Transactions of the Horticultural Society. 289 



preserve the more tender shoots of the vegetable kingdom 

 from the injurious effects of excessive exhalation. 



Radiation is the power of emitting heat in straight lines, 

 independently of contact ; it is a property of all matter, but 

 diners in different kinds of matter. Absorption is another 

 general property differing in a similar manner. To prove 

 the different radiating powers of bodies, on a clear night place 

 a thermometer upon a grass-plat, and another upon a gravel 

 walk, or the bare soil ; the temperature of the former will be 

 found many degrees below that of the latter. " The fibrous 

 texture of the grass is favourable to the emission of the heat, 

 but the dense surfaces of the gravel seem to retain and fix it. 

 But this unequal effect will only be perceived when the 

 atmosphere is unclouded, and a free passage is open into 

 space; for even a light mist will arrest the radiant matter in 

 its course, and return as much to the radiating body as it 

 emits. The intervention of more substantial obstacles will of 

 course equally prevent the result, and the balance of tem- 

 perature will not be disturbed in any substance which is not 

 placed 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." 



Radiation during night cools the air on the surface of the 

 ground to the temperature of 32° or lower, at least ten months 

 of the year, and often in July and August to 35°. Low vege- 

 tation suffers from this cold, but from the foliage of a tree or 

 shrub it glides off, and settles upon the ground. It also glides 

 off from eminences and settles in hollows. 



" Any thing which obstructs the free aspect of the sky arrests 

 in proportion the progress of this refrigeration, and the slightest 

 covering of cloth or matting annihilates it altogether. Trees 

 trained upon a wall or paling, or plants sown under their protec- 

 tion, are at once cut off from a large portion of this evil ; and are 

 still farther protected, if within a moderate distance of another 

 opposing screen. The most perfect combination for the growth 

 of exotic fruits in the open air would be a number of parallel walls 

 within a short distance of one another, facing the south-east 

 quarter of the heavens ; the spaces between each should be 

 gravelled, except a narrow border, which should be kept free 

 from weeds." 



The reasons for the gravel and the removal of weeds, the 

 gardener will deduce from what has been stated above as to 



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