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in the Magnetic Forces of the Earth. 22 1 



of dew deposited in any small interval of time will then depend 

 not only upon the reduction of temperature, which takes place 

 in this interval, but also upon the difference of temperature be- 

 tween the ground and the contiguous air, that obtains during this 

 interval. Now, this difference may increase, as the night ad- 

 vances, at the same time that the loss of temperature from radia- 

 tion continues the same ; and the increase may be more than suf- 

 ficient to compensate for the diminished temperature of the vapor, 

 and thus the dew may be more abundant. It will be seen far- 

 ther on, that even when a small quantity of dew falls during the 

 night, a portion of vapor is abstracted from the air to the height 

 of several hundred feet. We may learn from this fact that we 

 cannot derive the variations in the quantity of dew from the ob- 

 served variations in the falling of the dew point, noted only at 

 one particular height. The observations should be made at va- 

 rious heights to furnish the data required for such a calculation. 

 Whether the fact really be in accordance with our understand- 

 ing of the statement of Lame or not, there can be little question 

 that, in the average of weeks and months, there will be more 

 dew deposited after midnight than before ; for, besides that we 

 have the authority of Lame in support of the assertion, it will 

 be observed that it is only when the elevation of the tempera- 

 ture of the air above the dew point at 4 p. m., is equal to, or less 

 than, the amount that the temperature falls in the interval of 

 time between this hour and sundown, that the dew will begin to 

 fall at sundown, and it appears from observation, that at Phila- 

 delphia, fhe difference between the dew point and the tempera- 

 ture of the air, at 4 p. m., is, on the average, about equal to the 

 entire average nocturnal loss of temperature, and is accordingly 

 about 8°, more or less, greater than the fall of temperature down 

 to the time of sundown. Dew must therefore be much more fre- 

 quent after than before midnight. It is to be observed here, that 

 it is not necessary that the humidity of the air should be above 

 the average, in order that any dew may be deposited during the 

 night, as may perhaps be inferred from the above statement; for, 

 if it were not for the heat given out by the falling dew, the tem- 

 perature would fall much lower ; in other words, the actual loss 

 of temperature is that due to radiation diminished by the heat 

 given out by the condensed vapor. 



As to the relative frequency of dews at different seasons, it 

 must become greater, from the very nature of things, for the same 

 loss of heat by radiation, in proportion as the humidity of the air 

 becomes greater ; and therefore must increase from July or Au- 

 gust to December or January. It is true, that this increased fre- 

 quency in the occurrence of dew, is partially compensated lor by 

 the diminution in the amount of dew deposited from saturated 

 air m a given time, from the same reduction of temperature, as 



