264 



IklETEOROLOGY AND ALLIED SUBJECTS. 



inches) the extremest ultra-violet cadmium line is still visible. {Z. 0. 

 G. M., Vol. XV, 1880, p. 57.) ^ 



Marie-Davy has published a study upon the carbonic 'acid gas con- 

 tained in the atmosphere as observed at Montsouris, 187G to 1879. The 

 annual means are as follows : 



It is evident from this that of all the meteoroloi?ical elements the 

 clearness of the sky is one that has a direct connection with the quan- 

 tity of CO2, and that the ratio is such that the greater quantity of OO2 

 coincides with the least clearness. 



Again, by comparison with the winds, it is found that the southwest 

 bring a greater, but the northerly a smaller, quantity of CO^. Since 

 now the clearness cannot be directly influenced by the i)reseuce of 

 CO2, and since CO2 is perfectly diaphanous, like dry air, therefore 

 Marie-Davy concludes that from the quantity of CO2 we have a means 

 of predicting the clearness — that is to say, the weather — for a long time 

 ahead. {Z. 0. G. M., XV, 1881, p. 135.) 



Cornu has shown, from his investigation into the limit of the ultra- 

 violet portion of the spectrum at difl'erent altitudes (/t) above the horizon, 

 that the limit of visibility of the photograpl^ic spectrum, as defined by 

 the length (A) of the last wave on the photographic plate, is connected 

 with the altitude ot the sun by the formula 



log sin h = inX -f n 



where m and n are two constants. For a station whose altitude above 

 sea-level is z this formula becomes 



log sin li = m U -{• ~) + n 



where g is a constant whose value is approximately 8G8.2. If, now, the 

 absorbing power of the atmosphere is due to any substance distributed 

 in the atmosphere according to an unknown function of the altitude, 

 then Cornu shows that theoreticall}- the law of absorption must be a 



logarithmic function of the form: m_ =log Z— log c. 



If, now, we sub- 



stitute in this formula the value of m = — 0.018882, and 5 = 808.21, we 

 obtain 5; = 17761 x log c — log/. Xow the hj'psometric formuhi is : z = 

 18336 log bo— log b, whence it follows that the mass of the absorbing sub- 

 stance is at any given altitude proportioned to the height of the barometer, 

 and therefore has a constant ratio to the mass of the atmosphere itself. 



