72 Royal Society : — Mr. E. J. Stone on a Decennial 



results a correction for change of station, it has the great advantage 

 of securing immunity from disturbance 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 ad- 

 visable 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 se- 

 cular variation. Next in order comes the calculation of the semi- 

 annual 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 

 progressively from 3*5926 to 3*6178 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 semi- 

 annual 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 resulting ex- 

 cess 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 de- 

 cided variation 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 uncertainty arising from the 

 probable disturbing causes at the first magnetic station. 



It is hoped that a second series of observations at the new station 

 will throw greater light on the fact of the sun's influence on terres- 

 trial magnetism, by either confirming the results obtained above, or 

 by adding fresh weight to the conclusions arrived at by the President 

 of the Royal Society. 



March 23. — General Sir Edward Sabine, K.C.B., President, in the 



Chair. 



The following communications were read : — 



" On an approximately Decennial Yariation of the Temperature 

 at the Observatory at the Cape of Good Hope between the years 

 1841 and 1870, viewed in connexion with the Variation of the Solar 

 Spots." By E. J. Stone, F.R.S., Astronomer Royal at the Cape of 

 Good Hope. In a Letter to the President. 



Eoyal Observatory, Cape of Good Hope, Jan. 17, 1871. 



Dear Sir, — I enclose a curve of the variation of the annual mean 



