MAGNETICAL OBSERVATIONS IN IRELAND. 
161 
These results, however, are not entitled to much confidence. 
An attentive consideration of the formulae (IV.) and A .) will 
show that the direction of the resultant isodynamic lines will 
vary very widely with moderate variations in the values of 
x 
y,} y^) y (o) on it depends; or, in other words, that 
a small error in the position of the lines of dip or of horizontal 
intensity will entail a very great one in that of the lines of total 
force. Thus, if we were to take for the lines of dip those in¬ 
ferred from the observations of the year 1835 alone, we should 
find 
P = + *0001051, Q, = - *0000455, 
tan w -- — *4329, w — — 23° 25'; 
a result differing by more than 10° from the former. In these 
latitudes, therefore, very great accuracy is necessary in the de¬ 
termination of the lines of dip and of horizontal force before we 
can make, in this manner, even an approximation to the direc¬ 
tion of the lines of total force. For these reasons the results 
of the direct method, to which we now proceed, seem to be de¬ 
serving of more confidence. 
In the calculation of the isodynamic lines from the results of 
observation by the statical method, we shall take the number of 
W J 
observations at each station to represent the weight of the re¬ 
sult ; we have in this manner 
Limerick . . . weight = 4, 
Armagh .... —— = 2, 
Youghal . . . -= 2; 
the weight of each of the other determinations being unity. 
The following are the elements of the computation ; 
1835. 
