SECTION II 



DISCUSSION OF THE DAILY FLUCTUATION OF THE ATMOSPHERIC 



TEMPERATURE, 



TABLES OP HOUELY VALUES AND OP HOUKLY DIPPEEENCES PEOM THE 

 DAILY MEAN, FOE EACH MONTH AND THE YEAE, 



AT VARIOUS PLACES IN NORTH AMERICA. 



The Daily Fluctuation of the Temperature. — The daily variation of the tempera- 

 ture, due to the change in the sun's altitude, and dependent upon the length of the 

 day or time of insolation, is principally affected by the amount of aqueous vapor 

 suspended in the atmosphere, by the serenity or cloudiness of the sky, and by the 

 elevation of the ground. As an accumulative effect, the greatest heat will occur 

 some time after the sun has reached its greatest altitude, and the greatest cold some 

 time after its greatest depression. Even in midwinter, in the high latitudes of the 

 Arctic Regions and in the continued absence of the sun, this periodic fluctuation is still 

 perceptible, which may be accounted for by the progress of waves of heat and by 

 its transfer from more southern and still partly insolated regions. In midsummer, 

 when the sun remains above the horizon, the range of the daily fluctuation in the 

 Arctic Regions is very small owing to the small variation in the sun's altitude. As 

 an instance of a small daily fluctuation in a low latitude, Key West near the northern 

 tropic may be cited; here the great humidity of the air tends to confine the daily 

 amplitude within narrow limits. As an example of the opposite eifect or of an 

 excessive daily variation, Albuquerque in the valley of the Rio Grande may be 

 cited ; it is due to the dryness of the air and the great altitude of the place. 



For the investigation of the daily fluctuation hourly observations are quite suffi- 

 cient, but they should be continued for several years, whenever it is desirable to 

 bring out reliable values of the average daily amplitude for each month. It is in 

 these investigations that the want of self-registering instruments or thermographs 

 is most felt. Our records of temperatures, continued regularly during day and 

 night, even for a single year, are very scanty, and there are but three stations where 

 the observations continue over a sufliciently long period; these are Toronto, Canada, 

 and Mohawk, New York, with full hourly records extending over six years at each 

 place, and Sitkar, Alaska, with records over more than twice this period. To Dr. 



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