198 SOLAR KADIATION. 



is only 50° on the Ehine ; the harvest near London is matured with a 

 mean summer temperature of 62°, and in the same time at Upsal with 

 59°. When we take these phenomena into consideration, we must 

 conclude that they depend upon the presence or absence of that 

 important element of heat, solar radiation, by which the temperature 

 of opaque bodies is raised above that which they could receive from the 

 diffused heat of the atmosphere. When we also know that the absorp- 

 tion and assimilation of carbon, the substance of which about half the 

 mass of plants is composed, does not take place except under the 

 influence of light, and is proportionate to its intensity, we feel assured 

 that the determination of its effects must prove interesting to cultivators. 

 Under this impression, the Comte de Gasparin made various experiments. 

 In 1840 he communicated some observations on three Mulberry-trees, of 

 the same variety. One of these was fully exposed to the rays of the 

 sun, the second only till noon, and the third was whoUy in the shade. 

 The solid matter of the leaves of the first was 45 per cent, of their 

 weight ; that of the second, 36 per cent. ; whilst that of the third was 

 only 27 per cent. In 1852 he ctdtivated some Broad-beans on a plot 

 of ground divided into two equal parts by a partition which shaded one 

 half the ground from the rays of the sun. After being dried, the plants 

 grown on the south side weighed 21 ounces ; hut those grown on the 

 north side,, although much taller, weighed only twelve ounces. The 

 difference in their fructification was, however, still more remai'k- 

 ahle. The plants on the south side had 131 pods, those on the north 

 only 4*7. It is impossible to attribute these results to the simple 

 augmentation of heat. The plants in the above experiment had a mean, 

 atmospheric temperature of 59J° Fah. for 84 days, and 5|° was the 

 average d.aily amount of solar radiation. Certainly an additional 

 5J° of obscure heat would not produce such results. 



The laws indicated by Daniell plainly direct us to the 

 means we are to employ to moderate atmospherical temperature. 

 A screen, of whatever kind, interposed between the sun and a 

 plant, intercepts the radiant heat of the sun, and returns it into 

 space ; and thus, in addition to the diminution of perspiration 

 by the removal of a part of the stimulus that causes it, actually 

 tends, to lower the temperature that surrounds the plant. In 

 like manner, the interposition of a screen, however slight, 

 between a plant and the sky, intercepts the radiant heat of 

 the earth; and, instead of allowing it to pass off into space, 

 returns it to the ground, the temperature of which is main- 

 tained at a higher point than it otherwise would be. Hence it 

 is that plants growing below the deep projecting eaves of 



