10 PHILOSOPHY OF STORMS. 



half miles per hour, besides the motion which it may have 

 had towards the west at the earth's surface. 



27. As this air rolls off down the inclined plane of the 

 surface of the atmosphere towards the poles, it will, while 

 near the equator, fall a little west of the meridian, but it 

 will recede from the equator a few degrees only before by 

 the diminished diurnal velocity of the latitude to which it 

 has arrived, it will first move along the meridian, and then 

 east of it, and thus, perhaps, storms near the northern tropic 

 will be found to move towards the north, and storms near 

 the southern tropic towards the south. 



That the air does roll down an inclined plane in the 

 upper parts of the atmosphere from the equator towards the 

 poles will appear from the consideration that the mean tem- 

 perature of the air in the torrid zone is about 80 greater 

 than in the frigid zones, and as the mean temperature of 

 the air in the frigid zones is about zero, the air is 5 \ F of its 

 whole height higher at the equator than at the poles, or in- 

 deed very nearly that much higher at the tropics than at 

 the polar circles. 



The greater quantity of vapor, too, in the equatorial air, 

 will cause it to stand about ^ higher than the polar air, 

 and, from these united causes, if the polar atmosphere is 

 forty miles high, the equatorial will be about forty-eight 

 miles. 



It is of great consequence to meteorology, that the direc- 

 tion and velocity of these uppermost currents in the at- 

 mosphere should be ascertained. 



It has been thought by some that the fall of an immense 

 quantity of ashes on Barbadoes in 1812, upon an eruption 

 of a volcano in St. Vincent, proved that the current of air 

 at a great elevation in the torrid zone, is from west to east. 



But when it is considered that the force of a rapidly 

 rising column. of air is great enough to cause the upper 

 parts of this column to puff out even against a strong wind, 



