642 report— 1884. 



rather than the effect of the rapid -whirling motion, though in some respects, 

 indeed, these two conditions can he regarded as heing mutually causes and effects, 

 each heing essential to the maintenance of the other, while there are also some 

 further promoting causes or conditions not as yet here mentioned. 



It seems indubitably to be the truth that ordinarily for the genesis of a whirl- 

 wind the two chief primarily promoting 1 conditions are : — Firstly : A region of 

 diminished barometric pressure ; — this diminution of pressure being, it may be 

 presumed, due to rarefaction of the atmosphere over that region by heat and some- 

 times further by its condition as to included watery vapour ; and, secondly : A pre- 

 viously existing revolutional motion, or differential horizontal motion, of the 

 surrounding air ; such revolutional or differential motion being not necessarily of 

 high velocity at any part. 



The supposed accumulation of air rarefied by heat or otherwise, for producing 

 the abatement of pressure, may, the author supposes, in some cases extend upwards 

 throughout the whole depth of the atmosphere ; and in some cases may be in the form 

 of a lower warm lamina which somehow may have been overflowed or covered by 

 colder air above, through which, or into which, it will tend to ascend ; or the lower 

 lamina may in some cases be warmed in any of several ways, and so may get a 

 tendency to rise up through the colder superincumbent atmosphere. On this part 

 of the subject, the author believes, there is much scope for further researches and 

 advancements both observational and cousiderational — that is to say, by encourage- 

 ment of a spirit towards accurate observation, and by collection and scrutiny of 

 observed facts and appearances, and by careful theoretical consideration, founded 

 on observational results or suppositions. 



To the author it seems probable that the great cyclones may have their region 

 of rarefied air extending up quite to the top of the atmosphere; while often whirl- 

 winds of smaller kinds, many of the little dust whirlwinds for instance, which are 

 frequently to be seen, may terminate or gradually die out at top in a layer or bed 

 of the atmosphere different in its conditions, both as to temperature, and as to 

 original motion from the lower layer in which the whirlwind has been generated. 

 In many such cases the upper air may probably be cooler than the lower air in 

 which the whirlwind originates. On the subject of the actions going on at the 

 upper parts, or upper ends, of whirlwind cores, in most cases, the author feels that 

 he is able to offer at present little more than suggestions and speculative conjec- 

 tures. In very many descriptions of the appearances presented by those whirl- 

 winds with visible revolving cores, which are called waterspouts, it is told that the 

 first appearance of the so-called waterspout consists in the rapid shooting down 

 from a dense cloud, of a black, cloudy streak, seemingly tortuously revolving, and 

 swaying more or less sidewise. This is said rapidly to prolong itself downwards 

 till it meets the surface of the sea ; and the water of the sea is often imagined and 

 described as rising up bodily, or as being drawn up, into the partial vacuum, or 

 central columnar place of diminished pressure. The frequently entertained notion 

 • — a notion which has even made its way into writings by men of science, and of 

 authority in meteorology — that the water of the sea is sucked up as a continuous 

 liquid column in the centre of waterspout whirlwinds, is by some writers and 

 thinkers repudiated as being only >a popular fallacy ; and it is affirmed that it is 

 only the spray from the broken waves that is carried up. In this denial of the 

 supposition of the water being sucked up as a continuous liquid column, the 

 author entirely agrees : and he agrees in the opinion that spray or spindrift from 

 the sea, set into violent commotion by the whirlwind, is carried up in a central 

 ascending columnar core of air. 



On the other hand, the commonly alleged inception of the visible waterspout 

 phenomena, in a descending, tortuously revolving, and laterally bending or swaying, 

 cloudy spindle protruding from a cloud, the author supposes to be so well 

 accredited by numerous testimonies that it must be seriously taken into account in 

 the development of any true theory and explanation of the physical conditions and 

 actions involved. He ventures to hazard a suggestion at present — perhaps a very 

 crude and rash one. It is that the rising central core may perhaps, in virtue of 

 its whirling motion and centrifugal tendency, afford admission for the cloudy 



