- 



in the Magnetic Forces of the Earth. AT 



also, that the difference between the mean temperature of the air 

 and the temperature at the earth's surface appears to be, in general, 



from 3° to 5° C. lower at the end than at the beginning of a 



1 



night. This being done, and - being taken equal to 0*5, the- 



ft 



rates of cooling at the beginning and end of anight, upon which 

 the thermometer falls from 59° to 32°, are found to be nearly as 

 47 to 46, or very nearly equal. 



The experiments of Melloniand others have established, "that 

 the portion of the sky concerned in the radiation is included within 

 30° to 35° of the zenith ; — that clouds beyond this have but little 

 interfering effect." 



The air cools, by radiation into space and the upper regions of 

 the atmosphere, like the surface of the earth, by radiation to the 

 earth's surface which cools more rapidly than the air from its 

 superior radiating power, by contact with the earth and objects 

 connected with the earth, and by condensation. The difference 

 between the temperature of the earth's surface and of the air at 

 the height of four or five feet may amount to several degrees. 

 The most recent experiments upon nocturnal radiation, viz., those 

 of Melloni, have established, " that while under certain circum- 

 stances some bodies can be cooled to 8° C. below the tempera- 

 ture of the air four or five feet above, in general, the effect of ra- 

 diation is to reduce the temperature of vegetation, &c, not more 

 than 2° below that of the surrounding air." Ordinarily the 

 agitations of the air will be sufficient to establish very nearly an 

 equilibrium of temperature between it and the earth's surface. 



2. Heat is propagated from one particle to another of the earth's 

 mass by ordinary radiation. Hence when two contiguous parti- 

 cles have the same temperature they exchange, by reciprocal ra- 

 diation, equal quantities of heat, and when their temperature is 

 different, the one will gain and the other lose, in a given time, an 

 amount of heat proportionate to their difference of temperature. 

 The rate at which this gain or loss takes place in any body, con- 

 stitutes its conductibility. If we conceive the matter near the 

 earth's surface to be divided into layers of particles, of indefin- 

 itely small thickness, in the cooling of the earth at night the heat 

 lost by any one layer is gained by that next above it. The flow 

 of heat from below upward will diminish the fall of temperature 

 at the surface of the earth; and the velocity of flow will be pro- 

 portionate to the difference of temperature of the first and second 

 Jayer. As the cooling goes on this difference will increase, and 

 at the same time the difference between two consecutive differ- 

 ences will become less, and therefore the velocity of cooling will 

 diminish. If the night were to continue for an indefinite time, 

 this change would go on until the first differences attained to a 

 maximum value, and the second differences became zero, when 



