302 



METEOROLOGY. 



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different heights the influence which the cooling effect of the roots 

 and the crown of the tree exercises on the temperature of the same. 

 In experimenting, the cooling effects proceeding from the roots is pro- 

 duced by abundantly watering the ground ; and that of the crown of 

 the tree by suspended apparatus acting as a covering or shade. To 

 ascertain with exactitude the cooling effect thus produced, it is 

 required to have two trees, one of which may serve as a subject of 

 experiment, and the other under natural conditions to serve as a tree 

 for verification or comparison of results obtained. 



To find two trees of the same species, development, girth, and 

 height, and equally exposed to the action of light, presents some 

 difficulty. Others also presented themselves to Boehm and 

 Breitenlohner, arising from the necessity there was that there should 

 lie within a certain distance abundant water, and that the tree used 

 for verification should not be so remote from the subject of experiment 

 as to render impossible the comparison of fundamental conditions. 



Two birches were chosen, which, however, did not fulfil all the 

 conditions, as they were not in every respect exactly alike. The 

 tree selected for experiments was in every respect weaker than the 

 other which was to serve for comparison. In three different ways 

 could be carried out the idea of placing thermometers, one at the foot 

 of the tree, another half-way up the trunk, and the tliird in the 

 Ijranclies — viz., first, placing them vertically equidistant without 

 regard to the diameter of trunk or branches ; second, without regard 

 to equality of vertical distances, to seek for equal diameters ; third, 

 without regard to diameters, to place the thermometers at equal 

 distances, and at equal depths in the trees. It was necessary to 

 avoid a mixing of these three methods, in order not to complicate 

 and confuse the observations. 



Through the central layers of the birch constituting soft wood 

 through which circulates the sap, it presents a possibility of placing 

 the thermometer in the medula of the tree. By adopting the first 

 of the methods specified, the depth at which the thermometers 

 would have been placed in corresponding points in the two trees 

 would have been very different ; and by adopting the second, the 

 distances of the points of observation would have been rather great, 

 especially in the lower part of the trunk. The principal object to 

 be attended to, was the determining the influence of the temperature 

 (if the soil on the thermometric condition of the trunk in relation 

 with the ascending movement of the sap. Taking as a fundamental 

 fact that the temperature of the soil influences the internal tempera- 

 ture of the trunk the more, the less distant is the point from the 

 ground, and the younger are the wood circles, the experiments ought 

 to be made at equal distances from the ground. In the first case, the 



