280 THE ATMOSPHERE AND METEOROLOGY. 



CHAPTER X. 



SPEED OF THE EEVOLVING MASSES OF AIR. — SPEED OF THE CYCLONE. — FALL OF 

 THE BAEOMETRIC COLUMN. — IRREGULAKITIES OF THE WIND IN THE PATH OP 

 THE CYCLONE. 



It is not yet known what degree of swiftness the masses of air 

 cavried by the cyclones can attain, for it is in the upper regions of 

 the atmosphere, where the medium only offers a feeble resistance to 

 the aerial currents, that the storm -wind must have its greatest rapidity. 

 And it does not suffice to ascertain the progress of the particles of air 

 immediately at the level of the ground, or even slightly above it, to 

 form an idea of the speed at which the atmospheric mass carried by 

 the hurricane moves. In one of his ascents Mr. Coxwell made a 

 journey of 68 miles in 60 minutes, while below him the instruments 

 indicated a speed of hardly 14 miles in the same interval. Another 

 time Mr. Glaisher moved at 15 miles per hour, while at the Greenwich 

 observatory the same sheet of air only advanced 500 yards. How great, 

 then, is the speed of the cyclone at a certain height above the ground 

 when on the earth strewn with obstacles it progresses at the rate of 

 50 yards per second, or 100 miles per hour, four times the speed of 

 our locomotives ! This fearful rapidity of the air at the surface of 

 the ocean, and the friction of the aerial particles which results, ex- 

 plains perfectlj^, as Cicero remarked 2000 years ago, why the tem- 

 perature of the water rises during storms.* 



As to the pressure exercised by the aerial current which moves with 

 such speed, it is truly formidable. In a memoir on the Construction 

 of Lighthouses, Fresnel estimated the strongest pressure of the wind 

 at 616 lbs. per square yard, but it is very probable that in a number 

 of hurricanes this figure has been greatly surpassed. Not to mention 

 the effects produced by the great cyclones of the tropics, a number of 

 cases have presented themselves in the temperate zone where the 

 pressure exercised by the wind on a space of little extent was much 

 greater than meteorologists had foreseen. Thus, to cite but one 

 example, the storm of the 27th of February, 1860, coming from the 

 * Be Natura Beorum — Zeitschriff fiir Erdhoide, March, 1864. 



