250 



SCIENCE OF GARDENING. 



Part II. 



than the neighbouring air, condenses a part of the watery vapor of the atmosphere into 

 dew, the utility of which is too manifest to require elucidation. This fluid appears chiefly 

 where it is most wanted, on herbage and low plants, avoiding, in great measure, rocks, 

 bare earth, and considerable masses of water. Its production, too, tends to prevent the 

 injury that might arise from its own cause ; since the precipitation of water, upon tlie 

 tender parts of plants, must lessen the cold in them, which occasions it. The prevention, 

 either wholly or in part, of cold, from radiation, in substances on the ground, by the in- 

 terposition of any solid body between them and the sky, ai-ises in the following manner : 

 the lower body radiates its heat upwards, as if no other intervened between it and the 

 sky ; but the loss, which it hence suffers, is more or less compensated by what is radiated 

 to it, from the body above, the under surface of wliich possesses always the same, or very 

 nearly the same temperature as the air. The manner in wliich clouds prevent, or occa- 

 sion to be small, the appearance of a cold at night, upon the surface of tlie earth, is by 

 radiating heat to the earth, in return for that which they intercept in its progress from 

 the earth towards the heavens. For although, upon the sky becoming suddenly cloudy 

 during a calm "night, a naked thermometer, suspended in the air, commonly rises 2^ or 3 

 degrees : little of tliis rise is to be attributed to the heat evolved by the condensation of 

 watery vapor in the atmosphere, for the heat so exti'icated must soon be dissipated ; 

 whereas the effect of greatly lessening, or preventing altogether, the appearance of a su- 

 perior cold on the earth to that of the air, will be produced by a cloudy sky, during the 

 whole of a long night. 



1 1 99. Dense clouds, near the earth, reflect back the heal^hei/ receive from it by radiation. But 

 similarly dense clouds, if veiy high, though they equally intercept the communication of the 

 earth with the sky, yet being, from their elevated sitviatiou, colder than the earth, will ra- 

 diate to it less heat than they receive from it, and may, consequently, admit of bodies on 

 its surface becoming several degrees colder than the air. Islands, and parts of continents 

 close to the sea, being, by their situation, subject to a cloudy sky, will, from the smaller 

 quantity of heat lost by them through radiation to the heavens, at night, in addition to 

 the reasons commonly assigned, be less cold in winter, than countries considerably distant 

 from any ocean. 



1200. Fogs, like clouds, will arrest heat, which is radiated upwards by the earth, and if 

 they be very dense, and of considerable pei"pendicular extent, may remit to it as much as 

 they receive. Fogs do not, in any instance, furnish a real exception to the general rule, 

 that whatever exists in the atmosphere, capable of stopping or impeding the passage of 

 radiant heat, will prevent or lessen the appearance at night of a cold on the surface of 

 the earth, greater than that of the neighbouring air. The water deposited upon the earth, 

 during' a fog at night, may sometimes be derived from two different sources, one of which 

 is a precipitation of moisture from a considerable part of the atmosphere, in consequence 

 of its general cold ; the other, a real formation of dew, from the condensation, by means 

 of the superficial cold of the ground, of the moisture of that portion of the air, which 

 comes in contact with it. In such a state of things, all bodies will become moist, but 

 those especially which most readily attract dew in clear weather. 



1201. When bodies become cold by radiation, the degree of eflect observed must depend, 

 not only on their radiating power, but in part also on the greater or less ease with which 

 they can derive heat, by conduction, from wanner substances in contact with them. 

 Bodies, exposed in a clear night to the sky, must radiate as much heat to it during the 

 prevalence of wind, as they would do if the air were altogether still. But in the former 

 case, little or no cold will be observed upon them above that of the atmosphere, as the 

 frequent application of warm air must quickly return a heat equal, or nearly so, to that 

 which they had lost by radiation. A slight agitation of the air is sufficient to produce 

 some effect of tliis kind ; though, as has already been said, such an agitation, when the 

 air is very pregnant with moisture, will render greater the quantity of dew, one requisite 

 for a considerable production of tliis fluid being more increased by it, than another is 

 diminished. 



1 202. It has been remarked, that the hurtful effects of cold occur chiefly in hollow places. 

 If this ' be restricted to what happens on serene and calm nights, two reasons from 

 different sources are to be assigned for it. The first is, that the air being stiller in such 

 a situation, than in any other, the cold, from radiation, in the bodies which it contains, 

 will be less diminished by renewed applications of warmer air ; the second, that from the 

 longer continuance of the same air in contact with the ground, in depressed places than 

 in others, less dew will be deposited, and therefore less heat extricated during its 

 formation. 



1 203. An observation closely connected with the preceding, namely, that in clear and 

 still nights, frosts are less severe upon hills, than in neighbouring plains, has excited more 

 attention, chiefly from its contradicting what is commonly regarded an established fact, 

 tliat the cold of the atmosphere always increases with the distance from the earth. But 

 on the contrary the fact is certain, that in very clear and still nights, the air near to the 



