MAGNETIC FORCE IN IRELAND. 177 



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 observation , is the mean of the com- 

 mencement and end ; and the recorded temperature is also the 

 mean of those observed at the beginning and end of the observation. 

 It will appear from the preceding that the corrections employed in 

 deducing the corrected from the observed times are those due to 

 temperature and to the rate of the chronometer. 



In addition to the observations which follow there were 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 (b] 

 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 of Trinity 

 College, the place which was afterwards selected for all the Dublin 

 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 trust- 

 worthy as the rest, but chiefly because of the uncertainties of the 

 double comparison which they involve. 



