THi; KARTH. 



237 



In this manner, as a denser air produ<'es a wind, on one 

 hand; so will any accident, that contributes to lighten the air, 

 produce it on the other : for a lighter air may be considered as 

 a vacuity, into which the neighbouring air will rush -. and hence 

 it happens, that when the barometer marks a peculiar lightnes » 

 in the air, it is no wonder that it foretells a storm. 



The winds upon large waters are generally more regular than 

 those upon land. The wind at sea generally blows with an even 

 steady gale ; the wind at land puffs by intervals, increasing its 

 strength, and remitting it, without any apparent cause. This, 

 in a great measure, may be owing to the many mountains, towers, 

 or trees, that it meets in its way, aU contributing either to turn 

 it from its course, or interrupt its passage. 



The east wind blows more constantly than any other, and for 

 an obvious reason : all other winds are, in some measure, devia- 

 tions from it, and partly may owe their oiigin thereto. It 

 is generally, likemse, the most powerful, and for the same 

 reason. 



There are often double currents of the air. While the wind 

 blows one way, we frequently see the clouds move another. 

 This is generally the case before thunder ; for it is well known 

 that the thunder cloud always moves against the wind : the 

 cause of this surprising appearance has hitherto remained a se- 

 cret. From hence we may conclude, that weathercocks only 

 inform us of that current ot the air which is near the surface of 

 the earth ; but are often erroneous with regard to the upper re- 

 gions, and, in fact, Derham has often found them erroneous. 



"Winds are generally more powerful on elevated situations than 

 on the plain, because their progress is interrupted by fewer ob- 

 stacles. 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 v,e ai'e convinced by the deep snows that 

 croHii even the highest. 



Winds blowing from the sea are generally moister, and more 

 attended with rains, than those which blow over extensive tracts 

 of land ; for the sea gives off more vapours to the air, and these 

 are roiled forward upon land by the wind's blowing irom 



