186 OBSERVATIONS OF THE TERRESTRIAL 



and if h and h f be the horizontal intensities at the two places, the 

 numbers of the fifth column are computed from the formula 



Again, if A, denote the horizontal intensity in London, the ratio 

 - will be determined in the same manner ; and, multiplying by 



*' // 



it the numbers in the fifth column, we obtain the values of , 



or the ratios of the horizontal intensity at the places of observation 

 to that at London, as given in the sixth and last column. 



The stations with which all the other places in Ireland are 

 immediately compared are Dublin and Limerick ; and it will at 

 once appear that, as the ratios of the horizontal force at these 

 stations to that at London enter as factors in all the final results, 

 much accuracy is required in their determination. For this pur- 

 pose we have three distinct series of observations. In the first and 

 second the intensities of the horizontal force in Dublin and Lime- 

 rick are directly compared with that in London ; and in the third 

 these intensities are compared together. The results of these com- 

 parisons, given in Table II., are here put together, so as to be 

 seen at one view. 



I. Horizontal intensity in Dublin, the horizontal intensity in 

 London being unity. 



July, Aug., 1835, . . Cyl. R (c). . . Int. = -9456 



ii ,, . . R (d). . . -9421 



September, 1835, . . L (a). . . -9390 



, L (6). . . 9376 



Oct., Nov., 1835, . . L (a). . . '9319 



>i >i ii . . - L (6). . . -9319 



Mean = -9380 



II. Horizontal intensity in Limerick, the horizontal intensity 

 in London being unity. 



July, Aug., Sept., 1834, Cyl. S (i). . . Int. = -9396 



July, 1835, .... _ -9470 



July, August, 1835, . _ R ( c ). . . _ .946! 



>i ii - R (rf); . . -9513 



Mean = -9460 



