8 ON THE REDUCTION OF THE 
Guyot’s Tables D, XVI. and XIX.’, if extended, used with for- 
mula (i.), would give in this case 1.179, and Lieut. Dunwoody’s 
give 1.204. 
The value of Table A does not, however, consist so much in sup- 
plying a basis for working out isolated examples, as in furnishing 
data, in a convenient form, for the calculation of tables of reduction 
to sea level, for individual stations. To construct these all that is 
necessary is, first, to obtain the numbers _WV for every second 
degree of temperature, the value assigned to Z being the height of 
the cistern of the barometer above the sea; and then, to multiply 
these numbers by oS and tabulate the values of the reduction so 
obtained for values of 6, between convenient limits, and at larger or 
smaller intervals, according as the station is at a slight or consider- 
able elevation above the sea. The products for any given tempera- 
ture need not be obtained separately, but may be found, one from 
another, by continued addition, and the whole process may be very 
quickly performed with the aid of the Arithmometer of Thomas de 
Colmar, for use with which the table is specially adapted. 
The time occupied in forming a table in this way, is less than one 
half of what is required if the formula of Laplace (iii. of this paper) 
be employed. 
For stations more than 1100 ft. above the sea, Table B (from which 
Table A was deduced) may be employed. In this table the values 
000 : - : . 
of = are given; so that if V is the number in the table for 
t t 
temperature ¢, formula ili. becomes 
; B ~ 100,000 “e° 
Z 
or log B = i00.000 2 * log 8 
For isolated examples this form is sufficiently convenient; but, in 
constructing a table for any station, it is better to make # = 100. 
The formula then becomes 
log (100+ >.) = ‘mon +2, 
