December i8, 1891.] 



SCIENCE. 



343 



and widely observed, since it is believed that they are grad- 

 ually breaking up, so that probably in a very few years no 

 distinct traces of them may remain (see also O. Jesse on so- 

 called luminous clouds, in the journal Himmel und Erde, 

 vol. i., p. 263). 



Photonraphic results of the researches of O. Jesse are given 

 in Part xl. of the Transactions of the Berlin Academy of 

 Science for 1890, and Part xxvi. for 1891. It is very desira- 

 ble that such photographs should be taken in as many differ- 

 ent localities as possible, because from them we get the surest 

 basis for consideration of the situation and movements of the 

 clouds. But valuable ai i may be given by the co-operation 

 of numerous observers in various regions of the earth with- 

 out the aid of any apparatus. 



The principal points upon which stress is to be laid in this 

 inquiry are: — 



([J By what method can the so-called luminous clouds be 

 most surely distinguished from others, especially from the 

 ordinary cirrus cloud ? 



Clouds or cloud like formations which after sunset and 

 before sunrise stand out brightly from the dark ground of 

 the heavens, no earthly or unearthly sources of light being 

 present on the horizon, can only produce this effect by 

 means of their own light or else by light which they re- 

 ceive directly or indirectly from the sun or moon below the 

 horizon. 



Cloud-like formations which shine at night by their own 

 lighi have doubtless been formerly observed above the sur- 

 face of the earth. To these formations belong not only 

 thunder and lightning clouds, but also some polar light and 

 meteoric phenomena. 



But the so-cilled luminous clouds do not belong to the 

 various species of self-luminous clouds, for finer measure- 

 ments of their light are wanting, besides which the fact that 

 they are only seen within the zone of twilight proves that 

 the sun below the horizon is the principal source of their 

 light. 



It is well known that there are clouds within this twi- 

 light zone which resemble high mountain peaks, and which 

 in the first stages of twilight shine in tlie light of the sun, 

 though tlie latter is below the horizon of the observer. It is 

 easy to determine the relation between the position of the 

 sun belo.v the horizon, and the height of those layers of at- 

 mosphere which receive the sun's light and reflect it. 



But the laws which govern the whole course of twilight 

 are modified when the distribution of tlie sunlight-reflecting 

 particles in the atmosphere is altered to any great extent. If, 

 for instance, numerous minute atoms produced by volcanic 

 eruption or by the breaking-up of meteoric bodies find their 

 ■way into those heights above the earth's surface in which 

 usually the gaseous elements of the atmosphere are present 

 in a very scattered form, it may happen that such a layer, 

 which reflects the sunlight very strongly, may curiously 

 alter tlie course of the twilight. 



So long after sunset as the masses of air beneath such a 

 layer receive direct light from the sun and reflect it, the ob- 

 server will not distinguish any deviation from the usual 

 course of twilight. But as soon as the further sinking of the 

 setting sun gradually deprives the lower layers of air of the 

 ■direct light, the higlier layer of. dust still receiving light 

 from the sun stands out in astonishing brightness, the parti- 

 cles of dust having strong reflecting power, thus giving to 

 the close of twilight the curious effect of the sudden appear- 

 ance of shining (.louds on the broad surface of the heavens. 



The phenrnipna of the luminous clouds corresponded when 



first perceived to the above description. At present they are 

 no longer so strong or so extensive, but only form thin 

 whitish blue shining veils, similar in form to the so-called 

 cirrus or feather-clouds, occupying but a comparatively small 

 part of the floor of the heavens inside the twilight segment, 

 and in our zone mostly near the horizon. Probably, the 

 layers are now so thin that very near and exactly above us 

 they can no longer be seen. 



From the above considerations, it is clear in what way 

 these clouds differ from those situated nearer to us, and es- 

 pecially from the cirrus clouds floating scarcely more than 

 thirteen kilometres above the earth's surface. All these 

 lower clouds appear in the later twilight gray and shadowy 

 on a light ground, because the layers of atmosphere above 

 them are the chief source of the remaining twilight. The 

 luminous clouds differ too in shape and structure from the 

 other kinds of clouds. 



We must guard, however, against the error of mistaking 

 cirrus for luminous clouds, when, in exceptional cases, the 

 former look very bright, in consequence of receiving light 

 either directly or indirectly from the moon or other sources. 

 In this case, the question is decided by the relatively high 

 degree of stability in position and form of the very liiuh and 

 distant luminous clouds, as ordinary clouds lie lower and 

 nearer, and show much more rapid changes of position. 



(2) When convinced of beholding so called luminous 

 clouds, to what points shall attention be especially directed, 

 and what simple measurements of place, time, form, etc., 

 shall be carried out in order to aid most usefully in the in- 

 quiry ? 



In answering this question, we will first consider those 

 methods of research in which the observer can obtain no in- 

 strumental aid, except only a watch, which should be a 

 sufficiently good timekeeper to estimate the time of observa- 

 tion to one minute, when compared with the correct time 

 within eight or twelve hours after the observation. 



Such simple observations are the more useful, since it fre- 

 quently happens that in the well-fitted up and prepared sta- 

 tions, observation of the phenomena is prevented by bad 

 weather, or else that the phenomena stretch over too large an 

 extent of the earth's surface to be included in an organized 

 series of observations. The farther the statn^us are apart, 

 the more valuable are the most simple methods. For in- 

 stance, in order to get corresponding photographic observa- 

 tions from two stations, thirty-five kilometres apart, such as 

 Berlin and Nauen, the most rigid exactness, both as to time 

 and place, must be observed. 



If, however, observations are taken in East Prussia and la 

 the Rhine province respectively, a from twenty to thirty 

 times larger margin of difference as to time and place can be 

 allowed than in the foregoing case, without in any way les- 

 sening the value of the result. 



So, if without preparation and instruments to hand an ob- 

 server believes he beholds luminous clouds, he must not 

 imagine that he can render no service to science by examin- 

 ing them closely, for very possibly the most simple method 

 may, taken in conjunction with other similar observations, 

 prove to be of the greatest service. 



It is desirable, too, to look out for luminous clouds at all 

 seasons of the year, though, so far, they have only been 

 seen in summer. In the northern hemisphere they have 

 only been seen from the end of May to the beginning of 

 August, with greatest frequency and brightness in the month 

 of July. 



During these weeks, usually two stars are seen simul- 



