ON SUNSPOTS AND TERRESTRIAL PHENOMENA. 455 



and Balfour Stewart have reduced some observations made by Mr. 

 Campbell. 1 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 we 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 records of solar radiation. A 

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

 which after some alterations introduced by Professor Stokes win very 

 likely prove both efficient and easy to handle. Dr. Froelich 2 has in the 

 meantime made some very praiseworthy efforts in the same direction, 

 and has come to the 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 ; that is to say, the sun seemed hotter when it showed smaller signs 

 of sunspot activity. Dr. Froelich's experiments have been criticised by 

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

 to be said against the certainty of his result. Professor Langley objects 

 that the logarithmic law of increasing absorption with 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 be correct. Looking over the curves as given by Dr. 

 Froelich, it seems, however, that, although 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, and a very 

 slight error would produce considerable differences in the final result. 

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

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

 deserves further 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 atmospheric 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 radiation for a suc- 

 cession of several months would bring home most valuable results. 

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

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

 measure 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. 



Atmospheric Pressure. 



The relation between solar radiation and the atmospheric pressure at 

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

 plicated one, and must vary greatly with the meteorological conditions of 



1 Proo. Royal Soc. xxiii. p. 578 (1875). 



2 Electroti'chnisclie Zeitschrift, v. p. 3 (18S4). 



