[ 69 J 
IV. Magnetic Observations made at Stonyhurst College Observatory , from April 1863 to 
March 1870. By the Rev. S. J. Perry. Communicated by the President. 
Received January 23, — Read March 9, 1871. 
Results of Seven Years’ Observations of the Dip and Horizon tal Force. 
In a paper published in the Philosophical Transactions for 1863, the President of the 
Koyal Society enters into a discussion of a six years’ series of the Kew Magnetic Obser- 
vations, with the view of ascertaining whether the sun’s position with reference to the 
earth really produces a sensible semiannual inequality in the terrestrial magnetic ele- 
ments. The probability of the existence of such a disturbing cause, founded on a com- 
parison of the monthly determinations of the Dip and Horizontal Force taken at 
Hobarton and at Toronto, was made much more evident by Sir Edward Sabine’s dis- 
cussion of the Kew observations; and it was from a desire of bringing more data to bear 
upon this important question that the reduction of the seven years’ observations, just 
completed at this observatory, has been undertaken. With this object in view, it 
was considered of great importance to adhere closely to the plan laid down by Sir 
Edward Sabine, and to present the results, for the sake of comparison, in three Tables. 
The first contains the mean monthly determinations of the elements, with their deduced 
mean values and secular variation, the second presents a view of the semiannual inequa- 
lity, and the third gives the residual errors and the consequent most probable errors of 
any single observation and of the deduced mean values of the elements. 
Magnetic observations were first taken at Stonyhurst in 1858, but the continuous 
series of monthly determinations of the Dip, Declination, and Intensity were only com- 
menced in March 1863. The same instruments, i. e. a dip-circle by Barrow and a 
Jones unifilar, have been used throughout the whole seven years. These instruments 
were both tested at Kew before being sent to this observatory, and the dip-circle was 
again examined there last January with most satisfactory results. The constants of the 
vibration-magnet were determined by Mr. Welsh, and are given in the Proceedings of 
the Royal Society for February 1865. In calculating the value of the Horizontal Force, 
it has never been found necessary to apply any correction for the arc of vibration, which 
has always been small, nor for the Frodsham chronometer, whose rate has never ex- 
ceeded 2 s per day. 
The yearly adopted values of P are the following : — 
for 1863 
-0-00217 
for 1867 
-0-00254 
1864 
-0-00219 
1868 
-0-00271 
1865 
-0-00254 
1869 
-0-00210 
1866 
-0-00275 
MDCCCLXXI. 
L 
