149 



Europe from the time of the earliest observations, and that 

 the rate of this diminution is not uniform. It is, accord- 

 ingly, a question of considerable interest and importance to 

 determine the precise amount of the annual decrease, for a 

 given epoch, at any station. Conceiving that the observa- 

 tions of dip in Dublin, though extending over a very limited 

 time (three years), were yet sufficiently numerous to furnish 

 a close approximation to this amount, the author has put 

 them together with that view. The observations are thrown 

 into ^ve distinct groups, those of the same group having 

 been made nearly at the samie time. The following are the 

 results : 



The observations of M. Kupffer clearly show, that the 

 diminution of the dip is not uniform throughout the year ; 

 but that from December to May it is nearly stationary, the 

 whole diminution taking place in the remaining eight months. 

 For the convenience of calculation, we shall assume that the 

 diminution takes place at a uniform rate throughout these 

 eight months. It is evident then, that if S denote the un- 

 knotem dip at an assumed epoch, the 1st of January, 1836; 

 S^ the dip observed at any other time ; n the number of 

 effective months in the interval ; and t, the monthly decrease, 

 each of the above results will furnish an equation of condi- 

 tion of the form 



g + wezig^. 



Combining these five equations by the method of least 

 squares, we obtain two resulting equations which give the 

 most probable values of S and e. We thus find, 

 g z: 71«. 3'.84, fzz0'.97. 



