220 Wm. A. Norton on the Variations 



of all these considerations we may conclude, that the notion of a 

 variable flow of heat toward the surface of the earth fails entirely 

 to explain the unequal losses of temperature at night in different 

 seasons of the year, and that it is highly improbable that it has 

 any considerable effect to diminish the losses of temperature from 

 hour to hour during the night. Indeed all known facts connected 

 with the variation of temperature, favor the idea that the laws of 

 the nocturnal variation are the result of some cause at the earth's 

 surface tending to diminish the amount of heat lost there, and 

 producing the same effect as if the radiation varied between cer- 

 tain limits and according to certain laws. This cause can be 

 nothing else than the deposition of condensed vapor from the air 

 in contact with the earth. 



Let us now take up the independent inquiry as to what is the 

 testimony of observation and experiment in relation to the varia- 

 tions in the amount of dew deposited in different seasons and in 

 different hours of the night, the actual amount deposited at any one 

 season, and the quantity of heat given out in the condensation of 

 the vapor into dew. I find the following statement with respect 

 to the first of these points, in Lame's Cours de Physique. u Dew 

 is less abundant before midnight than during the hours which 

 precede the rising of the sun. It is more frequent in spring, and 

 especially in autumn than in summer." I conceive the meaning 

 of the first of these statements to be, that on any individual night 

 when the dew begins to fall early in the evening, it becomes 

 more abundant towards morning. If this be true, it may arise 

 from the fact that there will be sufficient displacement of the air, 

 even on what would be called a calm night, to bring about more 

 or less of an exchange of place between the air resting upon the 

 surface and that which is posited above this ; and as the cooling 

 goes on, these various masses brought in succession to the surface 

 will become more and more humid, and therefore more and more 

 likely to deposit a portion of their vapor before they give place in 

 their turn to other bodies of air. Such currents will be established 

 by inequality of surface and of radiation, the irregular action of 

 elevated objects, and perhaps other causes. When the air is per- 

 fectly tranquil, the deposition of vapor might increase towards 

 morning by the temperature of the air above the surface becom- 

 ing reduced below the dew point, in which case a portion of its 

 vapor would be condensed and fall in a fine mist. This undoubt- 

 edly sometimes happens, and must be more frequent towards 

 morning than before midnight. But there is another way in 

 which such a phenomenon may be conceived to arise. The air 

 in contact with the earth, and for a certain small distance above 

 it, when dew is being deposited is saturated with vapor. Its 

 temperature is probably somewhat higher than that of the ground, 

 for the variations of temperature of the ground will not be com- 

 municated instantaneously to the air just above it. The amount 



