TRANSACTIONS OF SECTION A. 643 



stratum to penetrate down as an inner core within that revolving ascending core 

 now itself become tubular. The cloudy stratum may be supposed not originally to 

 have been endowed with the revolutional motion or differential horizontal motion 

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

 originally endowed. The upper stratum of air from which the cloudy spindle core 

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

 cold enough 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 

 be 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 be possible, in some cases of whirlwinds, for the barometric pressure in the 

 central or axial region to become abated through the combined influences of rare- 

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

 one hand, and of whirling motion, on the other hand, very much beyond the abate- 

 ment that could be due to heat or heat and moisture alone, without the whirling 

 motion. He thinks it very likely that in great whirlwinds, including those which 

 produce the remarkable phenomena called waterspouts, it may be impossible for the 

 whirling action to bo confined to the lower region of the atmosphere ; but that, 

 even if commenced there, it would speedily be propagated to the top. It seems also 

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

 the whirling fluid, ascending by its levity, would drive outwards from above it all 

 other air endowed with less whirling energy, and would be continually clearing 

 away, upwards and outwards, the less energetic axial core which enters from below, 

 and any, if such there be, that has entered from above. He thinks the question 

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

 not go forward, growing more and more intense, promoted always by energies 

 from the thermal sources which have produced differences of temperature and 

 moisture in different parts of the atmosphere, and that thus a much nearer approach 

 to vacuum in the centre may be caused than would be due merely to the levity of 

 the superincumbent air, if net whirling. 



He also wishes to suggest that the dark and often frightful cloud usually seen 

 in the early stages of whirlwinds and waterspouts, and the dark columnar revolving 

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

 precipitation of moisture into the condition of fog or cloud, on account of abate- 

 ment of pressure by ascension in level and environment with whirling air, which, 

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

 inwards of the surrounding atmosphere. 



Addendum. — A few brief explanations and references will now be added to 

 assist in the understanding of some of the principles assumed in what has been 

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

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

 the fluid be impelled inwards by forces directed towards the axis, the absolute 

 velocity will greatly increase with diminution of distance from the axis. Thus in 

 the Whirlpool of Free Mobility, in which the particles are perfectly free to move 

 outward or inward, the velocities of the particles are inversely proportional to the 

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

 On this subject reference may be made to a paper by the author on ' Whirling 

 Fluids,' published in the ' Brit. Assoc. Eeport, Belfast Meeting, 1852,' part ii. p. 130. 

 Again, as to the inward flow caused, in a frictionally retarded bottom lamina of a 

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

 bottom on which the whirling fluid rests, reference may be made to a paper by the 

 author on the ' Grand Currents of Atmospheric Circulation,' in the ' British 

 Association Report, Dublin Meeting, 1857,' part ii. p. 38. 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 Royal Society for May 1876,' in respect to 

 the flow of water round bends in rivers, &c, with reference to the effects of 

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

 the 'Proceedings of the Institution of Mechanical Engineers, August 1879/ 

 p. 456, where the inward flow is explained as experimentally exhibited. 



t t 2 



