BAROMETER TO SEA LEVEL. 7 



small empirical correction, determined from accurate comparison of 

 reduced readings and actual observations, to be applied to Table II." 

 A formula is also given, wMch may be written E = (I^+If') Z, in 

 whicb N is the number from Table II., and N' that from Table III. 



If we compare this formula with (iv.), it is evident that some cor- 

 rection to N is necessary, since R does not vary as Z. The correction 

 should, however, depend on the reading of the barometer (/5) as well 

 as on Z and t; but the empirical correction N' is given without 

 regard to /?. 



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. Dun woody 's Tables have not, so far as I am aware, been 

 anywhere brought into iise. The results given by his Tables II. and 

 III. do not, however, differ much at moderate altitudes from those 

 ^ven by Table A, as will be seen from the following examples : 



EXAMPLES OP THE USE OP 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 ^^J^.^ = 3.0047, and the difi"erence for 

 800 46 



100 ft. =0.3819. 

 Hence the reduction, 



B = ( 3.0047 + — ^ 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. Dun woody '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 ^^^^^^^^ = 3.9071, and the difierence for 100 ft. is 0.3990, 



hence -B = (3.9071 + 0.3990) x 0-28 = 4.3061 x 0.28 



= 1.206. 



