1865.] Rev. W. Sidgreaves Magnetical Observations. 65 



The proposal having been seconded, the ballot was taken, and Lord 

 Dufferin was declared duly elected a Fellow of the Society. 



The following communications were read : 



I. " Monthly Magnetical Observations taken at the College Observa- 

 tory, Stonyhurst, in 1864." By the Rev. WALTER SIDGREAVES. 

 Communicated, with a Note, by the President. Received January 

 24, 1865. 



The Horizontal, Vertical, and Total forces are calculated to English mea- 

 sure, one foot, one second of mean solar time, and one grain being assumed 

 as the units of space, of time, and of mass. 



The Vertical and Total forces are obtained from the absolute measure of 

 horizontal force and the Dip. The measures of the Dip-angle obtained with 

 needle 2 have not been used in these calculations, as it appears from the 

 observations taken with this needle that the position of its axle is less true 

 than that of needle 1. 



For the observations of Deflection and Vibration, taken each month for 

 absolute measure of horizontal force, the same magnet has always been 

 employed. 



The moment of inertia of the magnet, with its stirrup, for different degrees 

 of temperature, and the coefficients in the corrections required for the 

 effects of temperature and of terrestrial magnetic induction on the magnetic 

 moment of the magnet, were determined at the Kew Observatory by the 

 late Mr. Welsh. 



The moment of inertia of the magnet, with its stirrup, using the grain 

 and foot as the units of mass and of linear measure, is 5-27303. Its rate 

 of increase for increase of temperature is 0'00073 for every 10 of Fahr. 



The weight of the magnet, with its stirrup, is approximately 825 grains, 

 and the length of the magnet is nearly 3'94 inches. The moment of inertia 

 was determined independently of the weight and dimensions, by the method 

 of vibration with and without a known increase of the moment of inertia. 



The temperature corrections have always been obtained from the formula 

 q(t 35) + q'(t Q 35) 2 , where t is the observed temperature and 35 Fahr. 

 the adopted standard temperature. The values of the coefficients q and q' 

 are respectively 0'0001128 and 0-000000436. 



The induction coefficient /* is 0-000244. 



The correction for error of graduation of the Deflection bar at I'O foot 

 is-f 0-00004 ft., at 1-3 foot +0'000064 foot. 



The observed times of vibration are entered in the Table without cor- 

 rections. 



The time of one vibration has been obtained each month from the mean 

 of twelve determinations of the time of 100 vibrations. 



The angles of deflection are each the means of two observations. 



In deducing from these observations the ratio and product of the mag- 

 netic moment in of the magnet, and the earth's horizontal magnetic inten- 



