April 23, 1896J 



NATL RE 



591 



seen to be agitated and to be forming centripetal spirals, and 

 producing a liquid cone several centimetres in height. Above 

 this cone a great number of little drops accumulate, which fall 

 hack in spirals. This attraction, at a distance, is even more 

 striking if the water is slightly heated ; the vapour then forms 

 a holloiv tube, of which the hollow part is distinguished by its 

 .lark colour and its geometrical regularity ; it shoots forth from 

 the water towards the small mill, causing light objects, such as 

 bits of straw, which are floating on the liquid, to be thrown up. 

 Such is the experiment which in 1887 was made in the open 

 air at the great works of the Weyher and Richmond Company. 

 With the reduced apparatus, now placed before you (Fig. 6), we 

 can repeat it in conditions quite as convincing. The small mill 

 is placed at the top of the case two metres high, closed on one 

 side by a glass; the water, slightly warmed and containing a 

 little soap, is placed at the bottom of the case in a basin. I 

 set the small mill going ; you see the agitation at once, the 

 soap-bubbles precipitate themselves at the foot of the column 

 of vapour. Soon the column takes the form already described, 

 and represents exactly the appearance of a real water-spout ; at 

 the foot is the Imisson — that is to say, the collection of bubbles 

 and little drops : at the top. the expanded hollow lube of vapour. 



Fig. 6. — Arti.lclal reproJ 



a water-spout. 



A light balloon placed at the surface of the water is first carried 

 to the centre, and rendered captive at its foot ; by quickening 

 the rotation (which increases the power of the whirlwind) the 

 balloon is raised by the water-spout, and sometimes follows the 

 ; iral the whole of its height. 

 The helicoidal movement of this light balloon, as well as the 

 aspect of the nebulous spiral, shows well the constitution of the 

 water-spout ; one sees the superposed rolls of helicoidal currents, 

 some ascending, others descending (Fig. 7) ; it is a perpetual 

 going and coming between the mill and the surface of the water. 

 As all the currents turn in the same direction if the ascending 

 ones screw to the right, the descending ones screw to the ie/t. 

 It is the absence of having recognised this double movement 

 of a.scent and descent which is at the bottom of the misunder- 

 standing between the partisans of ascending water-spouts and 

 those who maintain that they are only descending phenomena. 



The ascending movement of the light balloons caught up by 

 the water-spout, shows well the ascending velocities ; it is more 

 <Iifficult to put in evidence the descending region, declared in 



NO. 1382, VOL. 53] 



some theories to be the only existing one, because it occupies in 

 the reduced experiment a very small space ; it is confined 

 to the interior of the nebulous sheath, of which the hollow 

 centre is distinguished by its dark colour. I will, however, 

 show it to you with the help of a very simple artifice. Take 

 a body emitting smoke to the top of the water-spout ; we see 

 this aspirated smoke at once reach the interior of the sheath, 

 roll itself into a slender cone, and descend to the surface of 

 the water. This is exactly what is seen in nature when, in a 

 water-spout, the clouds descend in the form of a stream 

 which grafts itself in the middle of the btiisson formed by 

 the water at the surface of the boiling sea. This spiral is, so 

 to speak, the harmless part of the water-spout ; the terrible part 

 is invisible ; it is formed by the mass of air which rages round 

 the spiral. In the experiment before us it is contrary. The 

 raging mass is visible owing to the smoke which is supplied ; the 

 interior of the spiral remains dark ; it is by the introduction ot 

 the smoke that the existence and form are recognised. 



There still remains to show you that with a similar arrange- 

 ment a cyclone can be produced with all its characteristics — 

 variation of pressure a" its passage, barometric minimum, central 





FiG.*7. — Double direction of liquid currents in a water-spout. 



calm, brisk rising of wind, centre of the storm, &c. — which has 

 also been attained by M. Weyher. 



The following remarks have been subsequently added to the 

 lecture at the Royal Institution : — 



We will conclude by describing with some detail that experi- 

 ment which reproduces so accurately all cyclonic phenomena. 

 In reality a cyclone is nothing else than an immense aerial 

 whirlwind ; it only differs from a water-spout by its proportions, 

 and principally with respect to the height and the diameter ; in 

 a water-spout the diameter is very small in relation to the height, 

 whereas in the cyclone it is the contrary. But in both cases the 

 general movement is the same ; the aerial currents descend all 

 around, to remount immediately on the interior spirals, with 

 a diameter more or less great, but leaving, as in a water-spout, 

 a central region free, in which the descending movement is 

 equally to be found. 



Here is a flat rotating disc, about I metre in diameter, 

 mounted on the extremity of a crane 2 metres in radius ; by means 



