SOLAE RADIATION. 195 



in the clear aspect of the sky. A portion of a grass-plat under the 

 protection of a tree or hedge, will generally be found, on a clear night, 

 to be eight or ten degrees warmer than surrounding unsheltered parts ; 

 and it is well known to gardeners that less dew and frost are to be found 

 in such situations, than in those which are wholly exposed." {Sort. 

 Trans., vi. 8). 



This very important subject has received further explanation from 

 a writer, whose words we quote, with some omissions, from the 

 Gardener's Chronicle of 1853, pp. 679 and 627. The action of the 

 sun upon aU. things that receive his rays is a matter of common 

 notoriety. How important to the growth of plants, to the formation of 

 colour and taste, to the ripening of fruit, to the consolidation of all 

 vegetable tissues, is solar light it is needless to say. But few 

 persons are aware of the amount of that force, or of the views of modem 

 philosophers as to the manner in which it takes effect. We may view 

 the surface of a lake exposed to the sun's rays during a warm summer's 

 day, whilst the whole scene may seem to be one of the utmost tranquillity, 

 so that we might naturally conclude that no movement of any import- 

 ance was then going on. It will be found, however, that such in 

 reality is not the case ; for the rays of the sun exert a force of which 

 we can scarcely form any adequate idea. Supposing the lake is only 

 two miles square, it may be calculated that there wiU be raised from its 

 surface in one day more than sixty-four thousand tons weight of water 

 (64,821), by means of solar radiation. This is at least equal to the work 

 of 10 steam-engines of 200 horse-power each for the same space of time, 

 presuming that the above weight is only raised to an average height 

 of between 300 and 400 feet. To balance that weight, a hill of earth 

 would be required, 30 feet high, 100 feet wide, and 600 feet in length. 

 In making the calculations which have led to these statements, it has 

 been assumed that, in a hot day in summer, a quarter of an inch of water 

 would be evaporated from an exposed surface of a lake in twelve 

 hours, and this from an area of two miles square would amount to 

 2,323,200 cubic feet, which, at 62^ pounds per cubic foot, is equal to 

 64,821 tons. Now, a quarter of an inch is not a maximum amount 

 of evaporation. The Comte de Gasparin observed 0-59 inch ( Gardiner's 

 Chronicle, 1849, p. 757), and on five successive days the average 

 exceeded haK an inch. Howard, in his Climate of London, has 

 recorded as much as 0*39 inch in one day. It therefore appears that 

 0-25 inch, that which we have assumed, is not an exaggerated 

 quantity ; on the contrary, it is but one-half of that which, according 

 to good authorities, has been actually removed by evaporation, and 

 under a temperature of from 73° to 75° Fahr. Instead of 64,000 

 tons, facts would justify us in stating that 130,000 tons might be raised 

 in one day from a surface of water not exceeding two males square. 



Some idea may be formed from these statements of the immense 



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