May z\, i838] 



NATURE 



105 



some extent, owing to the earth's rotation), relative to 

 some central point. 1 



Given these conditions, the rest follow as necessary 

 consequences, viz. (1) a current ascending up the axis, 

 combined with rapid rotation round it ; (2) a hyperboloidal 

 funnel of rarefied air tapering* downwards, and reaching 

 the earth when the action is powerful, round the sides of 

 which a condensed vapour-, or so-called water-spout, should 

 usually prevail, owing to the sudden rarefaction of the air 

 entering the central area through the sides or at the base, 

 with the consequent lowering of the plane of condensation 

 from the cloud-level which it usually occupies. When, 

 therefore, it is said that " a waterspout is simply the cloud 

 brought down to the earth by the rapid gyratory motion 

 of the tornado," 2 it is not meant that the cloud is actually 

 carried downwards by an aerial current, since by theory 

 the motion is precisely in the opposite direction ; but that 

 the conditions of condensation are propagated downwards 

 from the cloud-stratum where they first commence. 

 Neglect of this consideration, as well as the physical fact 



that condensation can only occur under most exceptional 

 circumstances in a downward current, has led to many 

 false deductions from apparent circumstances. 



Theory, moreover, indicates that the current up the 

 axis, together with gyration round it, which, by the con- 

 servation of rotational momentum, may become exceed- 

 ingly rapid as the air approaches it, must combine to give 

 a spiral character to the movement near the axis, while 

 the conditions of continuity equally demand that there 

 should be a compensatory descending current somewhere 

 in the vicinity, gyrating spirally in the same sense, and of 

 only moderate velocity, owing to its greater distance from 

 the axis. 



At the base of a tornado, or its milder form of water- 

 spout, there should also be a rising up of the water at sea, 

 or of light objects on land, which are supported by the 

 ascending current until their collision or size carries them 

 outside the central area, when they fall back to the earth, 

 or to points where they are again brought within the 

 influence of the whirl-currents. These and many other 



Fig. i. 



minor characteristics of tornadic action are confirmed and 

 illustrated by M. Weyher's experiments. 



M. Weyher commences by examining the conditions 

 which prevail in an eddy produced in water, either by an 

 outflow through a sluice, or a momentary rotation im- 

 parted by the stroke of an oar. In the former case the 

 motion is well known, but in the latter it is somewhat 

 new to find that besides the rotation round a vertical axis 

 there is an interchanging vertical motion such that each 

 particle describes a descending helix down the axis of the 

 whirl, and ascends in a helix of the same sense to regain 

 the surface. 



Fig. 1 shows the same circulation produced by the 

 revolution of a tourniquet, A. 



If this figure be looked at upside down, it substantially 



1 Sergeant Finley found, in his review of 600 tornadoes in the United 

 States, that the direction of rotation round the axis was invariably cyclonic, 

 or against watch-hands ("Signal Service Notes," No. xii. p. 10). 



- " Recent Advances in Meteorology " (p. 301), by W. Ferrel. (Washing- 

 ton, 1885. 



depicts what is believed to be the motion of the air in a 

 whirlwind, waterspout, or tornado, and is precisely similar 

 to what is found to be the motion round those artificially 

 produced by M. Weyher. 



The important point to notice with respect to the 

 water eddies, which are introduced mainly to show their 

 analogy to air-whirls, is that, according to M. Weyher, 

 their source of action must be at some distance below the 

 surface. By artificially causing the liquid to rotate at its 

 surface only, he found it impossible to obtain the central 

 descending funnel of a complete water eddy. 



A similar condition is found to hold in an inverse sense 

 in the case of artificially- produced air-whirls. The action 

 must in their case originate in the upper part of the air- 

 column, whence the motion is communicated by degrees 

 to its lower boundary. The analogy, therefore, between 

 the water eddy with a descending motion round its axis 

 and the atmospheric whirlwind is completely inverse, 

 and not direct, as some have supposed. 



