458 PHILOSOPHY OF STORMS. 



an unusual tendency to rise in vortices as it became heated 

 in passing over the heated sands of Egypt; and contrary 

 to what is the fact, should there produce incessant rain. 1 

 3d. In like manner, how does it happen that at Marseilles, 

 during the depth of winter, when the wind shifts from the 

 north,' and begins to blow from the south, that is, blows 

 from the Mediterranean, and from a warm towards a com- 



1 This objection seems to be founded on an illogical deduction from my 

 doctrine, that because all rains and snows and hails are produced by an up- 

 moving current of air, therefore all up-moving currents of air must produce 

 rains, hails, or snows. This does not follow : for if any one will take the 

 trouble to look how cumuli are formed in a summer day, he will see them 

 sometimes swelling up to a great height, and then, not yet having got dense 

 enough to rain, their tops will be swept off by an upper current, moving in a 

 different direction from themselves, or with a different velocity ; and they 

 will thus become spread out along the heavens, and their up-moving power 

 destroyed. Now, when the north wind blows in Egypt, the current below is 

 almost diametrically opposite to what is known to be the direction of the cur- 

 rent above in that latitude. 



Besides, the current above contains all the caloric of elasticity which was 

 given out to it, during the great condensation of the vapor which produces 

 the mighty rains as it passes over the mountains in Abyssinia : so that it will 

 contain very much more caloric to the pound than even the hot air on the 

 surface of the ground in Egypt ; and, therefore, when the up-moving cur- 

 rents over that country rise to the height of this upper current which is flow- 

 ing off towards the north, they will enter a medium of less specific gravity 

 than themselves, and on that account they will cease to rise. 



Besides, I have long observed that if the dew point is more than 20 below 

 the temperature of the air, cumuli hardly form, though the day is entirely 

 clear, and up-moving columns forming as usual. This circumstance is easily 

 understood, when it is known, as it is by experiment, that these columns cool 

 about one degree and a third for every hundred yards that they ascend, whilst 

 the strata of the atmosphere itself are only one degree colder for every hun- 

 dred yards high. From these two facts it follows, that though the columns 

 start upwards by their specific levity from greater heat near the ground, they 

 are constantly, in their ascent, approximating nearer and nearer to a state of 

 equilibrium with the surrounding air at their own elevation, and finally must 

 cease to rise, unless they reach the point where cloud begins to form, and then 

 as the law of cooling is changed to about two degrees for three hundred yards, 

 the upward motion will be continued, unless hindered by some of the causes 

 mentioned above. 



