MAGNETIC FORCE IN IRELAND. . 197 



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 Eoss in London ; so that their cor- 

 rections may be 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 well 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 (1), 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 observa- 

 tions on account of the double number of readings), we find 

 69 7''3 as the mean value of the dip deduced from sixteen ob- 

 servations 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 + 10'*0. 



For the other needles employed in Ireland, we have 



Needle S (1), . . . . correction = + 6'-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 general very 

 widely, and as these angles are usually varied in different obser- 

 vations with the same needle, there is a presumption, at least, that 

 every constant error will be removed by repetition, and that the 

 differences of the separate results from the absolute dip will be 

 equal on the positive and on the negative side. This seems to be 

 confirmed by the amount of the final difference in the present in- 

 stance, which does not appear to be larger than may be fairly 

 ascribed to the errors of observation. It seems bettor therefore to 

 regard this needle as subject to no constant error. 



The degree of confidence to which these determinations are 

 entitled may now be estimated, by applying the corrections so 

 obtained to the observations made with these needles at Limerick 

 in 1833 and 1834, the only other station at which they have been 

 all employed. The observations in the former year are reduced 

 to the latter, assuming the annual decrease of dip in Ireland to 



