56 



NATURE 



[May 15, 1879 



Cyclones 



^^ Mr. Barham, in Nature (vol. xviii. p. 249), concludes that 

 " a cyclone is occasioned by the meeting and passing each other 

 of a northerly and a southerly current, so that they pass each 

 other on the left hand respectively." Supposing this to be true 

 for the northern hemisphere, we must for the southern hemi- 

 sphere substitute ri^if hand for /efi. 



There is nothing in this contrary to dynamical principles, but 

 the facts of the geographical distribution of cyclones appear to 

 show that the eddies or swirl? in which they originate are formed, 

 not by north and south currents passing each other, but by the 

 same meeting ^^.s^ other, that is to say, in the tone where the 

 north and south trade-winds meet, when this zone is at some 

 distance, from the equator. Cyclones are not formed on or near 

 the equator, because there the earth has no rotation relatively to 

 an axis drawn vertically to its surface. 



Mr. Blanford has written in Nature (vol. xviii. p. 328), 

 showing that Mr. Barham has not accounted for the motive 

 power of the cyclone, and explaining it by the liberation of 

 latent heat from the condensation of vapour in an ascending 

 current at the centre of the cyclone. This explanation was first 

 given by Espy in his " Philosophy of Storms," and is certainly 

 true. But the two questions are quite distinct, viz., what sets 

 the cyclone going? and, how is it supplied with motive power? 



Three conditions are needful for the formation of a cyclone- 

 first, an eddy produced where currents of air meet ; second, 

 sufficient "steam power of the atmosphere," as Espy calls it, to 

 produce a strong in-draft to the centre of the eddy ; third, a 

 position sufficiently far from the equator for the currents of air 

 towards the centre to be sensibly deflected by the earth's 

 rotation. 



But how is the in-draft first set up ? I reply, by the centri- 

 fugal force of the eddy causing a barometric depression at its 

 centre, whence follow cold, the condensation of vapour, and the 

 liberation of heat which had been latent. The liberated heat 

 expands the air in the upper strata, thus supplying the motive 

 power of the cyclone. Joseph John Murphy 



Old Forge, Uunmurry, Co. Antrim, April 20 



Showers of Rain and Gusts of Wind 



It is a matter of common observation that showers of rain are 

 usually accompanied by more or less violent gusts of wind, but 

 as far as I know no explanation of the fact has yet been offered. . 



The cause I am about to suggest must be productive of some 

 result, but whether it is sufficient to account for the whole 

 phenomenon I cannot say without quantitative observations, 

 though I guess it is. 



It is quite certain that the actual velocity of even large drops 

 of rain is very small compared with that which they would 

 acquire by falling through the same height in vacuo, and, practi- 

 cally, a drop may be considered to f5l with uniform velocity 

 during the whole time of its descent. 



Now, after a drop has ceased to accelerate, it leaves an 

 amount of water in air equal to the weight of the drop multiplied 

 by the distance through which it falls, and most of this mil appear 

 as a current of air accompanying and following the drop. 



In fact, when the drops which compose a shower ai-e falling 

 without accelerating, the air through which the shower falls is 

 acted on by a downward force equal to the weight of the shower, 

 and the downward current of air thus caused must spread 

 laterally as it approaches the ground. 



Thus, if a shower were to fall on a calm day, there ought to 

 be a wind on the ground blowing in all directions from the 

 shower as a centre. The rough experiment of throwing a 

 shovelful of sand into the air and watching the dust as it reaches 

 the ground serves very well to show the kind of thing which 

 must happen. But when there is a wind blowing at the 

 time the shower falls, which is generally the case, another, and 

 perhaps more potent, cause than the mere lateral spreading of 

 the downward current comes into operation. 



The wind at the surface of the ground is moving more slow ly 

 than that at a higher level, because of the retarding effect of 

 friction, and the effect of the shower will be to import air with 

 a high lateral velocity from the upper regions to the lower, 

 where the velocity is small. 



