202 



KNOWLEDGE 



[September 1, 1899. 



CLOUDS. 



By E. M. Antoniadi, f.r.a.s., and G. Mathiei , 

 of the National Agronomkal Institute, Paris. 



THE applicationof photography to thestudy 

 of meteorological phenomena enables us 

 to investigate with great advantage the 

 form and height of the various clouds, 

 the appearance of water-spouts, rain- 

 bows, halos, corona, etc., as well as to record 

 the fugitive sinuosities of the lightning flash. 



Nor does this mode of enquiry require any 

 modification of the camera. A good large- 

 angle object-glass with an ordinary shutter is 

 all that is needed. For clouds, however, floating 

 about the zenith a moveable-headed foot will be 

 found of use. 



The simultaneous photography of the same 

 cloud from two different stations will give its 

 height above the earth's surface. At his private 

 observatory of Trappes (Seine-et-Oise), M. Leon 

 Teisserenc de Bort utilizes for this purpose two 

 photographic theodolites of the type adopted by 

 Her ]\Iajesty's Government in India. The in- 

 struments are separated by a distance of one 

 thousand three hundred and eighteen metres, 

 while they are united by a telephone, destined 



If we look at the sky through a spectroscope, we see all 



Fio. 1.— "Mare's Tail" Cloud, 6(1. 2h. 2.oiii. loeal time, 

 photograplieil on 1899, May. 



to ensure the simultaneity of the operations. In this way, 

 M. de Bort has secured results of permanent value. 



Fia. 2 -" Mackerel Sky," 189S. September, 21d. 41i. 51m^ 



1 the colours of the spectrum, though blue is the predominant 

 colour ; but it is only the reflected light that is blue, the 

 transmitted light is orange or red. The action, therefore, 

 of the particles of our atmosphere on sunlight is apparently 

 dichroitic. A careful consideration of these phenomena 

 led the late I'rof. Tyndall to the statement that our gaseous 

 envelope behaves exactly as if it were laden with extremely 

 small particles, impinged upon by the minute waves of 

 ether. And, as " in the case of water, for example, a 

 pebble will intercept and reflect a larger fractional part of 

 a ripple than of a larger wave," similarly the small particles 

 of the air would exert their most conspicuous action upon 

 the smaller waves, a predominance of blue in the scattered 

 light being the consequence. By his classical experiments 

 on " actinic clouds," Prof. Tyndall brought to Hght * the 

 brotherhood existing between the blue colour and polariza- 

 tion of skylight and the phenomena observed on exceed- 

 ingly fine precipitates. But the sifting action exerted by 

 these particles on the constituents of white light being 

 solely a product of their exiguity, an increase in the 

 diameter of the particles must necessarily entail a whiten- 

 ing of the difl'used light ; and as the dimensions of these 

 bodies vary Inversely as their height above the earth's 

 surface, the sky ought to appear, and actually does appear, 

 whiter near the horizon than about the zenith. 



Thus much with the blue reflected light. The red hue 

 of the transmitted light is accounted for, in this theory, by 

 the assumption that " through its successive collisions with 

 the particles, the white light is more and more robbed of 

 its blue constituents." 



The predominance of violet rays in the reflected skylight 

 is, in the large majority of cases, a perpetual drawback in 

 cloud photography, since the photographic action of blue 

 light is almost identical with that of white light. Hence the 

 necessity of devising some means to quench the violet of 

 the sky, while preserving the light of the clouds. To this 

 end various methods have been devised. 



Prof. Kiggenbach has utilized the polarization of sky- 

 light. The maximum of polarization being observable at 



* " Fragments of Scienee," Vol. I., Chapters on " Artificial Sky " 

 and " The Sky." 



