SOLAR RADIATION. 197 



solar heat are actually put out of existence by the growth of plants, but 

 an equivalent of statical mechanical elTect is stored up in the organic 

 products, and may be reproduced as heat by burning them. All the 

 heat of fires obtained by burning wood grown from year to year is in 

 fact solar heat reproduced." And so, we may accordingly add, must be 

 the heat derived from the combustion of at least the portions which 

 have had a vegetable existence, of wood-coal and other matters. 



Professor Thomson has concluded, and with reference to an equivalent 

 conclusion by Sir John Herschel, that heat radiated from the sun 

 (sunlight being included in this term) is the principal source of 

 mechanical effect available to man. From it is derived the whole mecha- 

 nical efiect of animals working, water-wheels worked by rivers, steam- 

 engines, and galvanic engines, &c. Vegetation is the great support 

 of animal power, but vegetation could not be maintained without the 

 action of the sun's rays, received directly or indirectly. "Without such 

 powerful evaporation caused by the sun's rays, as we have endeavoured 

 to exemplify, the rivers would soon lose their general source. And it 

 has been already stated that combustible materials, without which 

 steam-power could not be generated, are stores of solar heat. 



This gives some idea of the immense power of solar radiation ; and 

 within their respective spheres of action, both horticulturists and 

 agriculturists may advantageously direct their attention to the subject. 

 For instance, the former would avoid watering at a time, and in a 

 manner, that would render nearly all the water he supplied liable to be 

 carried oflf by evaporation before it could reach the principal roots of his 

 plants ; and the farmer, knowing the effects of radiation on a moist 

 surface, would hesitate before he flooded say four acres with manure 

 water, at the risk of losing a hundred tons of it, together with its 

 portion of ammonia, by evaporation. 



The same subject is taken up by the Comte de Gasparin in a commu- 

 nication to the Institute, printed in the Comptes Mendus for June, 1853. 

 The effects, he observes, of solar radiation on vegetation are so apparent 

 and so well known that no one doubts their importance. When one 

 plants a Vine, he does not require scientific information to direct him in 

 choosing a southern aspect; nor to plant fruit-trees against a wall which 

 receives and reverberates the rays of the sun; nor to place exotic plants 

 under glass, which readily admits direct rays of heat and light, but 

 through which obscure heat, or that derived from a heating apparatus 

 in a hothouse, passes slowly, and thus an accumiilation of heat takes 

 place ; practical men do all these things as a matter of course. But 

 there are many other effects resulting from the same cause, which do 

 not come so directly under our senses. . The Olive is unproductive at 

 Agen, with a mean temperature 9f 57° Fahr., and fertile in Dahnatia 

 with 55^° ; the limit of the Viue is arrested by 54° mean temperature 

 on the banlcs of the Loire, but grapes ripen where the mean temperatuie 



