2 ON THE REDUCTION OF THE 



No formula is given for use with Table XIX.', but it is stated 

 that the table may be employed " for reducing barometrical observa- 

 tions to the level of the sea, and also to any other level by a similar 

 process." An example is, however, given, applying tables in French 

 measure, corresponding to XIX.', the method of which example may 

 be represented by the formula 



P 2^2; 1 



where i\^ is the number in the table corresponding to the baro- 

 metric reading* and temperature at the upper station, and J¥ that 



corresponding to those at the lower station ; an approximate reduced 

 barometric reading and temperature being employed in taking out 

 the latter quantity. 



Formula (i.) may also be employed with Table XIX.', b being any 

 height and JV the number in the table corresponding to b. No advan- 

 tage is, however, gained, by using this table instead of Table XYL 

 with formula (i.), unless b be taken nearly equal to /?, so that we 

 may have, nearly 



Laplace's formula for computing differences of elevation from baro- 

 metrical observations, from which each of the above is deduced, may 

 be written 



Z=A^logj, (iii.) 



where J. is a constant, dependiag on the mean between the tem- 

 peratures at the upper and lower stations. Strictly, it also depends 

 upon the latitude of the station, and on the height above the sea ; 

 but the variations due to these may be neglected, unless the height 

 is very considerable. 



Now the number , iV , in the above mentioned tables, for baro- 



t 



meter reading 6, and temperature t, is the difference of elevation 



* Throughout this paper, when a ■barometric reading is spoken of, the reading reduced to 

 temp. sa^'Fahr. is to he understood. 



