May 23, 1890.] 



SCIENCE. 



317 



the central line of movement, though thes(! tangents to the 

 major axis are not necessarily of equal length." 



Lieut. Finley summarizes his study of the relations of a 

 tornado to general atmospheric conditions as follows: 

 " There is a definite portion of an area of low pressure 

 within which the conditions for the development of torna- 

 does are most favorable, and this is called the dangerous 

 octant. Tornado regions are to the south and east of the 

 region of high contrasts in temperature (temperature 

 gradient) and in dew-points. The area of tornadic action is 

 to the south and east of the region of high contrasts of cool 

 northerly antl warm southerly winds, — a rule that seems to 

 follow from the preceding, and is of use when observations 

 of temperature and dew-point are not accessible. The rela- 

 tion of tornado regions to the movement of upper and lower 

 clouds shows that the former indicate the presence of the 

 cold north-west current, and the latter the warm south-west 

 current of air, which ultimately lead to the development of 

 the high contrasts of temperature so essential to the birth of 

 tornadic action. The study of the relations of tornado 

 regions to the form of barometric depressions shows that tor- 

 nadoes are more frequent when the major axis of the baromet- 

 ric trough trends north and south, or north-east and south- 

 west, than when it trends east and west." 



From Lieut. Finley 's " Scientific Resume of Tornado 

 'Characteristics," I have selected the following as touching 

 upon points not already mentioned. 



" The time of day, the time of year, and the peculiar hot 

 and stifling condition, indicate that heat is the physical 

 agent developing the tornado. By the rotary action of the 

 tornado-cloud the condensed vapor is whirled into a fine 

 mist, giving it the appearance of steam, and lighting the 

 interior of the cloud. The tornado is accompanied by a 

 rumbling noise (very peculiar), which never ceases while 

 the funnel-shaped cloud is upon the earth or a short distance 

 above it. The funnel form of the cloud is due to the peculiar 

 ascensional movement of air-currents, the vapor , being con- 

 , _ densed along the central line of movement by the cold of 

 ' elevation. The motive power of a tornado, and the agency 

 which lifts objects or carries them long distances, is that 

 motion of the air in tlie cloud set up by the variable heat 

 conditions of large masses of air over adjacent regions. 



" The tornado vortex may be formed either by an ascen- 

 sional movecnent of a mass of heated air, giving rise to un- 

 stable equilibrium, or by the meeting of opposite currents 

 with higli temperature gradients, or by a combination of 

 these. Two currents of air approaching each other from 

 opposite directions will not come directly together, because 

 of the influence of the relative motion of the earth. The 

 mass of air coming from the south would have a greater 

 velocity eastward than that coming from the north: there- 

 fore, instead of meeting each other in a direct line, the two 

 currents will form an angle at their intersection, and the 

 combination of the two masses will give rise to a rotation in 

 a direction contrary to the hands of a watch with its face 

 upwards. These conditions account for the spiral movement 

 of the air-currents and the formation of the vortex in the 

 tornado. The cold air from the northward will under-run 

 the warmer air from the southward, because of the difference 

 in density of the two masses, and as a result will aid in the 

 formation of the whirl. 



" The electrical tension of the air cannot, under the most 

 favorable atmospheric conditions, cause the movement of 

 oppositely elect^rified air masses, because of the excellent con- 

 ductivity of free air, which always tends to equalize electric 

 potential. The presence of ozone is usually detected in the 

 wake of the tornado. People are stripped of clothing, fowls 

 and birds denuded of feathers. The peculiar roaring noise 

 which accompanies the progress of the tornado cannot be 

 ascribed to the intervention of electrical forces : it is far more 

 reasonable to assert that the noise is produced by the resist- 

 ance which the rapid and violent indraughts of air en- 

 counter while passing into the tornado's vortex. The vortex 

 approximates a vacuum, and the air rushes into it at the 

 spout end near the earth with great violence, atltended by a 

 hollow, sucking sound of marked intensity. 



" The peculiar sensations of what are termed 'burning,' 

 ' scorching,' or ' stifling' heat, which are reported by those 

 who experience the violence of the tornado, must be due to 

 the latent heat of vaporization, which is given off in great 

 quantities by the extremely rapid condensation that attends 

 the tornado as a constant feature." 



A very interesting comparison may be made between 

 tliese later views and the appearances noted earlier. Lieut. 

 Finley seems to have slightly confused the conditions which 

 prevail at the centre of the general storm with those at the 

 tornado. Great contrasts of temperature and dew-point, 

 meeting of hot south with cold north winds, etc., can only 

 occur at the centre of the general storm, but that point is 

 four hundred miles from the tornado. It seems probable 

 that altogether too much emphasis has been given to the two 

 clouds, — one in the north-west, and the other south-west — 

 which are described as gathering themselves together as two 

 giants eager for the coming contest. We have already seen 

 that these clouds are an accompaniment of the tornado. 

 They are probably never more than one or two miles apart. 

 They seem to rush together, because, when first seen, a cloud 

 of dust hides the true tornado, and these appendages appear 

 prominently on either side. In a minute or two the ob- 

 server is enveloped in the dust, and the next instant he sees 

 the death-dealing funnel, and his heated imagination at once 

 leads him to the conclusion that the funnel is produced by 

 the forcible meeting of the clouds. 



Contrasts of Temperature. 



Lieut. Finley especially insists on such contrasts as a vera 

 causa in the formation of a tornado, but it would seem as 

 though he has entirely mistaken the mode of action gener- 

 ally ascribed to an unstable equilibrium. We are impressed 

 with the fact that the clouds and resulting contrasts are on 

 the same level, whereas, in order to have a contrast capable 

 of causing an upsetting in the atmosphere, it is absolutely 

 essential that the cold air be above the hot. Lieut. Finley 

 expressly declares against this disposition by stating that the 

 colder air, by its density, under-runs the warmer. It is a 

 matter of deep regret that other writers on this subject have 

 not seen that such a condition as the last mentioned en- 

 tirely negatives their theories of tornado formation. 

 Tornado Whirls. 



No one who has read all that precedes can be more aston- 

 ished than the present writer to Bnd that the evidence proving 

 a whirl is so inconclusive and conflicting. I had fully made 



