LUNAR INFLUENCES. 



and whatever be their temperature, emit continually rays of 

 heat, just as the sun or any luminous body emits rays of light. 

 The intensity with which this radiation takes place, depends 

 partly on the temperature, partly on the sort of matter, partly 

 on the state of the surface of the body. The higher the tem- 

 perature, other things being the same, the more intense will be 

 the radiation. Certain sorts of bodies are strong, while others 

 are feeble, radiators. Metallic bodies are examples of the latter, 

 and charcoal of the former. Polished surfaces are unfavourable, 

 rough surfaces favourable to radiation. 



All bodies are likewise capable of reflecting from their surfaces 

 the rays of heat which fall upon them. But different bodies 

 possess this quality of reflection in different degrees, according to 

 the state of their surfaces ; those which have the greatest power 

 of radiation having the least power of reflection. 



A clear and unclouded sky, being in fact empty space, cannot 

 reflect back to the earth any of the heat which is radiated by 

 bodies on the earth ; but if the sky be clouded, the heat thus 

 radiated will be reflected back to the earth in a greater or less 

 degree. 



If, therefore, the firmament at night be clear and unclouded, 

 all bodies on the surface of the earth radiating heat towards it, 

 and receiving back no part of that heat by reflection, will lose 

 temperature, will become colder ; and this fall of temperature 

 will be greater with bodies which are strong radiators than with 

 those which are feeble radiators. 



But if the firmament be covered with clouds, the heat which 

 all bodies on the surface of the earth radiate will be reflected 

 back to them by the clouds, and receiving as much or nearly as 

 much as they emit, their temperature will be maintained. 



So powerful is the cooling effect of an unclouded sky, that in 

 hot climates water is frozen by such exposure. It is placed in 

 porous earthen pots, under the clear sky. It loses heat at the 

 same time by radiation from its surface, by radiation from the 

 surface of the earthen pan, and by evaporation, especially from 

 the latter surface. The result of these combined effects is, that 

 the water in the pans is congealed, although the temperature of 

 the air and surrounding objects may be considerably above the 

 point of congelation. 



The leaves and flowers of plants are always strong radiators 

 of heat, and on a clear and unclouded night they lose temperature 

 continually by this radiation, not receiving at the same time any 

 return by reflection. But if, as has been explained above, the sky 

 be clouded, they will receive as much as they give, and their 

 temperature will not fall. 

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