82 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 65 



the Indio values. One may conclude from this that the volcanic dust, 

 which causes a decrease of about 40 per cent (Dorno) in the ultra- 

 violet radiation and about 20 per cent in the visible affects the rays 



Effective Radiation 



p mm. Lone Pine-Bassour 



4 — 0.004 



5 + °-°°5 



6 + 0.012 



7 + 0.015 



8 + 0.009 



9 — 0.003 



10^ 



nj "°-° 13 



.Mean + 0.003 



that constitute the nocturnal radiation less than 2 per cent. As 

 the nocturnal radiation has probably its maximum of energy in a 

 region of wave lengths at about 8 /a, this is a fact that in itself is 

 not very astonishing. Measurements in the sun's energy spectrum 

 show that even for waves not longer than about 0.8 jx, the trans- 

 mission of the atmosphere is very nearly equal to unity, the rays 

 being very slightly affected by changes in the scattering power of the 

 air. If we use the observations of Abbot or of Dorno in regard to the 

 weakening of the ultra-violet and visible light, and apply the law of 

 Rayleigh for the relation between scattering and wave length, we find 

 from these data, applied to the average wave lengths of the regions 

 concerned, that about 97 per cent of the radiation at 8 fi must pass 

 undisturbed by the dust particles. There are several objections against 

 a quantitative application of the theory of Rayleigh to the conditions 

 here considered, but at least it shows that our result cannot be re- 

 garded as unexpected. 



The fact that the nocturnal radiation has only decreased by about 

 2 per cent, when on the other hand the incoming solar radiation is 

 reduced to about 80 per cent of its former value, explains the inter- 

 esting relation between climate and volcanic eruptions pointed out 

 by Abbot and Fowle in their paper already referred to. That the 

 climatic effect is not larger, in spite of the great decrease in the inso- 

 lation, may be due to the large number of processes at work — so to 

 say — tending to balance or to weaken the consequences of a decrease 

 in the incoming radiation. It has been shown here that this decrease 

 is not to any appreciable amount counterbalanced by a decrease in the 

 outgoing radiation from the surface of earth. But there are other 



