ON SUNSPOTS AND TERRESTllIAL PHENOMENA. 



455 



Tins 

 ration, 

 i{i>rliu 

 of liu> 

 , these 

 Olio of 

 periods 

 oinls of 

 11 iU'cm- 

 10 to bi> 

 >[\ morn 

 5 in tlio- 

 iieiK'C of 

 tofrether 



iod. it is 

 ^erntnve. 

 •a(li<-tion 

 erster i>' 

 teresting 



X of pre-^ 

 n'aturo of 

 weeu the 

 o coTiiincs 

 equalities, 

 •terniineil. 

 al, us long 

 ;est reiluc- 

 lim to the 



pparcnt or 

 terrestrial 

 ronto aud 



lequalitics 

 [•onto tern- 



1 individual 

 lit is ionnA 

 lone of the 

 Dout seven 



llity of the- 

 Ihc peculiar 

 lus recoi'ds 

 1 may hope 

 Icrainst the 

 prs Koscoe 



,3U. 



and Balfour Stewart have reduced some obsei-vations made by Mr. 

 Campbi^;!.' There is a slight preponderance of strong radiation at times 

 of the maxima of spots. Mr. Blanford has come to the same conclusion 

 from the results of some Indian observations. But in both cases the 

 effects of atmospheric absorption could not be eliminated, and for this 

 reason v>o must not attach any very great importance to the result. 

 Professor Balfour Stewart has constructed an apparatus which it is hoped 

 will allow any observer to take continuous recoi-ds of solar radiation. A 

 committee of this Association has been formed to test the instrument, 

 which after some alterations introduced by Professor Stokes will very 

 likely prove both cflicient and easy to handle. Ur. Froelich '^ has in the 

 meantime made some very praiseworthy efforts in the same direction, 

 and lias come to tlie conclusion that the solar radiation differs at different 

 times by as much as eight or ten per cent. The range of observations 

 has hitherto only extended over the space of a few months. The radia- 

 tion seemed the stronger the weaker the daily range of magnetic declina- 

 tion ; tliat is to say, the sun seemed hotter when it showed smaller signs 

 of sunspot activity, jjr. Froelicli's experiments have been criticised by 

 Dr. H. Vogel as well as by Professor Langlev, and indeed there is much 

 to bo said against thr- cc'rtainty of his result. Professor Langley objects 

 that the logarithmic law of increasing absorption "ith increasing thick- 

 ness of absorbing layer is only true for a ray of monochromatic light, and 

 may not necessarily be true for a mixture of such rays. This is un- 

 doubtedly correct, but Dr. Froelich answers that he has by observation 

 proved the law to bo correct. Looking over the curves as given by Dr. 

 Froelich, it seems, however, that, althongli the law holds very nearly, a 

 very slight deviation from it might account for the differences observed 

 by him. We have, in fact, to use the formula to extrapolate by, anJ a very 

 sliglit error would produce considerable differences in llie iinal lesult. 

 But although the question of variability of the sun has in this way by no 

 means been proved as yet, I believe that Dr. Froelieh's method well 

 deserves farther development, and that he would obtain most valuable 

 results by taking up again his original intention of observing at high 

 altitudes. It is only by reducing the atmosiiheric effects as much as 

 possible that we can hope to eliminate it altogether. There are places in 

 the Western Himalayas, not very difficult to get at, where it would be 

 possible to camp out at an altitude of eighteen thousand feet. An expedi- 

 tion fitted out to take regular observations on solar I'adiation for a suc- 

 cession of several months would bring home most valuable results. 

 While wo are looking for changes in [he total radiation of the sun, we 

 ought not to forget the instruments devised by Sir Henry Koscoe to 

 r".easure and register the actinic effects. For it is quite possible that 

 the increase and decrease in radiation will make itself principally apparent 

 in the more refrangible rays. 



Almospheric Fressaro, 



The relation between solar radiation and the atmospheric prcFSure at 

 finy given place on the earth's surface must necessarily be a very com- 

 plieated one, and must vary greatly with the meteorological condit jnn of 



' Proc. Itinjal S'>c. xxiii. p. 578 (187o) 



« Ekctrotcchnischo ZeiUclmft, v. p. ;5 (1884). 



\ I 



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