1883.] H. F. Blanford — Siin-tlteiinometer ohservations. 47 



Society, Part I, No. 3, 1878, and to Mr. Wcstmacott's article on the Pal 

 kings in Vol LIX of the Calcutta lleview. 



The following papers were read — 

 1, Some further results of sun-the?'?nometer ohservations with reference 



to atmospheric alsorption and the supposed variation of the solar heat. 



(Postponed from last IIeetin(j.)~-By H. F. Blanfoud, F. R. S , 



Meteorological Beporter to the Government of India. 

 This paper is published in the Journal, Part II, 1882. 



Mr. J. BiiiDGES Lee said he proposed to offer only a few observa- 

 tions on that portion of Mr. Blanford's paper in which he referred to the 

 difficulty of distinguishing between atmospheric effects due to aqueous 

 vapour, mist or diffused cloud and dust haze. All these forms of matter 

 are powerful heat absorbers. All interfere more or less seriously with the 

 value of any thermometric observations of the heat of the sun's rays and, 

 having regard to the smallness of the periodic variations which are believed 

 to occur with some regularity over cyclical periods between maxima and 

 minima of sunspots, it is evident that the errors introduced by the irre- 

 gular and uncertain causes referred to may be sufficient to vitiate, if not 

 altogether to destroy, the valu€) of all observations made at low altitudes, 

 so far as they have any bearing upon the question of the variation of 

 radiant energy of the sun's rays. Mr. Lee suggested, however, that it may 

 be quite possible to separate and to distinguish between the effects due to 

 the before-mentioned causes. Aqueous vapour diffused through the atmos- 

 phere absorbs from the sun's rays both heat and light, but the absorption is 

 selective and a spectroscope shows certain well known and distinctly mark- 

 ed dark bands towards the least refrangible end of the spectrum. Aqueous 

 vapour, except possibly to a very small extent in the uppermost layers of 

 the atmosphere, does not reflect or diffuse heat or light. Aqueous mist 

 or diffused cloud, on the other hand, consists of myriads of tiny globules of 

 water. These globules diffuse light by reflection and the light reflected is 

 polarised at right angles to the incident solar rays. That polarised light 

 can be observed and roughly measured by any suitably constructed pola- 

 rlscope. Professor Tyndall was, Mr. Lee believed, the first physicist who 

 studied in any detail the characters of mist and fine cloud precipitated 

 upon a beam of (electric) light. Speaking from memory, only, Mr. Lee 

 was unable to give a reference to the paper in which his results were de- 

 scribed, but he had had the advantage of attending the reading of his paper 

 and following the experiments then performed. He experimented with the 

 vapours of certain ethers and Hj'drochloric Acid or Chlorine enclosed in 

 long glass cylinders with flat ends and concentrated upon the mixture a 

 conical beam of electric light. The result was that a very fine dark blue 

 mist was quickly deposited upon the beam. The mist then passed through 



