322 THE POPULAR SCIENCE MONTHLY 



process of condensation has been started, it may continue automatically, 

 the liberation of latent heat tending to produce convectional currents. 

 This perhaps fairly expresses the general view of the average person 

 at this point. However, having seen that the influence of forests upon 

 temperature and upon humidity is so slight, even among the trees, it is 

 unreasonable to expect that the influence upon rainfall over the forest, 

 and especially away from the forest, will be considerable. In the great 

 ascending, damp air masses of a general storm ; in the flow of the winds 

 across a mountain barrier; in the active convectional overturning of a 

 summer thunderstorm — what really significant effect can the slightly 

 damper and slightly cooler air of the forest play in the process of pro- 

 ducing or determining the amount of the rainfall ? We say, " the air 

 over the forest is damper; therefore there will be more rainfall," quite 

 forgetting that the damper air is useless as a source of precipitation 

 unless it is cooled to the dewpoint. Furthermore, this moisture is 

 constantly being carried away by the winds, and distributed through a 

 great mass of air, thereby giving up more and more of whatever rain- 

 producing effectiveness it may have had. 



Forests and Kainfall: the Observations and the Difficulties 

 Whatever may be our personal prejudices, and whatever may be the 

 theoretical considerations in favor of an influence of forests upon rain- 

 fall, what we really want is the facts, so far as they are at present avail- 

 able. Obviously, in a scientific study of this problem, the historical 

 method of treatment, previously referred to; all theoretical considera- 

 tions, and all prejudices, must give way before the results obtained by 

 means of actual observations, made under approved conditions, with 

 accurate instruments. There has been great difficulty in securing abso- 

 lutely trustworthy observations. Many of the older records are clearly 

 unreliable because of the improper exposure of the rain-gauges, the dif- 

 ferences in the elevation or exposure of the instruments being enough 

 to account for all the observed differences in their catch. Some excel- 

 lent series of observations have, however, been carried on during the 

 past twenty-five years or more in several European countries, by the 

 agricultural and the forest experiment stations. A system of parallel 

 or radial stations has been extensively used, these being located within 

 forests and in the surrounding open country. Simultaneous observa- 

 tions extending over as many years as possible are compared, the great- 

 est care being taken to have the best exposures, and to allow for the 

 effect of the wind on the catch in the gauges. 



The proper exposure of rain-gauges is one of the most perplexing 

 problems in observational meteorology. Rainfall has long been known 

 to be very " patchy," that is, there are considerable differences within 

 very short distances. Thus it happens that gauges which are near 

 together and under similar conditions of exposure often record quite 



