no DISCUSSION OF THE DAILY FLUCTUATION 



James Lewis, of Mohawk, is due the merit of having early brought into operation 

 a thermograph of his own invention. 



The collection of monthly values for daily fluctuation comprises the results from 

 bihourly, hourly, and semi-hourly observations at 18 stations, see first table accom- 

 panying this section of the paper. They are arranged according to latitude. From 

 these the second series of tables is derived as follows: For each month separately, 

 the daily mean temperature t is subtracted from the observed temperature at any 

 hour, and the difference is set down; a positive sign thus indicates a higher, and a 

 negative sign a lower temperature than that of the day. These tables of difierences 

 would furnish the true diurnal fluctuation, if the effect of the annual fluctuation was 

 fully eliminated, and if the daily mean was accurately known. The amount of the 

 annual fluctuation in one day is generally small when compared with the daily fluc- 

 tuation, and corrections for it need only be applied in extreme cases, as for instance 

 in the Arctic Kegions, where the daily range is small in comparison with the annual 

 range; at Van Rensselaer harbor and Port Kennedy the maximum effect for 24 

 hours amounts to a little more than half a degree (Fah.), on account of which the 

 maximum correction for midnight and the hour preceding it would be one-fourth 

 of a degree, and proportionally less for the intermediate hours. This correction is 

 greatest in April and October, and insensible in July and January. 



These tables of hourly differences furnish at once the means of correcting any 

 irregularly observed series, and the mean temperature thus corrected will be the 

 same as that found from an unbroken and regular series of hourly observations. 

 The chief value of these tables lies in this application, and in any special case we 

 have only to select the table for that locality where the thermal conditions may be 

 supposed the same, or at least most nearly resembling those at the locality for which 

 the interpolation or reduction is to be made. For the purpose of facilitating this 

 application, a series of mean values for certain selected combinations of hours is 

 added to each table — these require some further explanation. 



These combinations refer to those observing hours from Avhich most probably the 

 nearest approximation to the mean temperature of the day may readily be deduced, 

 not only for the entire year, but also for each month and for any locality, and apply 

 to the cases of record limited to two, three, and four entries a day. The tabular 

 corrections to the selected four hour combination specially, become serviceable for 

 self-registering instruments, when with the least labor (reading off the trace or 

 punctures at those four hours) we wish to obtain a reliable daily mean short of the 

 tedious process of operating on 24 equidistant records. 



About the year 1815, Prof. C. Dewey examined^ the hours 7 A. M., 2 and 9 P. M., 

 adopted by the Manheim^ Meteorological Society, with reference to their applica-. 

 bility to our climate, and in 1816 and 1817 instituted a short series of hourly 

 observations at Williamstown which proved the fitness of these hours for observa- 

 tion in the United States. These results he communicated to Secretary Calhoun, 



^ Annual Report of the Board of Regents of the Smithsonian Institution for the year 1857, p. 310; 

 also annual report for 1860, p. 413. 

 '^ In Baden, Germany. 



