TRANSACTIONS OF SECTION A, 



643 



i<;i' whirl- 

 that the 

 mo- down 

 viiifr, and 

 .ownwai'ds 

 •int'd and 

 acuum, or 

 \ niition 

 ICC, and of 

 mtiunmis 

 •it(n'9 and 

 that it is 

 al ol' tlie 

 lunu, the 

 idvil't I'roni 

 a central 



ivatorspont 

 swavinsr, 

 f;o well 



accinnit ni 



lition.s and 

 ps a very 

 virt\n! of 



the cloudy 



stratum to penetrate down as an inner cove within that rev(dvin;^ a>ioending core 

 now itself hef'ome tubular. The cloudy stratum may he supposed not orifritially to 

 have been endowed with the revolutional motion or dillbiential horizontal motion 

 with which the lower stratum of thermally expanded air has been assumed to bo 

 orifrinally endowed. The up])er stratum of air from which the (doudy spindle core 

 is here taken to protrude down into tlie tubular funnel, is not to be supposed to bo 

 cold enoufrh to tend to sink by mere gravity. Though it were warm enough to 

 allow of its floating freely on the thermally expanded air below, it could still 

 he sucked down into the centre of the revolving ascending core of the whirlwind. 



The author wishes further to put forward the question as to whether it may 

 not bo possible, in some cases of whirlwinds, ior the barometric pressure in the 

 central or axial region to Itecome abated through the combined inlluences of rare- 

 faction by heat (increased, perhaps, by conditions as to included moist nre), on the 

 (ine hand, and of whirling motion, on the other hand, very uuudi beyond tlie abate- 

 ment that could be duo to beat or beat and moisture alone, without the whirling 

 motion, lie thinks it very likely tliat in great whirlwinds, including those which 

 profluoc the remarlcable phenomena called waterspouts, it may be impossible for the 

 whirling action to ho confined to tlie lower region of the atmosphoi'e ; but that, 

 oven if commenced there, it would s[eedily bo propagated to the top. It seems also 

 not unlikely, and in some trains of tlionglit it comes to appear very probable, that 

 llio whirling fluid, ascimding by its levity, would drive outwards from aljove it all 

 (itlier air endowed with less whirling energy, and would Ijc contiirially clearing 

 iiwiiy, upwards and outwards, the less energetic axial core whicli enti-rs from below, 

 and any, if such there be, that has entered from aljove. lie thinks the question 

 should at least be kept open as to wlietlier the whirling and scouring action may 

 not go forward, growing more and more intense, ])V(imnled always by energies 

 from the thermal sources winch have produced diflerences of temperatui'e and 

 moisture in ditl'erent parts of the atmosphere, and that thus a nuich nearer ajipi'oacli 

 to vacuum in the centre may lie caused than wouhl 1)0 due merely to the levity of 

 tlie superincumbent air, if jirl whirling. 



lie also Avishes to suggest that the dark and often Iriglitfnl cloud usually seen 

 in th(! early stages of wliirlwinds and waterspouts, and the dark colunniar revolving 

 core often seen ajiparently protruding downwards from the cloud, may be due to 

 preci]>itation of moisture into tlie cijiidition of fog or cloud, on account of aljate- 

 ment of pressure by ascension in level and (Uivironment witli whirling air, wdiich, 

 by its ctnitrifngal tendency, acts in protecting the axial region from the pressure 

 inwards of the surrounding atmosphere. 



Athloidiim. — A few brief I'xplanations and references will now bo added to 

 assist in the understanding of sonu* of tlie principles assumed in what has beim 

 already said. It is to he clearly understood that in a whirling iluid, even if the 

 velocity of the whirling motion be very snudl at great distances from the axis, if 

 the fluid be impidled inwards by fm-ces directed towards tlie axis, tlie absolute 

 velocity will greatly increase with diminution of distance from the axis, TIius in 

 the ir/iiripool of Froe Mohi/ifi/, in whicli the particles are perfectly free to move 

 outward or inward, the velocities of the ]>articles are invtn'sely proportional to the 

 distances from the axis, the fluid being understood to be inviscid or frictionless. 

 On this subject refereiu'o may be nnide to a paper by the author on ' Whirling 

 Fluids,' published in tlie ' Krit, Assoc, Iteport, i?elfast fleeting. LSol',' part ii. p. I'-U). 

 Agiiin, as to the inward flow causinl, in a frictionally retard 'd bottom lamina of a 

 whirlwind or whirlpool with vertical axis, by the frictional retardation from the 

 Imttom on which the whirling fluid rests, reference may be made to a paper liy the 

 author on the ' f xrand (Currents of Atmospheric Circulation,' in the 'British 

 Association Report, Dublin Meeting, 1^57,' part ii, p, ^iS, On another case of the 

 manifestation of the same principle, reference may be made to a paper by the 

 author in the 'Proceedings of the Koyal Society for .May 187G,' in respect to 

 the flow of water round bends in rivers, i^-c, witli reference to the eliects of 

 frictional resistance from the chaTmol in the bends, and to another paper by him in 

 the 'Proceedings of the Instituti(m of Mechanical Engineers, August 1879,' 

 p. 400, where the inward flow is explained as experimentally exhibited, 



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