28 FOREST INFLUENCES. 



soldered at tlie ends, and a galvanometer is introdncod into the circuit. 

 By the i)rinciples of thenno-electricity whenever the two soldered ends 

 are at the same temperature, there is no current and the galvanometer 

 is not affected. If one of the junctions is warmer than the other, theu 

 a current is produceol in the one or other direction, and the needle of 

 the galvanometer swings to the corresponding side. If now one junc- 

 tion is i^laced in the medium the heat of which is to be measured, and 

 the other is left accessible and is warmed or cooled until the needle 

 stands at zero, the temperature of the latter junction is the same as that 

 of the former. One junction can be placed in a flame, or in an aninml 

 or ])lant, or at the top of a tree, or at a depth in the ground, and its 

 temperature can be ascertained by mani])ulating the other. 



This method requires many precautions, and can not be conveniently 

 used for maximum and minimum temperatures, but incompetenthands 

 it is very serviceable and is capable of minute accuracy. 



In the observations of the Messrs. Becquerel it was so arranged that 

 a change of one degree of temperature would cause a deflection of ten 

 degrees in the needle. The needle was brought to zero with the aid of 

 a, lens, and an accuracy of reading to a tenth of a degree centigrade in 

 the galvanometer was abundantly assured. Every precaution was 

 taken. For instance, a junction exposed to the rays of the sun was 

 covered with a triple reflector which screened from the sun's rays, but 

 permitted free ventilation. The results have every guarantee of ac- 

 curacy and are worthy of unusual confldence. 



M. Becquerel's first memoir relating to the forest meteorological 

 problems was not published until 1864. Meantime other students of 

 the subject were showing signs of activity. The French forest inspect- 

 ors Contegril and Bellot took many observations in 1859 and 18G0 on 

 the disposal of rainfall in woods as compared with that in open fields, 

 and similar observations were taken in southern France. Hofmannat 

 Giessen, and Baur at Hohenheim had been taking forest observations 

 for some time. In 1802 and 18G3, Krutzsch, in Saxony, established 

 nine forest stations; several were established in Bavaria, and Count 

 Berchem-Hainhausen, in his private capacity as a large landowner, 

 sup])orted two in Bohemia. In 180G-G8, in Posen, Rivoli, also a private 

 landowner, made many simultaneous comparative observations in for- 

 ests and fields, including temperature, humidity, and evaporation. His 

 observations on individual features of forest action were conflned to 

 the season at which they were greatest. The fertile idea in modern 

 forest meteorological observations consists in having comparative sta- 

 tions at which observations are taken simultaneously. These stations 

 must be as nearly as ])ossible alike, except that one is to be in the 

 woods, the other outside. This happy idea, already employed by Bec- 

 querel and Rivoli, and donbtless by others, but not by all, was first 

 put to systematic use under governmental auspices at stations near 

 Nancy. The French forest administration undertook to set at rest the 



