MAGNETICAL OBSERVATIONS IN IRELAND. 145 
It will easily appear, from the second of the preceding for- 
mulee, that when the coefficient is so small as that just assigned, 
the variations in the values of s, resulting from moderate changes 
in the angles on which it depends, will be inconsiderable. In 
the observations in Ireland, for example, the entire change in 
the amount of the correction is a small fraction of a minute. In 
this and other similar cases, therefore, the correction may be 
regarded as constant; and its value may be inferred from any 
series of simultaneous observations made with the needle to be 
corrected, and with some other vdiose correction is already 
known. In this manner it has been found that the mean differ¬ 
ence of the results of needles L (1) and L (4) is — 8 IV = + l K 5 ; 
being somewhat smaller than that assigned above. But if 5 denote 
the absolute dip, we have already fcund that £ — 8 r = + 10'*0 ; 
and adding these differences, the correction of needle L (4) is 
$ - Sxv = + ll'*5. 
The correction of needle S (2) is inferred from the observa¬ 
tions made with that needle in Limerick, as given in Table IV. 
Limerick, July 1835 . . . dip = 71 16*9 
Dec. ... 71 14*6 
Mean ... 71 15*8 
This mean corresponds, in time, to the middle of October 1835. 
But the true dip in Limerick (July, August, 1834,) was found 
to be 71° 9''5; and when reduced to October 1835 (assuming the 
annual decrease to be 3',) it is 7l° 6'*0. The correction of the 
needle is therefore — 9 r, 8. 
The corrections of the needles being determined, w^e may now 
proceed to deduce the values of the absolute dip at the several 
places at which observations have been made. These values are 
given in the following table (Table V.). In the first and second 
columns are written the place and date of the observation. The 
third and fourth columns contain the corrected values of the dip 
deduced from the results of Tables III. and IV. by the applica¬ 
tion of the corrections now explained, and the fifth and last 
column contains the mean dip inferred from the tw T o preceding. 
In taking this mean, double iveight has been assigned to the 
results obtained with needles of the ordinary construction, the 
number of readings with these needles being double of that made 
with the needles whose poles are unaltered. 
1835. 
L 
I 
