METEOROLOGY. NIGHT COOLING. 487 



ly cold, a fact which is proved by the rapid diminution \)f tempera- 

 ture experienced on ascending mountains, or by rising into the air 

 in balloons. The internal temperature of the globe, the tendency 

 of which would be to compensate the loss experienced by the body 

 which radiates, has scarcely any efTect in lessening the cooling, be- 

 cause it is propagated with extreme sU)wness, in consequence of the 

 indifferent conducting powers of the earthy substances of which its 

 crust is composed. The air, lastly, which surrounds the radiating 

 body, does not warm it save in the most minute, inappreciable degree, 

 and rather by its contact than by transmitting to it rays of heat, tor 

 the gases have only very limited emissive powers. It is even in 

 consequence of the small amount of this power that the stratum of 

 air in contact with the surface of the ground, does not by any means 

 sink in tlie same proportion as the surface upon which it lies. Thus, 

 in the circumstances which I have indicated, a thermometer laid 

 upon the ground always indicates a temperature lower than that 

 which is proclaimed by one suspended in the air ; and the difference 

 is by so much the greater as the radiating power of the bodies ex- 

 posed is more decided, or as it may take place into a greater extent 

 of the heavens. Every cause which agitates the air, which disturbs 

 its transparency, which contracts the extent of the visible sphere, 

 interferes with nocturnal radiation, and therefore with cooling. A 

 cloud, like a screen, compensates either in whole or in part accord- 

 ing to its proper temperature, for the loss of heat which a body upon 

 the surface of the earth experiences in radiating into space. Wind, 

 by continually renewing the air which is in contact with the surface 

 of bodies tending to cool by radiation, always diminishes its effect to 

 a certain extent. It is for this reason that a cloudless sky and a 

 calm atmosphere, when nocturnal radiation attains its maximum, are 

 most dangerous or injurious to our harvests. 



In a night which combines all the conditions favorable to radiation, 

 a thermometer of small size laid upon the grass will be found to 

 mark from 10° to 14" or 15" Fahr. below the temperature of the sur- 

 rounding atmosphere. Thus in the temperate zone in Europe, as 

 Mr, Daniell has observed, the temperature of meadows and heaths 

 is liable to fall during ten months of the year by the mere effect of 

 nocturnal radiation to a temperature below the freezing point of 

 water; this is particularly apt to happen both in spring and autumn, 

 when the destructive effects of radiation are most to be apprehend- 

 ed, the nocturnal radiations of those seasons frequently lowering the 

 temperature several degrees below the freezing point. 



A few observations which I made upon nocturnal radiation at dif- 

 ferent heights in the Cordilleras, seem to indicate that its effects 

 there are less decided than in Europe, in consequence perhaps of 

 the greater quantity of heat acquired by the ground in the course of 

 the day. It appears that in this mountain range it rarely freezes at 

 a height less than 6560 feet above the level of the sea ; although 

 there are certain circumstances there which favor nocturnal radia- 

 tion sc much, that it is really impossible to indicate any very precise 

 limits. In a general way it may be said that the crops of those 



