1826.] Mr. Daniell on Climate. 121 



intensity of this action increases with the distance from the 

 equator to the poles ; as the lowest depression of the thermome- 

 ter which has been registered between the tropics, from this 

 cause, is 12°, whereas in the latitude of London, it not unfre- 

 quently amounts to 17°. But however this may be, it is certain 

 that vegetation in this country is hable to be affected at night 

 from the influence of radiation by a temperature below the 

 freezing point of water, ten months in the year ; and even in the 

 two months, July and August, which are the only exceptions, a 

 thermometer covered with wool will sometimes fall to 35°. It is, 

 however, only low vegetation upon the ground which is exposed 

 to the full rigour of this effect. In such a situation the air 

 which is cooled by the process, lies upon the surface of the 

 plants, and from its weight cannot make its escape ; but from 

 the foliage of a tree or shrub, it glides off and settles upon the 

 ground. 



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 further 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 on each side, which 

 should be kept free from weeds and other short vegetables. On 

 the southern sides of these walls, peaches, nectarines, figs, &c. 

 might be trained to advantage, and on their northern sides 

 many hardier kinds of fruit would be very advantageously 

 situated. Tender exotic trees would thus derive all the benefit 

 of the early morning sun, which would at the earliest moment 

 dissijiate the greatest accumulation of cold which immediately 

 precedes its rise, and the injurious influence of nocturnal 

 radiation would be almost entirely prevented. Upon trees so 

 trained, the absolute perpendicular impression could have little 

 effect, and this little might even be prevented by a moderate 

 coping. 



Mats or canvass, upon rollers to draw down occasionally in 

 front of the trees, at the distance of a foot or two from their 

 foliage, would, I have no doubt, be a great advantage in certain 

 dry states of the atmosphere before alluded to, and in the case 

 of walls which are not opposed to others, would be a good 

 substitute for the protection of the latter. 



Experience has taught gardeners the advantages of warding 

 off the effects of frost from tender vegetables^ by loose straw or 

 other litter, but the system of matting does not appear to be 



