fides of the tube give a refiftance, ancl retard 

 its motion, in a proportion that is not eafily calculated. 

 From this increafe of the wind's denfity in blowing 

 through narrow pafiages, it is that we fee the dorms fo 

 very violent that fometimes blow between two neigh- 

 bouring hills. It is from this, that when caugbt in 



