XX11 INTRODUCTION. 



the prevailing direction of the trade winds is north easterly, and 

 there the hurricanes move towards the north west. It is probable, 

 however, they sometimes bend out of their course, to pass over an 

 island, lying near the track, which they would otherwise pursue. 



If a tornado should stop in its motion for a few seconds, as it 

 might do, on meeting with a mountain, it would be likely to pour 

 down an immense flood of water or ice, in a very small space, for 

 the drops which would be carried up by the ascending current 

 would soon accumulate to such a degree as to force their way back, 

 and this they could not do without collecting into one united stream 

 of immense length and weight, and of course on reaching the 

 side of the mountain ; this stream, whether it consisted of water or 

 hail, would cut down into the side of the mountain a deep hole, 

 and make a gully all the way to the bottom of the mountain, from 

 the place where it first struck. (192 to 200.) 



As the air spreads out more rapidly above than it runs in below, 

 there will be a tendency in storms to increase in diameter, and 

 this tendency will be greater on the north side than on any other, 

 for the efflux above finds less resistance on that side, for a reason 

 assigned in the next paragraph ; therefore it is probable that storms 

 become elongated north and south, and then, if they move to- 

 wards the east, they must travel side foremost. 



At the equator, or at least those parts of it where the trade winds 

 are constant from east to west, it is probable tornadoes travel from 

 east to west. (147.) For as the air in the torrid zone is about 80 

 in temperature at a mean, and the air in the frigid zone is about 

 zero, the air in the torrid zone is constantly expanded by heat about 

 A ? f i ts whole bulk in the frigid zone. This will cause the airat 

 the equator to stand more than seven miles higher from the sur- 

 face of the earth to the top of the atmosphere than at the north 

 pole. The air, therefore, will roll off from the torrid zone both 

 ways towards the poles, causing the barometer to fall in low lati- 

 tudes, and rise above the mean in high latitudes. This will cause 

 the air to run in below towards the equator, and of course rise 

 there. Now from the principle of the conservation of areas, it 

 will recede more and more to the west as it rises, and of course 

 the upper current of the air, at the equator, probably moves to- 

 wards the west. However, as the air rolls off above, towards the 

 north, it will be constantly passing over portions of the earth's sur- 

 face, which have a less diurnal velocity than the part from which 

 it set out, and as from the nature of inertia it still inclines to retain 

 the diurnal velocity towards the east, which it originally possess- 

 ed, when it reaches the latitude of about 20 or 25 degrees, it will 

 then probably be moving nearly towards the north, and beyond 

 that latitude its motion will be north easterly. The effect of the 

 air rolling off above from the equator towards the poles, when 



