414 



NA TURE 



[September 23, 1922 



knows of the two places. Does the sort of coal 

 make a difference, or the length of time it is kept 

 before consumption ? Or is much of the London dirt 

 dust from other sources than coal fires, dispersed more 

 widely than in the damper Manchester atmosphere ? 

 A. E. Boycott. 

 Medical School, University College Hospital, 

 London, W.C., August 28. 



Prof. Boycott's statement is rather surprising ; 

 but I cannot think that the explanation is to be found 

 in the larger amount of domestic smoke in Manchester. 

 One would rather expect the reverse, and I can only 

 suppose that the difference between Manchester and 

 London dirt is due to the larger amount of dust not 

 arising from smoke, as Prof. Boycott suggests. Any 

 difference in the quality of coal used in Manchester 

 and London would scarcely have the effect he de- 

 scribes. 



The point is an interesting one and I think could 

 be settled by microscopic examination of specimens 

 from the two towns. Soot is easily identified in this 

 way. J. B. Cohen. 



Thwaite Cottage, Coniston Lake, Lancashire, 

 September 1, 1922. 



Waterspouts. 



Waterspouts on Lake Victoria are very commonly 

 seen from Entebbe, but at a long distance away, and 

 though I have worked on the lake shores for nearly 

 four years it was only two days ago that I first saw 

 one near enough to be of real interest. 



1 was in camp on the north end of Bugalla, the 

 largest island of the Sese Archipelago. The camp lay 

 about 300 yards from the shore of a small bay. At 

 daybreak on June 30 there were very lowering black 

 clouds and every indication of an immediate heavy 

 storm. While looking out from the tent I suddenly 

 saw that a waterspout was travelling obliquely to- 

 wards us, and as it eventually came to within about 

 100 yards of the shore a very good view was obtained 

 for about five minutes before it came to an end. 



The pedicle arose from a well-marked circular area 

 on the water, which was otherwise onlv faintly rippled 

 by the preliminary puff of wind before the approaching 

 storm. 



This circular area was evidently very violently dis- 

 turbed as a cloud of vapour, greatly agitated, rose 

 from it for a little distance. 



The pedicle was extremely narrow at its lower end, 

 and not quite straight, being sinuous in outline. It 

 broadened out gradually into a column which went 

 up into the low cloud ; the core of this column was 

 much less dense than the periphery, and the violent 

 upward spiral ascent of the water could be clearly 

 seen. 



So far I have described nothing unusual, but the 

 following was quite new to me and seemed of great 

 interest. 



Surrounding the central core, but separated from 

 it by a clear narrow space, was a sheath, the lower 

 end of which faded away some distance above the 

 water. The profile of this sheath was undulating, it 

 being thicker in some places than others. A curious 

 point is that this sheath seemed to pulsate rhvthmic- 

 ally, but I could not say whether the appearance 

 of pulsation might not have been an illusion caused 

 by waves travelling up its outer surface. 



This pulsation gave an uncanny suggestion of a live 

 thing, which was aided by the violent spiral move- 

 ment upwards in the central core, the clouds of 

 vapour boiling round its base, and the movement of 



NO. 2760, VOL. I io] 



the whole across the water — indeed, we watched it 

 spellbound until the pedicle dissolved away at the 

 bottom, and the ascent of the part above brought the 

 phenomenon to an end. 



M v wife watched with me, and is in entire agreement 

 about the curious appearance of pulsation of the outer 

 sheath. 



Fig. 1 is a reproduction of a pencil drawing which 



mav give some idea of what we saw. I cannot estimate 

 the height to which the column rose. Its cessation 

 was followed by violent rain and thunder. 



G. D. Hale Carpenter, 

 Uganda Medical Service. 

 Entebbe, Uganda, July 1. 



Dr. Hale Carpenter's letter brings out one feature 

 which has never, to my knowledge, been noted in a 

 waterspout, namely, the sheath, separated from the 

 main body of the whirl by a clear space. Wegener, 

 in his book on " Wind- und Wasser-hosen in Europa," 

 gives illustrations of a large number of waterspouts, 

 but in no case is there mention of two trunks one 

 within the other. The nearest approach to the 

 phenomenon noted by Dr. Hale Carpenter is the not 

 infrequent occurrence of waterspouts which show two 

 clearly defined parts, an upper thick column with a 

 lower whirl of much smaller thickness. 



The accepted explanation of waterspouts is that 

 they consist of whirls in rapid rotation with a dis- 

 continuity at the outer boundary. The rotation pro- 

 duces a rapid lowering of pressure within the whirl, 

 and consequently a lowering of temperature, which 

 may easily be sufficient to bring the air in the whirl 

 down below its dew point. This is sufficient to 

 explain the main features of the typical waterspout. 

 The amount by which the temperature is lowered 

 decreases outward from the " axis " of the whirl, 

 while the difference between the air-temperature and 

 dew point normally increases downward from the 

 cloud level. The thickness of the visible column or 

 zone of condensation therefore diminishes downward, 

 giving the form of an inverted cone of irregular 

 shape. Near the water the air is again near satura- 

 tion, and the difference between air temperature and 

 dew point is small, so that the base of the whirl is 



