On the Nature of Solar Spots. 237 



hourly intensities, giving the daily mean chemical intensities of the 

 place, a rough method of integration is resorted to : this consists in 

 determining the weights of the areas of paper inscribed between the 

 base-line and the curve of daily intensity, that chemical action being 

 taken as 1000 which the unit of intensity would produce if acting 

 continuously for twenty-four hours. The remarkable differences 

 observed in the chemical intensity on two neighbouring days is 

 noticed on the curves for the 20th and 22nd of June 1864: the 

 integrals for these days are 50 '9 and 119, or the chemical actions on 

 these days are in the ratio of 1 to 2 34. 



The chemical action of light at Manchester was determined at the 

 winter and summer solstices, and the vernal and autumnal equinoxes : 

 the results of these measurements are seen by reference to the accom- 

 panying curves. The integral for the winter solstice is 4*7, that of 

 the vernal equinox 36*8 ; that of the summer solstice 119, and that 

 of the autumnal equinox 29' 1 . Hence, if the chemical action on the 

 shortest day be taken as the unit, that upon the equinox will be 

 represented by 7, and that upon the longest day by 25. 



The results of simultaneous measurements made at Heidelberg 

 and Manchester, and Dingwall and Manchester, are next detailed. 



From the integrals of daily intensity the mean monthly and 

 annual chemical intensity can be ascertained, and thus we may 

 obtain a knowledge of the distribution of the chemically acting 

 rays upon the earth's surface, such as we possess for the heating rays. 



January 26, 1865. — Major-General Sabine, President, in the Chair. 



The following communication was read : — 



"Researches on Solar Physics. — Series I. On the Nature of Solar 

 Spots." By Warren De la Rue, Ph.D., F.R.S., Balfour Stewart, 

 A.M., F.R.S., &c. and Benjamin Loewy, Esq. 



After giving a short sketch of the history of their subject, the 

 authors proceed to state the nature of the materials which had been 

 placed at their disposal In the first place, Mr. Carrington had 

 very kindly put into their hands all his original drawings of sun-spots, 

 extending from November 1853 to March 1861. In the next place, 

 their materials were derived from the pictures taken by the Kew 

 heliograph. A few pictures were taken by this instrument at Kew 

 Observatory in the years 1858 and 1859. In July 1860 it was in 

 Spain doing service at the total eclipse. In 1861 a few pictures 

 were taken at Kew, while from February 1862 to February 1863 the 

 instrument was in continuous operation at Mr. De la Rue's private 

 observatory at Cranford, and from May 1863 until the present date 

 it has been in continuous operation at Kew under Mr. De la Rue's 

 superintendence. A Table was then given from which it was deduced 

 that the number of groups observed at Kew from June to December 

 1863 inclusive was 64, while that observed by Hofrath Schwabe 

 during the same interval was 69. In like manner, the number at 

 Kew between January and November 1864 inclusive was 109, while 

 during the same interval Hofrath Schwabe observed 126. It thus 

 appears that Schwabe' s numbers are somewhat larger than those of 

 Kew ; but probably, by means of a constant corrective, the one series 

 mav be made to dovetail with the other. 



