354 



SCIENCE. 



[Vol. XV. No. 384 



gradually extend downward ; for the gyrations cause a great 

 diminution of tension and of density, and the air conse- 

 quently in the centre rushes up with great velocity, and that 

 below of the still unagitated strata is drawn in to supply its 

 place, which likewise runs into gyrations around the centre, 

 so that the gyrations in a very short time extend down to 

 the earth's surface. The whole column of gyrating air is 

 like a tall flue containing very rarefied air, the centrifugal 

 force of the gyrations acting as a barrier to prevent the 

 inflow of air from all sides into the interior; and if the 

 gyrations at the earth's surface were as rapid as those above, 

 it would be similar to such a flue with all the draught cut 

 off. 



"But very near the earth these gyrations, and conse- 

 quently the centrifugal force, are very much diminished on 

 account of the friction at the surface, and this allows the 

 air to rush in quite near the surface to supply the draught 

 of the interior ascending current " (see American Journal 

 of Science, July, 1881). 



These views are repeated or amplified in "Recent Ad- 

 vances in Meteorology" (1886), with the following sugges- 

 tions added: " When the air expands as it is heated, it re- 

 quires a greater quantity of heat to raise its temperature 

 through 1°, since in this case work is done, and it is done 

 at the expense of heat supplied. An additional amount of 

 heat, therefore, the equivalent of the work done, has to be 

 supplied." "The complete temperature conditions of a 

 cyclone, therefore, rarely extend down to the earth's surface, 

 but the interchanging and gyratory motions, commencing 

 first up in the cloud regions, are soon propagated downwards 

 tp the earth's surface by the action through friction of the 

 upper strata upon the lower ones." Speaking of unstable 

 equilibrium, we find, " Currents of air at the earth's surface 

 which come from a warmer latitude are caused to flow 

 under the colder upper strata, where the normal motion is 

 nearly eastward. In the south-east octant within the 

 cyclone, the surface currents are from the south, bringing 

 warm and moist air northward under the cold-air currents 

 above from the north. This inci'eases the temperature 

 below, and decreases it above, and gives rise to the large 

 vertical gradient of temperature, decreasing with increase of 

 altitude, which is necessary to the unstable state. " 



A few final quotations are made from "Popular Treatise 

 on the Winds" (1889) :— 



"The pressure near the centre of tornadoes becomes very 

 much diminished, and in their passage over a place there is 

 sometimes a very sudden change in pressure. Corks fly 

 from empty bottles, cellar-doors are burst open against the 

 force of a strong wind blowing against them on the outside, 

 the walls of houses are thrown outward on all sides." "The 

 direction of the general drift of the air is very nearly that of 

 the progressive motion of the tornado, and so mostly from 

 south-west to north-east. The velocity of this is always con- 

 siderable in comparison with, though generally much less 

 than, the gyratory velocity of the violent part of the tor- 

 nado." Professor Ferrel quotes a statement regarding the 

 fall of trees in the Gentry County tornado: "Those on the 

 south (right-hand) side of the centre were pointing to the 

 east and north-east, and even north-west when very near 

 the centre. On the north side they were pointing north- 

 west, west, south-west, and south-east." 



Hail-Storms. 



"A hail-storm is simply a tornado in which the ascending 

 currents are so strong, and reach so high up into the upper 

 strata of the atmosphere, that the raindrops are carried up 

 into the cold regions above, where they are frozen into hail." 

 The theoretical velocity needed to keep up a hail-stone 2.58 

 inches in diameter is one hundred miles per hour. The fall 

 of rain and hail is said to always precede the tornado by ten 

 to thirty minutes. 



Thunder-storms. 



"The fundamental conditions of thunder-storms, as of cy- 

 clones and tornadoes, are the state of unstable equilibrium, 

 at least for saturated if not for dry air, and a high relative 

 humidity. In what are usually called thunder-storms, the 

 conditions are nearly or quite absent which give rise to a 

 gyratory circulation over a large area, such as takes place in 

 the case of cyclones, and usually the conditions are wanting 

 which give rise to small local and violent tornadic gyrations, 

 though most tornadoes are thunder-storms. According to 

 Finley, "of 473 cases in which the atmospheric conditions 

 preceding tornadoes were observed, 410 were reported as vio- 

 lent thunder-storms." If the air is in the unstable state, and 

 over a given circular area is a little warmer and lighter 

 than that of the sui-rounding parts, there is set up a vertical 

 circulation, with an ascending current in the interior, and 

 an incoming current from all sides in the lower part of the 

 air, to supply the ascending current. In the interior as- 

 cending current the height of incipient condensation and of 

 the base of the cloud depends upon the depression of the 

 dew-point of the air; and the aqueous vapor above that 

 height is condensed, falls as rain, and cools the air through 

 which it falls, until its temperature is lower than that of the 

 surrounding air. This central cooled air, being now header 

 than the surrounding air, both on account of its greater 

 density and the amount of falling rain pressing on it, now 

 gradually settles down, and causes an outward current in all 

 directions from the centre. If there were at any one time 

 rain and hail falling with uniform velocity equivalent to a 

 rainfall of 13.6 millimetres in depth, it would increase the 

 barometric pressure 1 millimetre; and from this alone would 

 arise a squall with a velocity of about 34 miles per hour, 

 making no allowance for friction." 



More than two thousand pages have been perused in mak- 

 ing these quotations. While no one person, perhaps, could 

 give a perfect resume of such a mass of matter, yet it is hoped 

 that no important theory has been omitted. 

 Faye's "Views. 



It is necessary to mention one other theory that has been 

 maintained by M. Faye of Paris. He has made a study of 

 the appearances on the sun around his spots, and has been led 

 to conclude that action in terrestrial storms is analogous in 

 many respects. He thinks that this action originates in the 

 upper atmosphere, and is propagated downward to the 

 earth. A storm is practically a whirl in the swift-moving 

 easterly current, similar to whirls in streams where an ob- 

 struction exists. This theory calls for a single remark. 

 There does not seem to be a cause existent in the atmosphere 

 in any degree adequate to set up such a whirl, unless we ap- 

 peal to some force outside of the motion of the upper cur- 

 rent. The transfer of the origin of the storm to the upper 



