MAGNETICAL OBSERVATIONS IN IRELAND, 143 
the same dimensions and were used with the same circle, and 
yet their results, as we see, are widely discordant. We must 
seek, then, in the needles themselves the cause of these perplex¬ 
ing discrepancies, and we are forced to conclude that there may 
exist, even in the best needles, some source of constant error 
which remains uncorrected by the various reversals usually 
made; and that accordingly no repetition of observations with 
a needle so circumstanced can furnish even an approximation to 
the absolute dip. If this error be due to the incomplete adjust¬ 
ment of the needle (such as deviation of centre of gravity from 
the axle, &c.) its magnitude will be a function of the dip, and 
of the force, which may be assumed to be constant where the 
variations of these elements are not considerable. Hence, to 
determine its amount for any particular needle, it is necessary 
to make a careful series of observations with it at some station 
for which the dip has been accurately determined (from the 
mean of several needles); and the difference will be a constant 
correction to be applied to all future results within certain 
limits. 
It fortunately happens that the two ordinary needles used in 
the present series of observations in Ireland were among those 
employed by Captain James Ross in London ; so that their cor¬ 
rections maybe considered to be accurately known. The mean 
difference of the values of the dip as given by needles L (1) and 
L (4) having been w r ell determined by observations elsewhere, 
the results obtained with the latter needle in London may be 
grouped with those of the former. Thus, the mean of seven 
observations made with needle L (4), September and October, 
1835, (when reduced to needle L(l),) is 69° 9'*8. If then we 
combine this with the direct result of the nine observations with 
needle L (1), viz. 69° 6'*3, (allowing double weight to each of the 
latter observations on account of the double number of readings,) 
we find 69° 7^3 as the mean value of the dip deduced from six¬ 
teen observations with the two needles, and reduced to needle 
L (1) as the standard. Comparing this with the mean result of 
the eight needles, the correction of needle L (1) is found to be 
+ io'-o. 
For the other needles employed in Ireland, we have 
Needle S (1) .... correction = + 6 f, 0. 
Meyer’s needle.— 2 / *3, 
With respect to Meyer’s needle, however, it is to be observed 
that as the angles from which the dip is deduced differ in ge¬ 
neral very widely, and as these angles are usually varied in dif¬ 
ferent observations with the same needle, there is a presumption, 
