26 FOREST INFLUENCES. 



differ by several times as luucli as the differeuce wliicTi could be reason- 

 ably expected between that on prairie and on forest. ■ In addition to 

 this, there is no certainty, generally, that any change of climate which 

 is satisfactorily proven is necessarily due to a change in the forest. 

 Changes are constantly going on in the climate of all stations and en- 

 tirely without obvious connection with surrounding forest conditions. 

 In fact, their causes are beyond our present knowledge. 



These are the secular changes, so called; they are probably periodic, 

 and their periods may be tens, scores, or hundreds of years. It is hardly 

 possible to find a long series of observations in which traces of these 

 secular changes are not evident. 



The historical metliod has really given but meagre results of a reliable 

 character. On the other hand, the extravagant results to which it has 

 sometimes led have cast discredit on the whole subject of the influence 

 of forests on climate. It will not be used in what follows except in a 

 single case, in which all the conditions which can reasonably be required 

 seem to be fairly fulfilled. 



There are also recognized results of forest action which are entirely 

 distinct from the meteorological influences with which we are here con- 

 cerned and which refer to soil and water conditions. Such is the in- 

 fluence of forests on the flow of surface water and the occurrence of 

 floods, on the amount of ground- water and the preservation of springs, 

 on the holding of movable soils or reclaiming of swampy ones. These 

 are all of the highest importance, but they will not be considered here. 

 The questions of temperature and of the changes of the air are the only 

 ones belonging to forest meteorology proper. 



SYSTEMATIC OBSERVATTOISrS IIS^ FOREST METEOP.OEOGY. 



There are several series of sporadic observations on forest meteor- 

 ology made before the beginning of this century, as, for instance. 

 Hunter's observations on the temperature of trees in 1775 and 1778, 

 and Schaeplf's observations on the same subject in New York in 1783. 

 The first systematic observations seem to be those taken at Geneva from 

 1 790 to 1800 by Pictet and Maurice. They referred to tree temperatures 

 and were made ou a horse-chestnut tree two feet in diameter. 



The thermometer was placed in a hole in the north side of the trunk 

 six inches deep, and the space around the stem was closed up with wax, 

 so that the water and air from the outside would not influence the tem- 

 perature shown by the instrument. The observations were made with- 

 out interruption at sunrise, 2 o'clock p. m., and sunset. Comparative 

 observations were made at the same time on a thermometer to the 

 north, in the shade, and on four ground thermometers at depths of 3 

 inches, 1 foot 5 inches, 4 feet 3 inches, and 12 feet 8 inches. The ob- 

 servations were made with precautions similar to those now considered 

 necessary, but not generally practiced at that time. 



