278 rHii.osoPiiv OF storms. 



equator ihan at the j^oles. The greater quantity of vapor, loo, iti the 

 equatorial air, will cause it to staud about 1-90 higher ihan the polar air, 

 and, from these united causes, if the polar atmosphere be forty miles 

 high, the equatorial will be about forty-eight miles. — 3. Herschel says, 

 that since the earth revolves about an axis passing through the poles, 

 the equatorial portion of its surface has the greatest velocity of rotation 

 and all other parts less in the proportion of the radii of the circles of 

 latitude to which they correspond. The healed equatorial air, while it 

 rises and flows over towards the poles, carries with it the rotatory velo- 

 city due to its equatorial situation in a higher latitude, where the earth's 

 surface has less motion. Hence, as it travels northward or southward, 

 it will gain continually more and more on the surface of the earth in its 

 diurnal motion, and assume constantly more and more a icesterly rela- 

 tive direction, until, as the atmospheric elevation and rotatory velocity 

 diminish towards the poles, the air, as it rolls oflf down the inclined 

 plane of the surface of the atmosphere towards the north, will be con- 

 stantly passing over portions of the earth's surface which have a less 

 diurnal velocity than the part from which it set out, and, as from the na- 

 ture of inertia it still inclines to retain the diurnal velocity towards the 

 east, which it originally possessed, it will veer gradually round, and 

 when it reaches the latitude of about 20 or 25 degrees, it will then pro- 

 bably be moving nearly towards the north, and beyond that latitude its 

 motion will be north-eastwardly 5 while the air towards-lhe south will 

 first veer round towards the south, and then south-easlwardly. This will 

 be rendered plain to any person who will take up the terrestrial globe 

 and examine the operation of these two forces, bearing in mind at the 

 same time that the surface of the earth at the ecjuator moves at the rate 

 of 1000 miles in an hour, while at 60 degrees of latitude it revolves only 

 at the rate of 500 miles in the same lime. 



That such is ihe necessary operation of these causes is satisfactorily 

 proved by the cirrus-cloud, which forms at great elevations, and always 

 indicates the course of the upper current. 



]n our latitude this cloud always comes from the west, or rather a lit- 

 tle south of west; in the torrid zone it comes from the east; in north 

 latitude 25 degrees it comes from the south; and in the same latitude 

 south it comes from the north. A tornado, therefore, in Pennsylvania, 

 and probably throughout the northern and southern temperate zones, be- 

 ing guided by this upper current, in which the cirrus-cloud appears, will 

 move towards the east, or to a point a little north of east; in the torrid 

 regions it will move towards the west; and in intermediate latitudes it 

 will move towards the north and south respectively. Indeed, they will 



