INFLUENCE ON HUMIDITY. 105 



The relative humidity decreases upwards in forests, but there is au 

 occasional reversal of this at night. Over the oi^eu lields it decreases 

 upwards with wet soil, but increases with dry. It is almost invariably 

 greater about the forest, above as well as inside, over the field station, 

 and it is higher above the forest than it is at 5 or G feet above the 

 open field. The small significance of the vapor poured into the air 

 by forests in raising the absolute humidity suggests the question of 

 the rehitions of the forest to other constituents of the air. The forest 

 takes up large quantities of carbonic acid, retaining the carbon and 

 rejecting the oxygen. Does it sensibly change the relativ^e quantity of 

 either constituent so that the composition of the air is slightly differ- 

 ent in the woods? Repeated observation shows that each constituent 

 is curiously uniform in quantity in the free air. It has been thought 

 the carbonic acid is quite variable, but the introduction of better 

 methods of observation shows that, except in confined places where 

 the gas is produced, the variations are very small. 



A study of the numbers involved will show that the carbonic-acid gas 

 taken u]) by a forest is a vanishing quantity compared witli that which 

 passes the forest in the same time with the moving air. Graudeau 

 gives the annual product of carbon by a fi)rest of beeches, spruces, 

 or pines as about 2,700 pounds per acre. This corresponds to 0,900 

 l)Ouuds of carbonic-acid gas, or 00,300 cubic feet. ISTow, if the average 

 motion of the air is 5 miles per hour (a light breeze, a low estimate), 

 and the layer of air from which tlie gas is taken be estimated at 100 

 feet thick, there would pass over an acre 050,01)0,000 cubic feet in one 

 liour. This air nuist contain about tliree parts in ten thousand of car- 

 bonic gas, and the total amount of tlie latter per hour is l(;r),0!H) cubic 

 feet. But this is 2|, or more tlian twice as nuich as that taken up by 

 the trees in the entire season, so tliat the air could provide in thirty 

 minutes for the wants of the trees for the entire season. If this sea- 

 son is taken at 5 months, or 150 (laj^s, the amount of air passing is 

 55 X 10' X 21 X 150 = 10S X 10'" cubic feet, and tlie amount of carbonic-acid 

 gas is 501x10*^. Tlie ratio of ('arl)onic acid used to that furnished is 



•^"^•^ 5!I4000000' ^'" «"" ^''''' ^" ""'^^^•^- 



The additions of oxygen to the air would form a still smaller percent- 

 age of the oxygen already present, for this gas makes up a quarter of 

 tlie air instead of a thirtieth of 1 per cent. 



The case is somewhat different with ozone, as it is formed iu certain 

 definite localities and its eagerness for combination is sncli that it can 

 not pass far from its sonrce ))efoi-e it will disappear. The resins of 

 coniferous woods should be an abundant sonrce of ozone, and although 

 the decaying matter in and about woods provides abundant occasion 

 foi- its disa]>pearance before ])assing otf into the outside air, still the 

 amount of ozone in such woods should be sensibly greater than outside. 

 Observation has not shown this une<|uivocally, but that may be due to 

 the numerous imperfections of the method employed. 



