OF THE EARTH’S MAGNETISM. 525 
Where (1842) January represents the mean force for that month; 2 February, 
twice the force for February, and so on; the difference betwixt the yearly means 
thus obtained and the corresponding monthly means were then taken, and these 
differences, plus a constant quantity (that all may have the same sign*), are entered 
in Table XII. This method, it is evident, is equivalent to making a line parallel 
to that of the yearly mean values, the axis of z; and we may consider the ordi- 
nates from this line to that of the monthly mean differences as varying with 
season. 
* I adopt this in preference to giving the quantities with the algebraic signs 4 and —, since 
some persons in glancing at such results may fail to perceive a maximum if it has the negative sign 
prefixed. 
