1871.] Observations at Stony hurst College Observatory, 369 



in the English survey in 1858, greatly encouraged the undertaking of 

 monthly magnetic observations, and the Rev. A. Weld procured in conse- 

 quence the instruments still in use. Only occasional observations were 

 made with these instruments for some years, and it was only in 1863 that 

 a continuous series of monthly determinations of the magnetic elements 

 was started by the Rev. W. Sidgreaves. He observed regularly until Sep- 

 tember 1868, when I returned to my former post at the Observatory, and 

 I have continued the same work ever since. 



A stone pillar was at first erected for the magnetic instruments in the 

 open garden, and this remained in use from 1858 until the beginning of 

 1868, when a most convenient hut of glass and wood was built for the in- 

 struments in a retired corner of the College garden. This alteration was 

 rendered necessary from the placing of iron rails in the vicinity of the old 

 pillar ; and although it introduces into the results a correction for change 

 of station, it has the great advantage of securing immunity from dis- 

 turbance for the future. 



Considering the object in view in drawing up this reduced form of the 

 dip and horizontal-force observations, I have judged it advisable to adhere 

 strictly to the tabular forms in which the matter has been presented in 

 previous discussions of a similar nature. Each element is the subject 

 matter of these tables. In the first are the monthly values of the element, 

 the deduced mean value, and its secular variation. Next in order comes 

 the calculation of the semiannual inequality. The residual errors, and 

 consequent probable weights of the observations and results, compose the 

 third and last Table. 



The yearly mean values of the horizontal force are found to vary pro- 

 gressively from 3*5926 to 3 • 6 1 78 in British units, the mean for Oct. 1st, 

 1866, being 3*6034, with a secular acceleration of 0-0042. Calculating 

 from the monthly Tables the mean value of the horizontal force for the six 

 months from April to September, and for the semiannual period from 

 October to March, we find the former to be 0*0005 in excess over the 

 latter, showing that this component of the intensity is greater during the 

 summer than during the winter months. Treating the dip observations in 

 a precisely similar way, we obtain 69° 45' 21" as the mean value of this 

 element for October 1st, 1866, subject to a secular diminution of 1' 49''*2 ; 

 the extreme yearly means being 69° 48' 47" and 69° 37' 52". The result- 

 ing excess of 10" for the winter months in the computed semiannual 

 means is so small, that the observations tend mainly to show that the 

 effect of the sun's position is not clearly manifested by any decided varia- 

 tion in the dip. Deducing the intensity from the above elements, we 

 obtain for the summer months the value 10*4136, whilst that for the 

 winter months is 10*4128. The intensity of the earth's magnetic force 

 would thus appear to increase with the sun's distance, but the difference is 

 not large enough to have more than a negative weight in the question 

 under discussion. This weight, moreover, is lessened by the slight uncer- 



