May 1, 1899.] 



KNOWLEDGE 



105 



those vague, thin, lofty clouds resembling tufts or branches 

 of curled hair, so characteristic of the true cirrus type. 



Cirrus clouds and their compounds seem to play an 

 important part in the production of thunderstorms, 

 although the exact action taking place in the formation of 

 the latter is not, perhaps, clearly understood. The cirrus 

 clouds, bristling all over with pointed ice spicules, if 

 brought into the neighbourhood of a cumulo-nimbus 

 cloud highly charged with electricity, would tend to lower 

 the electrical potential of such a cloud considerably by the 

 numerous point discharges. Now the experiments of 

 Lord Eayleigh in connection with this brought out some 

 interesting results. Experimenting with a water jet, he 

 found that by electrifying it feebly, the separate water 

 drops coalesced instead of rebounding from one another as 

 they did before electriiication. If, however, the jet was 

 strongly charged — say, with a Ley den jar— the effect was 

 just the opposite. The colliding drops no longer coalesced, 

 but were scattered even more widely than when the jet 

 was unelectrified. 



Assuming, then, that the reverse of this action is true — 

 that the separate particles of a cloud when highly charged 

 rebound from one another to a greater degree than when 

 less charged — the action of a cirrus cloud invading the 

 territory of a cumulo-nimbus, or vice versa, will perhaps be 

 more clearly understood. The drops will coalesce more 

 when the electrical potential is lowered, and the large-size 

 rain drops accompanying a thunder shower will result. 



Coming now to the conditions affecting cloud formation, 

 many circumstances determine when and where the water 

 vapour is condensed. It may rise only a comparatively 

 short height above sea level before the change takes place, 

 or circumstances may require its ascension to high altitudes. 

 For instance, suppose a cold north-east wind, or a south- 

 east wind already laden with moisture, or a lowering of 

 the barometric pressure to set in ; all these will tend 

 towards an early formation of cloud. Upon a dry, calm 

 day, however, the vapour wiU generally have to travel up 

 a long way before it wiU change its condition. In fact, 

 sometimes, under these circumstances, it only changes 

 into water drops to be directly transformed again into ice 

 particles, resulting in the delicate, fleecy, cirrus variety. 



Again, water vapour possesses a good heat absorbing 

 power. A cloudy sky, therefore, or an atmosphere already 

 charged with vapour will, in one respect, check condensa- 

 tion by preventing free radiation of the heat out into 

 space. 



But there is yet another and most important item in 

 the programme, the presence or absence of which materially 

 aids or prevents any cloud being formed at all. This 

 essential factor in the atmosphere is — dust. Dust, or 

 minute particles of matter of some description in the air 

 to act as nuclei, is apparently as necessary to the 

 formation of a cloud as the water vapour itself. It is 

 quite possible that a minute water drop dm be formed 

 without the agency of such a nucleus, but, as has been 

 sho^\-n by Clerk Maxwell and .J. J. Thomson, unless the 

 water vapour finds something to act as a nucleus aroimd 

 which to form a water drop, it will evaporate away again 

 before it has had time to grow. 



Most exhaustive has been the work by Mr, John 

 Aitken, f.r.s., upon the importance, both quahtatively 

 and quantitatively, of dust particles or other nuclei in 

 cloud formation. One of Aitken's experiments illustrating 

 the part played by dust is so striking that, although it will 

 doubtless be familiar to some, it wUl bear repetition. 

 Referring to the experiment illustrated in Fig. 1, it wUl 

 be remembered that with ordinary dust-contained air a 

 thick fog was formed in the vessel D when the air 



contained in the latter was expanded into C, The long 

 tube E F in Fig. 1 is packed with cotton wool, and one 

 end, F, connected to the vessel D by means of a thick 

 walled india-rubber tube. The pinchcock G is now 

 screwed up tight to prevent air flowing in through E F, 

 the stopcock B opened, and the vessels C D then exhausted 

 of afr by means of the pump A. B is then shut and G 

 opened slightly to let the outside air flow slowly in at E 

 and get filtered of its dust in traversing the tube. By 

 going through the whole process two or three times, the 

 final air in D will be quite dustless. The pinchcock is 

 then again screwed up and the air in D expanded by 

 opening the stopcock. No fog or cloud will be observed, 

 and the brilliantly coloured halos obtained before wUl be 

 conspicuous by their absence. Large drops of water wiU 

 trickle down the sides of the vessel because the latter wlU 

 offer a surface upon which the vapour can deposit its 

 burden. 



In view of the above result it can be seen why the well- 

 known Scotch mists are so different to many others ; why 

 the drops are so big and at the same time few. The 

 atmosphere in some parts of Scotland being so pure, so 

 comparatively free from dust and dirt, there are very few 

 particles of the latter round which the vapour can 

 condense. 



Illustrative also of this point, but In the other extreme, 

 the dense, thick fogs of our cities tell their own tale of 

 manufactories and fires and of the products of combustion 

 in the air. With nuclei in abundance and other conditions 

 satisfied, the water vapour has no difficulty in finding rocks 

 upon which to build its numerous water-drop castles. 



Although it is conclusive that water vapour must have 

 nuclei around which, as centres, it can condense ; and, not- 

 withstanding, owing to the immense number of solid dust 

 particles present in the air condensation takes place upon 

 them, yet they are not absolutely essential. It has been 

 shown by Aitken, E. von Hemholtz and others, that in the 

 absence of dust cloudy condensation occurs, but the solid 

 nuclei are replaced by other foreign substances. These 

 may be the molecules of the vapour of sulphuric acid, or 

 the particles given off by anhydrous sulphuric acid, or 

 even from metal surfaces when heated or electrified. 



There is yet one further point to be considered, and one 

 which Tyndall first described in his heat lectures. This is 

 the direct influence of sunshine in producing condensation. 

 Tyndall illustrated this by showing the beautiful clouds 

 that result when a beam of light is allowed to pass 

 through a long tube full of dustless, satiu-ated vapour. 

 Furthermore, Aitken has recently found, from similar 

 experiments, that many of the vapours which are called 

 impurities in the atmosphere, such as ammonia, chlorine 

 and several acids, give rise to nuclei of condensation when 

 acted upon by sunshine. Indeed, sulphurous acid and 

 chlorine are thus active, even in the dark, but to a much 

 less extent than in sunshine. 



Thus the great wheel of Nature turns — from the sea to 

 the sky, from the sky to the earth, and back to the sea 

 again ; this Interchange, this complete cycle, working for 

 our good and for the good of everything in general, is ever 



in action. 



♦ 



A NEW STAR IN SAGITTARIUS. 



ANEW star appeared in the constellation Sagittarius 

 early in the year 1898, or possibly in the latter 

 part of the year 1897. It was found from the 

 peculiarities of its spectrum, by Mrs. Fleming, 

 during the examination of the Draper Memorial 

 photographs. The approximate position for 1900, derived 



