b GN" THE REDUCTION OF THE^ 



The foregoing formulse being all either inconvenient, or not suffi- 

 ciently accurate except for small elevations, I have formed the accom- 

 panying tables (A and B), to facilitate the calculation of the reduction. 



It will be noticed from the form of (iii.) that, at any place, the- 



tempei-ature being constant, the reduced reading, and therefore also 



the reduction, varies as /3. It is, therefore, sufficient to calculate the 



reduction N , for one barometer reading (&) only; from which that 

 Z t 



for any other reading may be obtained by a simple proportion. It 

 is immaterial whether the value adopted for b be one which could be 

 attained, or not ; it may therefore be chosen with reference to con- 

 venience alone. In Table A, 6 is taken equal to 100 inches, so thai 

 the reduction for any reading (;3) of the barometer, may be obtained 

 by the formula 



Table A was calculated by means of formula (iii.), the value of Af 



(t — S2 \ 

 1 + — I . In this table is given the 



quantity iV , for values of Z equal to lOOj 200, 300, &c. feet, 

 Z t 



for every second degree of temperature from —40° to 100° Fahr,,. 



and also, the difference for the next 100 feet at each height. It is- 



sufficient to employ first differences only, in using the table. 



Table B is intended to diminish the labour in applying formula 

 (iii.), as will be explained in the sequel. 



Since calculating these tables, my attention has been called to a 

 paper by Lieut. H. H. C. Dunwoody, U. S. Army, in the Report of 

 the Chief Signal Officer, Washington, 1876. In this paper tables are 

 given, based in part on observations taken by direction of the Chief 

 Signal Officer, TJ. S. A., on Mount Washingtcm, Mount Mitchell, and 

 Pikes Peak. 



In the first table is given the decrease of temperature for each> 

 100 feet of elevation at each hour in the day. In the second table 

 is given the " weight of a column of air 100 feet high, at different 

 barometric pressures and temperatures, expressed in decimals of an 

 inch, calculated for north latitude 40°." The third table " shows a 



*See Gujpt's Paper D, pp. 9 and. 88. 



