Book III. HEAT AND LIGHT. 343 



2247. Two opinions are current req)ecting the nature of heat. By some philosophers it 

 is conceived to be a peculiar subtile fluid, of which the particles repel each other, but 

 have a strong attraction for the particles of other matter. By others it is considered as a 

 motion or vibration of the particles of matter, which is supposed to differ in velocity in 

 different cases, and thus to produce the different degrees of temperature. Whatever 

 decision be ultimately made respecting these opinions, it is certain that there is matter 

 moving in the space between us and the heavenly bodies capable of communicating heat ; 

 the motions of which are rectilineal : thus the solar rays produce heat in acting on the 

 surface of the earth. The beautiful experiments of Dr. Herschel have shown that there 

 are rays transmitted from the sun which do not illuminate, and which yet produce more 

 heat than the visible rays ; and Ritter and Dr. Wollaston have shown that there are other 

 invisible rays distinguished by their chemical effects. 



2248. Heat is radiated by the sun to the earth, and if suffered to accumulate. Dr. 

 Wells observes, would quickly destroy the present constitution of our globe. This evil 

 is prevented by the radiation of heat from the earth to the heavens, during the night, when 

 it receives from them little or no heat in return. But through the wise economy of means, 

 which is witnessed in all the operations of nature, the prevention of this evil is made the 

 source of great positive good. For the surface of the earth, having thus become colder 

 than the neighboring air, condenses d, 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 the 

 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 

 interposition of any solid body between them and the sky, arises in the following man- 

 ner : 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 radi- 

 ated to it, from the body above, the under surface of which possesses always the same, 

 or very nearly the same temperature as the air. The manner in which clouds prevent, or 

 occasion to be small, the appearance of a cold at night, upon the surface of the 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 this rise is to be attributed to the heat evolved by the condensation of 

 watery vapor in the atmosphere, for the heat so extricated must soon be dissipated ; 

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

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

 the whole of a long night. 



2249. Dense clouds, near the earth, reflect back the heat they receive from it by radiation. 

 But similar dense clouds, if very high, though they equally intercept the communication 

 of the earth with the^ky, yet being, from their elevated situation, colder than the earth, 

 will radiate 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 situations, 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 con- 

 siderably distant from any ocean. 



2250. Fogs, like clouds, will arrest heat, which is radiated upwards by the earth, and if 

 they be very dense, and of considerable perpendicular 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 neighboring air. The water deposited upon the 

 earth, during a fog at night, may sometimes be derived from two different sources, one 

 of wiiich 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 condens- 

 ation, 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 be- 

 come moist, but those especially which most i-eadily attract dew in clear weather. 



2251. When bodies become cold by radiation, the degree of effect 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 warmer 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 



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