6 ON THE REDUCTION OF THE 
The foregoing formule 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 
temperature being constant, the reduced reading, and therefore also 
the reduction, varies as 8. It is, therefore, sufficient to calculate the 
reduction ee , for one barometer reading (5) only; from which that 
for any other reading may be obtained by a simple proportion. It 
is immaterial whether the value adopted for 6 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 that 
the reduction for any reading (f) of the barometer, may be obtained 
by the formula 
Table A was calculated by means of formula (iii.), the value of A, 
being taken as* 60345.51 (1 ae = 
. In this table is given the 
quantity Z, , for values of Z equal to 100, 200, 300, &e. feet, 
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, U. 8S. A., on Mount Washington, 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 Guyot’s Paper D, pp. 9 and. 88. 
