344 



Prof. G. F. Armstrong. Diurnal Variation [Apr. 29. 



from experiments extending over the previons four or five years, in 

 1828. Among these were nine day and an equal number of night 

 observations. The latter showed an excess of 0*726 vol. of C0. 3 in 

 10,000 vols, of air over those of the day, and it was upon this some- 

 what limited basis that he was led to affirm the existence of a diurnal 

 variation. His exact figures are, for the day, 5*037 vols., and for the 

 night, 5-764 vols. C0 2 in 10,000 of air. 



In 1830 he published a further set of results, among which were 

 some experiments not made with the primary object of establishing- 

 this variation, but for the purpose of finding the difference between 

 the amounts of carbonic acid in the air of a town (Geneva) and of 

 the open country.* There were in all 13 such experiments extending 

 over parts of two years (1827-28), eight of which had reference to the 

 day and five to the night. Upon examining these a diametrically 

 opposite result to his former one is obtained. The mean of those 

 made in the country gives, for the day, 4 - 45 vols., and for the night, 

 4'02 vols. COn in 10,000 of air ; thus showing an excess of 0'43 vol. in 

 favour of the day. 



The experimental evidence as to the existence of a diurnal variation 

 in the amount of atmospheric carbon dioxide was therefore incon- 

 clusive, and I accordingly undertook, at Dr. Thorpe's suggestion and 

 with his collaboration, to carry out a series of observations with 

 direct reference to the question at Grasmere in Westmoreland, 

 during the summer and autumn of 1879. 



Up to this time the state of our knowledge concerning a diurnal 

 variation in the amount of atmospheric carbon dioxide may be said to 

 be comprised in the following statement. 



Yols. of C0 2 in 10,000 of Air. 



Excess. 



Day. 



ISight. 



Day. 



Night. 



Land-air. 



De Sanss-ure : 



1828 



1830 



5-037 

 4 56 

 4-45 



5-764 



4-81 



4-02 



0-727 

 0-250 



P. Truchot : 



0-43 



3- 14 



4- 15 



3-78 

 6-49 



0-64 

 2-34 



Sea -air. 



T. E. Thorpe 



3-011 



2-993 



0-02S 



* " Ann. de Chim. et de Phys.," vol. xliv (1830). 



