INFLUENCE ON HUMIDITY. 



103 



may be dnc to the siuTonii dings of tliis station, which is on the Liino- 

 bnrger heath, an ininicnse tract of moorland occnpying tlie eastern i>art 

 of Hanover. There is no distinct ditference between tree crown and 

 base nor between glade and plain. In the individual stations the mean 

 is frequently larger than the average of all, and is sometimes consist- 

 ently positive, in other cases regularly negative. It appears, as might 

 be expected, that there is no general diHerence between the absolute 

 humidity in woods and outside, and the differences in individual sta- 

 tions may be due more to the surroundings of the station in oi>en tields 

 than to the forest. The lack of a characteristic absolute humidity in 

 forests can be completely explained by the fact, already shown, that 

 the forest is not an especially active producer of the vapor of water. Its 

 position in this respect is intermediate; and even if it did produce a 

 peculiarly large or a specially small amount of vapor it would not be 

 apt to show a peculiar absolute humidity because of the rajiid connec- 

 tion and mixture of gases. A gas spreads in all directions and with 

 great rapidity usually from the point where it is produced. An obser- 

 vation by M. Becquerel illustrates the fact that a source of vapor need 

 not give the air about it a higher absolute humidity. lie compared, on 

 a summer's day, the psychrometer readings in the air at 5 feet, just over 

 a tree, just over garden vegetables, and just over a stream, and found 

 the absolute humidities to be practically identical. However much 

 these surfaces may hiive difl^n'ed in the production of vapor, the rapid 

 connection and mixture prevented this difference from being appreciable 

 as absolute humidity of the air. 



While the absolute humidity depends only on the amount of the 

 vapor of water in the air, the relative humidity depends on this and on 

 the temperature also. As a difference in temperature between woods 

 and open fields has already been shown, there must be a difference in 

 relative humidity, and as the temperature of the woods is lower the 

 relative humidity must be higher, or the value of W—0 must be posi- 

 tive. The followhig table shows that this is the case: 



There is no great diflerence in the different cases. The snrjdus in 

 the tree crown is a litth^ smaller because the temperature is higher 

 there. 



M. Fautrat has carried on some observations of humidity above trees, 



