THE LIBRARY. 81 



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 pirt of the trunk. 

 The principal object to be attended to, was the determin- 

 ing the influence of the temperature of 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 

 temperature of the trunk the more, the less distant the 

 p.rint is 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 

 temperature of the tree is intimately connected with the 

 volume of the trunk or branches, and with variations in 

 the themometric conditions of the air. And the effects of 

 temperature of this and of the sunshine propagate 

 themselves transversely, while the temperature of the 

 ground is propagated from below upwards. The direct 

 comparison of the temperatures of the trees in the first 

 two cases being excluded, this determined the naturalists 

 Boehm and Breitenlohner to adopt the third method 

 possible, that is, equal heights and equal depths. The 

 two birch trees selected presented the following 

 measurements : 



Horizontal distance between the two trees, 



Height of birch experimented on, 



Height of birch of verification or comparison, 



Distance of lowest point of observation from ground, 



Distance of middle point irom lowest. 



Distance of highest point from middle one, 



55 metres. 



15-5 



12 



6-3 



6 



6 



