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 five distinct groups, those of the same group having 
been made nearly at the same time. The following are the 
results : 
Date. No. of Obs.| Dip. 
I. | Oct. 21, 1833, l (Up eee 
II. | Sept. 9, 1834, 10 TOF; 4: 
Ill.| Sept. 18, 1835, 16 71°. 5. 2. 
IV.| April 25, 1836, 8 Z19%3",.9. 
V. | Aug. 5, 1836, 4 64 OR ee a 
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 6 denote the un- 
known dip at an assumed epoch, the Ist of January, 1836; 
8, the dip observed at any other time; m the number of 
effective months in the interval ; and «, the monthly decrease, 
- each of the above results will furnish an equation of condi- 
tion of the form 
S+ne= Oo, . 
Combining these five equations by the method of least 
squares, we obtain two resulting equations which give the 
most probable values of 6 and «. We thus find, 
$= 71°. 3'.84, e= 0.97, 
