26 REPORT OF THE SECRETARY. 



the twenty-four, continuously for a number of years. Such tables were 

 furnished by the observations at Toronto, Mohawk, New Haven, and 

 Philadelphia. The tables of this section will be of much importance to 

 those interested in meteorology, as furnishing data for the special cli- 

 matology of the various portions of the United States. The institution 

 has been frequently applied to by State authorities, agricultural and 

 medical societies and others, for information as to the temperature of 

 peculiar localities, which can now be more readily supplied than before 

 the tables were printed. 



The tables of the second section relate to the daily fluctuation of the 

 temperature during the year. The regular variations of the tempera- 

 ture due to changes in the sun's altitude and the length of the day are 

 affected by perturbations from day to day on account of aqueous vapor 

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

 and by the direction of the wind. As a general rule the maximum heat 

 occurs some time after the sun has reached his greatest altitude, and the 

 greatest cold somewhere between midnight and sunrise. Even in mid- 

 winter in high latitudes, during the continued absence of the sun, these 

 periodic fluctuations are still perceptible ; which may be accounted for 

 by supposing waves of heat transported from more southern regions 

 where the sun still rises above the horizon. 



The tables of the third section relate to the annual fluctuations of the 

 temperature and the perturbations from the curve derived from obser- 

 vations made during a series of years. These are of importance in de- 

 termining questions as to the secular changes in climate. 



The work is illustrated by three charts of isothermal spaces, one for 

 summer, another for winter, and the third for the year ; also two plates 

 and a number of wood-cuts. 



The next series of meteorological observations which remains to be 

 discussed and analyzed is that which relates to the pressure of the atmos- 

 phere, which, it is evident from what has been said in regard to the 

 winds, is an important element in determining the peculiar climatology 

 of the North American Continent and its relation to that of the other 

 parts of the world. This work will be commenced during the latter part 

 of the present year. 



Another work will be continued — that on the collation and discussion 

 of the observations on thunder-storms and other casual phenomena. The 

 observations in regard to these phenomena have been copied from the 

 original records and arranged in tables previous to their discussion. 



The miscellaneous work done by our computers in regard to meteor- 

 ology during the year 1875 is as follows : 



Manuscript temperature tables were prepared for the first seven months 

 of 1871 and' for March, 1872, these records not having been accessible 

 before. Many additions were made to the general tables from new ma- 

 terial sent by observers and others. The collection and tabulation of 

 the observations for rain-fall at the Smithsonian stations for 1874 has 



