184 



EIGHTH REPORT — 1838. 



Table LXXVIII. 



In deducing the position of the isodyiiamic lines from these re- 

 sults, equal weights have been assigned to all, for the reason al- 

 ready given. The following are the results of the computation : 

 L = 1-0256, M = -t- -000091, N = + -000067 ; 

 u= — 36° 29', r = -000113. 



The results which have been above obtained respecting the 

 position of the isodynamic lines in Ireland, are combined in the 

 following Table : 



Table LXXIX. 



In deducing the mean values from the preceding results, 

 we cannot, consistently with the character of the observations, 

 assign to each a weight in proportion to the number of stations 

 from which it is derived. If we compute the probable value 

 of the intensity at each station, and compare it with that ob- 

 served, we shall find that the differences are in general smaller 

 in Captain Ross's observations than in those of the two earlier 

 series ; so that the individual results are entitled to a greater 

 weight. This superiority is due, in great measure, to the cir- 

 cumstance that, in the latter series, all the observations were 

 taken by the same observer, with the same instrument, and 

 about the same time. On instituting a similar comparison 

 between the results of the two methods, it will be found that, 

 in Captain Ross's two series, the weight due to the results of 

 the statical method is very nearly double of that in the method 

 of vibration ; the probable errors being, nearly, in the ratio of 

 1 to V2. The same disparity between the methods is not 

 found in the results of the two earlier series, a fact wiiich 



