118 Mr. Darnell on Climate. {Feb. 



would often prevent the sudden injury which so frequently arises 

 at this period of the year. 



Some of the present practices of gardening are founded upon 

 experience of similar effects, and it is well known that cuttings 

 of plants succeed best in a border with a northern aspect pro- 

 tected from the wind; or if otherwise situated, they require to 

 be screened from the force of the noon-day sun. If these pre- 

 cautions be unattended to, they speedily droop and die. For 

 the same reason, the autumn is selected for placing them in the 

 ground, as well as for transplanting trees ; the atmosphere at 

 that season being saturated with moisture is not found to 

 exhaust the plant before it has become rooted in the soil. 



Over the absolute state of vapour in the air we are wholly 

 powerless, and by no system of watering can we affect the dew- 

 point in the free atmosphere. This is determined in the upper 

 regions ; it is only therefore by these indirect methods, and by 

 the selection of proper seasons, that we can preserve the more 

 tender shoots of the vegetable kingdom from the injurious 

 effects of excessive exhalation. 



Radiation, the second cause which I have mentioned as pro- 

 ducing a sudden and injurious influence upon the tender 

 products of the garden, is one that has been little understood, 

 till of late years, by the natural philosopher; and even to this 

 day has not been rendered familiar to the practical gardener ; 

 who, although he has been taught by experience to guard 

 against some of its effects, is totally unacquainted with the 

 theory of his practice. Dr. Wells, to whose admirable " Essay 

 upon Dew," we are so much indebted for our present knowledge 

 upon this important subject, thus candidly remarks upon this 

 anticipation of science : " I had often, in the pride of half 

 knowledge, smiled at the means frequently employed by gar- 

 deners to protect tender plants from cold, as it appeared to me 

 impossible that a thin mat or any such flimsy substance could 

 prevent them from attaining the temperature of the atmosphere, 

 by which alone I thought them hable to be injured. But when 

 I had learned that bodies on the surface of the earth become, 

 during a still and serene night, colder than the atmosphere, by 

 radiating their heat to the heavens, I perceived immediately a 

 just reason for the practice which 1 had before deemed useless." 



The power of emitting heat in straight lines in every direction, 

 independently of contact, may be regarded as a property com- 

 mon to all matter, but differing in degree in different kinds of 

 matter. Co-existing with it, in the same degrees, may be 

 regarded the power of absorbing heat so emitted from other 

 bodies. Polished metals, and the fibres of vegetables, may be 

 considered as placed at the two extremities of the scale upon 

 which these properties iu different substances may be measured. 



