402 FOEESTS AND THEIR CLIMATIC INFLUENCE. 



and for wliat reason veg-etables are overtaken by frost in low situations 

 and are exempt from its effects at heights a little more elevated. M. 

 Martins observed a ttict of this kind in the Botanic Garden of Mont- 

 l)ellit'r. Laurels, figs, olives, nearly all perish in the lower parts of this 

 garden, while, under conditions of shelter wholly similar, they are safe 

 at an eleviition greater by only a few metres. Do we not ]i;now also 

 tliat vines planted on the acclivities of hills i)roduce better wine than 

 those which grow at the bottom, on account of a more perfect maturity ? 

 Experiments which we have made with the electric thermometer 

 evince quite satistactorily this remarlvable property, that the teinpera- 

 tiire of the air rises from 1'".33 (-4 feet 4 inches) above the soil to 21'".25 

 {6S feet 10 inches) at the summit of a chestnut tree, and probably from 

 this summit upward to a certain height whose limit has been fixed by 

 M. Martins and other meteorologists 5 for the leafy periphery of trees 

 must be supposed to act like the soil covered with low plants, by reason 

 of its great absorbing and emissive powder. The mean differences 

 between the temperatures of the two stations have been verified at the 

 Jardin des Plantes, during several years: 



From 1™.:?3 to 16 metres 0o.420 



From 16 metres to 21™.25 0o.580 ; 



clearly showing the influence exercised by low plants and the periphery 

 of trees on the temperature of the ambient air through the effect of 

 calorific radiation. Let us now inquire what the influence of the body 

 of the tree — that is to say, of the trunk and branches — may amount to. 

 Every body, not excepting trees, immersed in air grows warm or cold, 

 and partakes consequently, more or less, in tlie variations of the temper- 

 ature experienced by the ambient medium, the effects produced depend- 

 ing on the state of the siufaces of the body, on its conducting power, 

 and its specific heat.. The experiments whose results we are about to 

 report, as detailed in several memoirs which we have had occasion to 

 present to the academy", (ISGl-'Gl,) furnish the clearest proof of the 

 above proposition. Some of those results are as follows: 



On examining the variations of temperature in the interior of a maple, 

 0"\J: (IJ inch) in diameter and situated in a group of other trees, it was 

 found that, during the months of August, September, and October, the 

 mean temperatures presented no sensible difference with that of the air 

 except in September, although the variations in the tree were in amount 

 very nearly one-half of those of the air. 



The temperature in a tree is far from being the same in all its ])arts. 

 If the leaves and branches promptly assume an equilibrium of tempera- 

 ture with the air, not the less promptly does the trunk conform tliere- 

 with,and that to a depth of 0"M, (f inches.) The effects are ditterent in 

 trees exposed to the solar radiation, according to the j)roximity or re- 

 moteness of objects which absorb and radiate heat : a plum tree i)laced 

 near a wall, 2 metres (7 feet) thick, was covered, in the month of July, 

 with leaves and fruit; the tree was metres (20 feet) in height, and in 

 diameter, 0'".33, (13 inch.) The difference between the maximum and 

 minimum of temperature had, for several days, been from 21^ to 25^", 

 and the temj^erature at the interior of the tree had ascended to 37^, 

 (99^" E. ;) such a state of things could not long exist without weakening 

 the tree, hence the leaves withered by degrees, the fruit fell off", and 

 everything announced the approach of death, which took place a month 

 later; thus ^vas produced an effect which gardeners call a heat stroke, 

 {coxp do dial cur.) 



We see, therefore, that a tree becomes heated in the air like an inert 

 body, and the more rapidly in proportion as its mass has less volume 