With regard to the magnitude of the force which the shower 

 exerts on the air, if we take the rainfall as 2 centimetres per 

 hour, the velocity of the falling drops as 7 metres per second, 

 and the height of the cloud as 700 metres, not improbable num- 



bers for a thunder-shower, the pressure per square metre of 

 shower is 560 grammes ; but the downward velocity which this 

 force would generate in the air would depend on the total area 

 of the shower, and would vary from place to place in the shower 

 itself. A. Mallock 



May 8 



Phosphorescence 



The study of phosphorescence has lately received a consider- 

 able impetus, mainly from its having been made use of in 

 commercial form for clock faces, door plates, &c., but as Prol 

 Morton lately remarked, the present producers must have dit 

 covered some method of greatly increasing the luminosity of the 

 sulphide used. After great difficulty I succeeded in getting a 

 small quantity from the French makers, the same that I know 

 Mr. Crookes had in obtaining it from the same source. Its 

 luminosity is undoubtedly infinitely superior to that of any 

 of the old methods of production, and gives hopes that 

 further advances may be made. My reason for troubling you 

 with this letter is to put on record a curious fact that has come 

 under my notice ia making some experiments detailed in the 

 Photo News for May 2, with a view to making luminous photo- 

 graphic images in various ways — that is, that not only light, but 

 heat produces the phosphorescent light. If we take a sheet of 

 card and dust it over with the powder, after coating it with a 

 sticky varnish and allowing to dry, we have a surface that when 

 exposed to daylight for a fevr seconds will remain luminous for a 

 certain time afterwards. If we place a transparent positive in 

 front we have an evamescent photographic image presented to us 

 on removing to a dark room, but if after simply exposing the 

 sheet to daylight we place the tips of the fingers against the back 

 of the card, spots of two or three times the luminosity will 

 appear at these places showing that the heat from the hand has 

 a great increasing action. The same occurs if the paper has 

 not been exposed to light, the mere warmth of the hand being 

 sufficient to render the sulphide luminous. Here we have light 

 produced by warmth o« a small scale. That phosphorescence is 

 yet in its infancy I am convinced, and also that it will yet have 

 a greatly extended future. Walter B. Woodbury 



Manor House, South Norwood, S.E. 



A Large Egg 



One of my Houdan fowls has laid an egg weighing 7 oz. ; 

 her ordinary eggs weigh 2^ oz. I ordered my man to blow the 

 egg in order to preserve the shell, when I was surprised to find 

 that, besides a small yolk and much white of egg, it also con- 

 tained a perfect ordinary -sized egg. This is now lying loose 

 within the large shell, which latter measures Si inches and 9J 

 inches in its two principal circumferences. E. L. 



Barham, Ipswich, May 6 



is^l 



THE IRON AND STEEL INSTITUTE 



THE annual spring muster of this young and vigorous 

 society, now in the tenth year of its age, and 

 numbering close upon a thousand members, was held, 

 according to custom, in the house of the Institution 

 of Civil Engineers in Great George Street, last week 

 This gathering has been awaited with considerable interest 

 for some time past, as communications of more than usual 

 importance were expected upon the problem of dephos- 

 phorising ordinary brands of cast iron, such as arc 

 smelted from the stratified ores of Cleveland, Lincolnshire, 

 and Northamptonshire, sufficiently to be able to produce 

 from them steel of fair merchantable quality ; and the 

 attendance fully justified the expectations, the large meet- 

 ing room being, as a rule, filled to overflowing each day 

 within a very few minutes after the opening of the pro- 

 ceedings. With the exception of a morning devoted to 

 complimentary and formal business, including the address 

 of the new president, Mr. Edward Williams, who succeeds 

 Dr. Siemens, and, in a few pages, presented a bold and 

 rapid sketch of the progress of the malleable iron and 

 steel industries since 1855, the year of Bessemer' s great 

 invention, and the presentation of the medal to Mr. Peter 



