$0 FOfiJESTHY IN SiAlti. 



the temperature of the same. In experimenting, the 

 cooling effects proceeding from the roots is produced 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 be 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 

 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 halt-way up the trunk, and the third in 

 the branches 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 medulla of 

 the tree. By adopting the first of the methods specfied, 

 the depth at which the thermometers would have been 



