1880.] in the Amount of Carbon Dioxide in the Air. 353 



on the other hand, is fully 20 miles distant from the sea along the 

 most direct line that can be drawn. From this, then we may infer 

 that there may possibly be other causes which tend to reduce the 

 amount of carbonic acid present, supposing, that is. that the results in 

 question do materially differ from the normal state ; an assumption 

 which, as regards the open country, has not yet been clearly esta- 

 blished. 



The most distinct climatological peculiarity of Grasmere may be 

 said to be its rainfall, which, during such a year as last when the 

 observations in question were made, might have risen to 120 or 130 

 inches, and it is situated, moreover, within 6 or 7 miles of a point on 

 the Sty-Head Pass which has the largest rainfall in the British Isles, 

 an average, that is, of 175, or in bad years such as last, of 200 inches 

 per annum, while the general average for Great Britain is under 

 30 inches. A glance at tbe temperature columns in the tables will 

 show how frequently the point of saturation was practically reached 

 during the observations. 



It is hardly possible, however, that the diminished quantity of the 

 carbonic acid is due to the solution of this gas in rain-water. It is 

 well known that the amount of any gas absorbed by water is a function 

 of the pressure which it itself exerts ; and it may be readily shown 

 that the actual pressure of the carbonic acid in the atmosphere is far 

 too small to exert any sensible influence upon its amount in the air. 

 If this explanation, therefore, of a diminished quantity of carbonic 

 acid is no longer available, may we not possibly turn to the operation 

 of such an exceedingly active vegetation as is found in some parts of 

 the English Lake District as a more feasible cause of any observed 

 reduction in its amount ? For we are not without evidence, as some 

 further experiments shortly to be described as well as those of 

 M. Beiset, near Dieppe, appear to indicate, that this may after all be 

 the chief disturbing agent. 



But to return to the point of chief importance in connexion with the 

 experiments at Grasmere, the unquestionable evidence, that is, they 

 afford of an appreciable diurnal variation in the amount of atmospheric 

 carbonic acid. 



The only results with which, in reference to this point, it is worth 

 while to compare them, are those of M. Truchot made at Clermont 

 Ferrand, Auvergne, in 1873. With these, although they agree as to the 

 main fact of the existence of a diurnal difference, they nevertheless 

 disagree considerably as to its amount. 



M, Truchot, for instance, as shown by the table (ante, p. 344), 

 has detected a diurnal variation in air taken in the neighbourhood 

 of vegetation and in sunshine — in a position that is presumably 

 identical with that in which the Grasmere observations were made — 

 of as much as 2*34 vols. C0 2 in 10,000 of air, while in a position even 



