Page:Popular Science Monthly Volume 82.djvu/327

Rh different amounts of rainfall or of snowfall. Further, the catch of a gauge is markedly influenced by the exposure. In the open field, for example, where there is a free sweep of the winds across the top of the gauge, more rain-drops and especially more snowflakes, are carried over the gauge than in a more protected location, where the drops and flakes can fall more nearly vertically. Thus, a gauge in a forest clearing where the wind velocity is somewhat reduced by the trees, ought to record more precipitation than one in the open country, although the actual fall might be identical in the two cases. A difference of a few feet in the elevation of a gauge will also often result in a catch varying considerably in two neighboring gauges. Furthermore, forests affect wind directions, and this also may influence the catch in the gauges. An element of great uncertainty is thus inherent in all the earlier results obtained by observation, and indeed to some extent in the later ones also, but it should be distinctly emphasized that every effort is now made to "correct" the results for just such errors. In the majority of places where parallel stations exist, the gauges in the forest have actually shown an excess over those in the surrounding open country. Whether this is a real excess of rainfall, or only a difference in the catch, is the disputed point.

There are four cases which have been frequently cited as showing an influence of forests upon rainfall. There is the famous Lintzel case, first cited by Müttrich. At Lintzel, on the Luneburg Heath, in Germany, the rain-gauges used to show a rainfall smaller than the average at a number of the neighboring stations. In 1877 a considerable planting of young trees was undertaken around Lintzel, until several thousand acres were covered. As time went on, the rainfall at the Lintzel station (in an open field surrounded by the forest) showed an increase as compared with that of the surrounding stations. There are, however, reasons against accepting these apparently conclusive results at their face value. The probability of error, the chance of discovering which is greatly diminished by the "smoothing" of the generalized results; the failure to make allowance for the protective effect of the increasing tree-growth; a recent change in the location of the rain-gauge; the shortness of the record, and the general variability and uncertainty of rainfall as a whole, are all considerations which, on the best of authority, may be urged on the other side.

Then there is the Nancy case, from France. This is a case of four stations (in two pairs), two in the forest and two in the open, within a small area, the altitudes and the general condition of one pair being,