MAGNETICAL OBSERVATIONS IN IRELAND. 1 28 
the place chosen as the base of reference, at the termination of 
each series of observations, as well as at their commencement. 
If it is then found that the needle has lost any small portion of 
its force, or if the time of vibration has augmented, the amount 
of the correction due to each result may be found by assuming 
the change to have been regular , or proportional to the time 
elapsed. When the loss is very small, however, (as was the case 
in the observations which form the subject of this paper,) the 
correction may be disregarded, provided we take as the time of 
vibration at the base of reference the mean of the times observed 
at the commencement and end of the series. 
The needles used in the present series are the cylinders 
L («), L (Z>), made by Dollond, and belonging to Mr. Lloyd; 
cylinder S (Z>) belonging to Captain Sabine, and cylinders R ( c ) 
and R ( d) in the possession of Captain James Ross. All the cir¬ 
cumstances of the observations are given in the annexed Table. 
The first, second, and third columns contain the place , day of the 
month , and hour of the observation. In the fourth column is 
set down the observed time of 100 vibrations, or the immediate 
result of observation divided by 3. The fifth column contains 
the terminal arc , the initial arc being in all cases 20°. In the 
sixth column is given the chronometer's rate ; in the seventh the 
temperature ; and in the eighth the deduced or corrected time . 
The hour, set down in the third column as the hour of obser¬ 
vation, is the mean of the commencement and end; and the re¬ 
corded temperature is also the mean of those observed at the be¬ 
ginning and end of the observation. It will appear from the pre¬ 
ceding that the corrections employed in deducing the corrected 
from the observed times are those due to temperature and to the 
rate of the chronometer. 
Beside the observations which follow there were some others 
of an earlier date, made for the purpose of comparing the hori¬ 
zontal intensity at Dublin and Limerick, the two stations with 
which all the other places in Ireland have been immediately com¬ 
pared. In the observations alluded to, the rate of cylinder S (Z>), 
was observed in the Philosophy School, Trinity College; and the 
local attraction of the building was determined by subsequent 
comparisons of the force there with that in the garden. Trinity 
College, the place which was afterwards selected for all the Dub¬ 
lin observations. These earlier comparisons, as well as some 
other imperfect ones obtained previously to the autumn of 1834 
with two other cylinders, have not been included in the annexed 
Tables ; partly because the needles employed do not seem to be 
as trustworthy as the rest, but chiefly because of the uncer- 
tainties of the double comparison which they involve. 
