120 Mr. Dcniell oji Climafe. [Peb. 



the surface, but will be dispersed by known laws throughout the 

 mass ; and if a solid body be a good radiator but a bad conduc- 

 tor of heat, the frigorific effect will be condensed upon the face 

 which is exposed. So upon the surface of the earth absolute 

 stillness of the atmosphere is necessary for the accumulation of 

 cold upon the radiating body ; for if the air be in motion, it dis- 

 perses and equalizes the effect, with a rapidity proportioned to 

 its velocity. 



It is upon these principles that Dr. Wells has satisfactorily 

 explained all the phenomena connected with dew or hoar frost. 

 This deposition of moisture is owing to the cold produced in 

 bodies by radiation, which condenses the atmospheric vapour 

 upon their surfaces. It takes place upon vegetables, but not 

 ■upon the naked soil. The fibres of short grass are particularly 

 favourable to its formation. It is not produced either in cloudy 

 or in windy weather, or in situations which are not perfectly 

 open to the sky. It is never formed upon the good conducting 

 surfaces of metals, but is rapidly deposited upon the badly con- 

 ducting surfaces of filamentous bodies, such as cotton, wool, 8tc. 



In remarking that dew is never formed upon metals, it is ne- 

 cessary to distinguish a secondary effect which often causes a 

 deposition of moisture upon every kind of surface indiscrimi- 

 nately. The cold which is produced upon the surface of the 

 radiating body is communicated by slow degrees to the surround- 

 ino- atmosphere, and if the effect be great and of sufficient conti- 

 nuance, moisture is not only deposited upon the solid body, but 

 is precipitated in the air itself; from which it slowly subsides, 

 and settles upon everything within its range. 



The formation of dew is one of the circumstances which 

 modify and check the refrigerating effect of radiation ; for as 

 the vapour is condensed, it gives out the latent heat with which 

 it was combined in its elastic form, and thus, no doubt, prevents 

 an excess of depression which might in many cases prove inju- 

 rious to vegetation. A compensating arrangement is thus esta- 

 blished, which, while it produces all the advantages of this gentle 

 effusion of moisture, guards against the injurious concentration 

 of the cause by which it is produced. 



The effects of radiation come under the consideration of the 

 Horticulturist in two points of view : the first regards the 

 primary influence upon vegetables, exposed to it ; the second 

 the modifications produced by it upon the atmosphere of parti- 

 cular situations. To vegetables growing in the climates for 

 which they were originally designed by nature, there can be no 

 doubt that the action of radiation is particularly beneficial, from 

 the deposition of moisture which it determines upon their foliage ; 

 but to tender plants artificially trained to resist the rigours of an 

 unnatural situation, this extra degree of cold may prove highly 

 prejudicial. It also appears probable from observation, that the 



