M. BECQUEREL, ON THE CLIMATIC EFFECTS OF FORESTS. 319 



tion cannot be bad in valleys wliile they succeed upon the hills, and the 

 reasons why vegetation may be touched by frost in the low grounds 

 while it is not at higher elevations. M. Martins has noticed a fact 

 of this kind in the botanical garden at Montpelier, where the laurels, 

 figs, and olives perished in all the low grounds in the botanical gardens, 

 while they were spared at some meters of higher level, the conditions 

 as to shelter being just the same, and there being no other differences 

 than altitude. Do we not likewise know that vines growing on the hills 

 yield better wine than those on the flats, and the reasons why they ripen 

 their fruits more completely? 



The experiments we have made with the electric thermometer bring 

 the evidence of this fact strongly to light. The temperature of the at- 

 mosphere rises from 1™.33 above the ground to 21™.25 at the top of a 

 horse chestnut tree, and probably to a certain height above, the distance 

 of which limit has been fixed by M. Martins and other meteorologists; 

 for the leafy covering of the trees should have the same effect as the 

 soil covered with low vegetation, by reason of its great absorbing and 

 evaporating power. The mean differences between the temperatures of 

 the two stations have been determined at the Jardin des Plantes, dur- 

 ing several years, as follows: 



From in^.SS to 16 meters 00.420 (C.) 



From IG meters to 21'".25 0^.580 



"We therefore clearly see the effect whicb low vegetation and the foliage 

 of trees exert upon the temperature of the ambient air, through the influ- 

 ence of the radiation of heat. We will now inquire as to which it may 

 be, with regard to the body of the trees — that is to say, the trunk and 

 branches. All bodies, and trees the same as other bodies, become cool 

 or warm, according to the air around them, and participate to greater 

 or less extent in the variations of temperature of the ambient air. The 

 effects produced depend on the state of the surface of bodies, its con- 

 ducting powers, and specific heat. The experiments leading to these 

 results are described in several memoirs which we have presented to 

 the Academy of Sciences,^ and furnish the most convincing proofs. 

 The following are some of the results observed : 



In seeking to find the variations of temperature within a maple of 0™.4 

 (15| inches) in diameter in the midst of a mass of trees, it was found 

 that during the months of August, September, and October the aver- 

 age temperatures did not vary sensibly from that of the air in Septem- 

 ber, although the range in variation was scarcely half as great within 

 the tree as in the air. 



The temperature within the tree was far from being the same in every 

 part. If the leaves and branches put themselves promptly in equili- 

 brium with the temperature of the air, the trunk did not delay to do the 

 same to a depth of 0™.l (about 4 inches). These effects are different in 

 trees exposed to the sun, and according-as these are near to or far from 

 objects that absorb and radiate heat. Near a wall 2 meters thick a plum- 

 tree was growing, 6 meters high and .35 in diameter, and covered with 

 leaves and fruit in July. The difference between maximum and mini- 

 mum was in some days from 24^ to 25° (C), and the temperature within 

 the tree arose to 37°. Such a condition could not exist long without 

 enfeebling the tree, and as a consequence its leaves perished little by 

 little, its fruits fell, and everything appeared to indicate the approach 

 of death, which came a month later, and was caused by what gardeners 

 call a "eoi(p de chaleur^ (stroke'of heat). 



1 Mdmoires de VAcade'mie des Sciences, 1861-'64. 



