HEAT ABSORPTIOIT. 



81 



forest, and the amouiit of mineral matter held in solution by the ground- 

 water will enable us to comijute the amount of water which has been 

 evaporated. The required data, particularly the latter, are often known 

 with some exactness. Taking, with Grandeau, the average amount of 

 of ash in the plant at 5 per cent and the quantity of mineral matter in 

 solution as two parts in ten thousand, the plant must transpire five 

 thousand times the weight of its ash or two hundred and fifty times its 

 own direct weight. Grandeau gives 6,497 kilograms as the annual pro- 

 duction of a beech forest (wood and foliage), 0,442 as that of Norway 

 spruce, and G,420 as that of pines, for every hectare covered. These 

 numbers multiplied by 250 would give the quantity of water transpired, 

 which, reduced to thickness of the sheet of water over a hectare, gives 

 the depth or rainfall equivalent. In these cases it gives 0.4, 0.3, and 

 0.3 inches, respectively. The numbers given by Grandeau are for cen- 

 tral Europe, as were the preceding. The results by the two methods 

 are 0.5 and 0.3 inches, which, by chance, are remarkably close to each 

 other. Knowing the amount of water transpired, and the temperature 

 at which transpiration takes place, it is easy to get the amount of heat 

 used up in the process. The evaporation of any given weight of water 



. , . . . ... ■ 000.5 — 0.095^ ^. 



would heat by 1° C. a weight of air which is times as 



great, where t is the centigrade temperature of the water evai)oratcd. 



As water is 773 times as dense as air at the standard temperature (32'^ 



F.) and standard pressure (30 inches), the evaporation of alayer of Avater 



an inch thick takes up as much heat as would warm by 1° G. a layer of 



, . , . , . C00.5 — 0.095 t ^^^ 

 air of standard density and ot a thickness oi _ X < iS= 



(006.5—0.095^) 3255 inches. For different temperatures the thickness of 

 the layer of a homogeneous atmosphere which might be cooled 1 degree 

 for each inch of the layer of water evaporated is as follows : 



The cooling equivalent to the annual forest growth, is therefore 

 about 0.4 times that exi)ressed in this table and takes place through the 

 entire season, but is greatest in late spring in sunshine, least in dark- 

 ness in late summer. It acts, however, continually, and when the 

 enormous thickness of the air layer is divided by 150 days (about the 

 length of the active season) and this by the number of seconds in a day, 

 the result per secon«l does not appear so very large. As a convenient 

 general expression it may be said that the evaporation (►f any depth of 

 water would take up enough heat to cool by one degree (Fahrenheit in 



12444— No. 7- 



-G 



