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SCIENCE. 



[Vol. XVII. No. 430 



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TORNADOES: A STORY OF A LONG INHERITANCE.' 



After illustrating the effects of a number of tornadoes by 

 lantern-slides, the lecturer defined a tornado as a violent whirling 

 storm of small dimensions, rapid progression, and brief duration, 

 and then considered the origin of its destructive winds. Follow- 

 ing the generally accepted theory that the tornado whirl is devel- 

 oped in a eonvectional up-draught, it was shown, by analogy with 

 the eddy of water running from a basin by a vent at the bottom, 

 that if tornadoes did not whirl, they would lose most of their 

 violence. But they all whirl, and nearly all in the same direc- 

 tion, — from right to left. The general possession of so well- 

 marked a feature implies that it has been inherited from some 

 antecedent condition, and it was therefore asked, where are 

 tornadoes formed? The records of the Signal Service leave no 

 room for doubt on this point : tornadoes are nearly always formed 

 in the south-eastern quadrant of the large cyclonic storms or areas 

 of low pressure, so characteristic of our daily weather-maps, and 

 to whose passage across the country we owe most of our weather- 

 changes. The cyclonic storms are vast whirls, their winds sweep- 

 ing over great spirals as tbey gradually approach the centre of low 

 pressure, but generally without destructive velocity, at least on 

 land. The spirals of our cyclonic storms universally turn from 

 right to left, and in this motion we undoubtedly have the reason 

 for the general rigbt-to-lef t whirling of the tornadoes ; for, when 

 a little whirl springs up in a great whirl, the turning of the two 

 will be in the same direction. This may suffice to show why 

 tornadoes turn ; but it may next be asked why cyclonic storms 

 turn. An answer will be found by examining the region of their 

 occurrence. They are developed in the belt of prevailing westerly 

 winds, which, taken as a whole, form a vast whirl from right to 

 left around the north pole. When the cyclonic disturbance arises 

 Ln this polar whirl, it must turn in the same direction as the polar 

 whirl turns; that is, again from right to left. Tornadoes may 

 therefore be said to have inherited their habit of turning from 

 iheir grandparent, the general circulation of the winds of the 

 northern hemisphere around the north pole. 



But why do the winds whirl around in this way ? Why not the 

 other way? Why do they whirl at all? The sun warms the air 

 at the equator, while it is cooled at the poles ; the expanded equa- 

 torial air flows away aloft north and south, and for this reason we 

 should expect to find caps of high pressure around the poles ; but 

 it must be remembered that the interchange between equator and 



1 Abstract ol a lecture before the Jobns Hopkins University Travellers' 

 Club, Jan. 27, 1891, by Professor William Morris Davis of Harvard Univer- 

 8lty. 



poles was established in an atmosphere that was already rotating 

 with the earth on which it lay. It possessed this rotation along 

 with the oceans in the youth of the earth, when all was still 

 glowing and molten with heat; and it was only later on, when the 

 earth had cooled somewhat, that the sun began to determine our 

 climatic zones, and start an atmospheric circulation: hence, as 

 the equatorial overflow runs poleward, it approaches the axis about 

 which it rotates. In accordance with the principle of the conser- 

 vation of areas, it must take on a whirl around the pole from west 

 to east, or, as the North Star would say, from right to left; and 

 this whirl is so much faster than the rotation of the earth that the 

 high pressure expected at the poles as a result of low temperature 

 is reversed into low pressure, due to excessive centrifugal force. 

 We thus learn that the prevailing winds whirl around the pole 

 because they had a way of turning with the earth ; that the cy- 

 clonic storms possess a spiral circulation from right to left because 

 they are formed in a whirling atmosphere ; and that the tornadoes 

 whirl because they are generated in whirling cyclones. 



But why does the earth rotate? On inspecting the planets of 

 our system, we find that rotation apj)ears to be a common charac- 

 terislic of all. The sun, the moon, Mars, Jupiter, and Saturn all 

 turn one way, these being the only bodies of pur system whose 

 direction of rotation has been surely observed. Moreover, they al! 

 turn on their axes in the same way as they revolve around the sun 

 in their orbits. Saturn's rings turn in the same direction. Let 

 us imagine what would happen if these rings were clotted some- 

 what at a certain point : the parts behind the clot would be hurried 

 on, and thus gaining a greater orbital velocity, and consequently 

 a greater centrifugal force, would tend to pass outside of the clot ; 

 the parts ahead of the clot would be retarded, and, thus losing 

 some of the centrifugal force that they had before, would be drawn 

 by the planet somewhat inside of the clot; the parts outside of the 

 clot would be drawn inwards, and, thus approaching the centre of 

 their orbital revolution, they would be accelerated, and would 

 tend to run ahead of the clot ; while the parts on the inside of the- 

 ring would be drawn outwards, and would lag behind the clot. 

 All these parts thus conspire to set up a whirling around the clot 

 as a centre, still maintaining their orbital motion around Saturn. 

 As a result, when all the matter of the rings is concentrated at the 

 clot, it will form a mass possessed of an axial rotation; and this 

 rotation will be in the same direction as its orbital revolution. It 

 has therefore been supposed that the planets once existed as rings 

 around the sun ; that the rings were not so evenly balanced as are 

 those of Saturn, which survive as rings even to this day; and that 

 the planetary rings gradually coalesced into rotating balls, and thus 

 gained their community of rotation. And yet why should the 

 planetary rings have all rotated the same way? For no reason, 

 unless they inherited their movement from a common ancestor. 

 This ancestor is thought to have been a vast nebula, whose inward 

 spiral falling together gradually produced the rings, all turning 

 one way around the great central mass, which later formed the 

 sun. But why did the nebula turn around ? Why. did its parts 

 not simply fall together in radial lines ? Because the nebula came 

 from chaos, and we must not imagine that chaos possessed so 

 specialized an arrangement as no motion, or as precisely such 

 motions as would neutralize all tendency to rotation while its parts 

 were falling towards their common centre of gravity. Any thing 

 but this in chaos. There must have been motions of all kinds, and, 

 their resultant being unbalanced with respect to their centre, they 

 necessarily developed a whirl as they coalesced into the primeval 

 nebula; and this whirl, thi-ough rings, planets, winds, cyclones, 

 and tornadoes, has never been lost. 



It is not simply to the imagination that we must trust for our 

 realization of these past stages of our history. The sun, being vastly 

 larger than the earth, still retains a glowing temperature, such as. 

 the earth has long since lost. Saturn's rings, evenly balanced, 

 marvellous examples of retarded development, illustrate a stage 

 long out of date with the unevenly arranged rings of the planets. 

 Most of the nebulse of the distant sky are still in the chaotic stage;, 

 but the great nebula of Andromeda, when finely photographed, 

 shows a series of incurving spirals, such as the North Star saw in 

 our nebula so long ago. It is the inheritance of this early habit 

 that makes our tornadoes whirl. 



