414 PHYSICAL GEOGRAPHY OF THE SEA, AND ITS METEOROLOGY. ' 



806. The vortex of a cyclone is often and aptly compared to a 

 Cyclones in the Mis- Hieteor. I liavo often observcd the paths of such 

 sissippi Valley. through the forests of the Mississippi Valley, and 

 the path of one of these " whirlwinds " as they are there called 

 has in no instance that has fallen under my observation been 

 more than a few hundred yards broad. There the track of 

 these tornadoes is called a " wind-road," because they make an 

 avenue through the wood straight along, and as clear of trees as 

 if the old denizens of the forest had been felled with an axe. I 

 hcve seen trees three or four feet in diameter torn up by the 

 roots, and the top, with its limbs, lying next the hole whence the 

 roots came. Nevertheless, the passage of the meteor, whose nar- 

 row path was marked by devastation, would create a great com- 

 motion in the air, and there would be high winds raging for 

 several miles on either side of the "wind-road." But (§ 799) let 

 us consider for a moment the effect of the diurnal rotation of the 

 earth upon one of these revolving discs 1000 miles in diameter: 

 its height would scarcely be two miles, and its thickness would not 

 be as great, in proportion to its diameter, as half the thickness of 

 this leaf is to the length of an inch. Now the difference in rate of 

 the diurnal rotation between the northern and southern limbs of 

 the disc would be sufficient, irrespective of any other power, to 

 break it up. Suppose its southern limb to be in 20° N., its 

 northern limb would be 1000 miles, say 17°, farther north, that 

 is, in 37°. Diurnal rotation would carry to the east the air in 

 the southern limb at the rate of 845 miles an hour ; but when 

 this same air comes round on the northern limb, diurnal rotation 

 would carry it eastward at the rate only of 720 miles. Because the 

 wind hauls in a particular way, it does not follow, as by diagram 

 (§ 799) it has been shown, that it is blowing in a circle, or that 

 the centre of the storm is at right angles to its line of direction. 



807. In the extra-tropical regions of each hemisphere furious 

 Kxtra-tropicai gales, galcs of wiud also occur. Ouo of tliose, remarkable 

 for its violent effects, was encountered on the 24th of December, 

 1853, about three hundred miles from Sandy Hook, latitude 39° 

 north, longitude 70° west, by the " San Francisco," steam-ship. 

 That ship was made a complete -wreck in a few moments, and she 

 was abandoned by the survivors, after incredible hardships, exer- 

 tions, and sufferings. Some months after this disaster I received 

 by the California mail the abstract log of the fine clipper ship 

 " Eagle Wing " (Ebenezer H. Linnell), from Boston to San Fran- 



