BAROMETER TO SEA LEVEL. T 
small empirical correction, determined from accurate comparison of 
reduced readings and actual observations, to be applied to Table IT.” 
A formula is also given, which may be written R = (V+ WN’) Z, in 
which JV is the number from Table II., and NV’ that from Table ITI. 
If we compare this formula with (iv.), it is evident that some cor- 
rection to JV is necessary, since & does not vary as Z. The correction 
should, however, depend on the reading of the barometer () as well 
as on Z and ¢; but the empirical correction WV’ is given without 
regard to f. 
The constants and formula, on which Table II. is based, are not 
given; and the rate of variation of the numbers, with the pressure, 
seems to deviate more than it should, from Boyle’s Law. 
Lieut. Dunwoody’s Tables have not, so far as I am aware, been 
anywhere brought into use. The results given by his Tables IT. and 
IIT. do not, however, differ much at moderate altitudes from those 
given by Table A, as will be seen from the following examples : 
EXAMPLES OF THE USE OF TABLE A. 
Example (1).—At a station 815 ft. above the sea, the reading of 
the barometer being 29.112 in., the temperature of the air 46° Fahr., 
to find the reduced reading. 
From Table A we find : oo. ‘can 3.0047, and the difference for 
100 ft. = 0.3819. 
Hence the reduction, 
R= (3.0047 + a x 0.3819 } x 0.29112 = 3.0620 x 0.29112 
= 0.891, 
and the reduced reading is 30.003. 
Guyot’s tables D, XVI. and XIX.’ used with formula (i.), each 
give, for this reduction, 0.876 in. Lieut. Dunwoody’s tables (ii.) and 
(iii.) give 0.890. 
Example (2).—At a station 1100 ft. above the sea, the reading of 
the barometer being 28 in., the temperature of the air 30° Fahr., to 
find the reduction to sea level. 
Here 1000" 30 = 3.9071, and the difference for 100 ft. is 0.3990, 
hence R = (3.9071 + 0.3990) x 0°28 = 4.3061 x 0.28 
= 1.206. 
