8 ON THE REDUCTION OF THE 



Guyot's Tables D, XYI. 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 iV for every second 



Z t 



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 -r^, and tabulate the values of the reduction so 



obtained for values of h, 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 



of — ^ — are given ; so that if iV^ is the number in the table for 



t t 



temperature t, formula iii. becomes 



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 



