[ 220 ] 

 XXXII. Proceedings of Learned Societies. 



ROYAL SOCIETY. 

 [Continued from p. 153.] 



February 5, 1874. — Joseph Dalton Hooker, C.B., President, in 



the Chair. 



r PHE following communication was read : — 



■*■ "Ona Self-recording Method of Measuring the Intensity of 



the Chemical Action of Total Daylight." By H. E. Eoscoe, F.E.S. 



The object of the present communication is to describe an instru- 

 ment by which the varying intensity of the chemically active rays, 

 as affecting chloride of silver paper of constant sensitiveness, can be 

 made self-recording. The method described by the author in the 

 Bakerian Lecture for 1865, although it has been the means of 

 bringing into notice many important facts concerning the distribu- 

 tion of the sun's chemical activity throughout the atmosphere, as 

 well as in different situations on the earth's surface, has not as yet 

 been introduced as a portion of the regular work of meteorological 

 observatories, owing to the fact that, in order to obtain a satisfac- 

 tory curve of daily chemical intensity, at least hourly observations 

 need to be made, and this involves the expenditure of more time 

 and labour than it has been found possible to give. In the pre- 

 sent communication a method is described, which, whilst preserving 

 untouched the principles and accuracy of the former method, re- 

 duces the personal attention needed for carrying out the measure- 

 ments to a minimum, and thus renders its adoption in observatories 

 possible. 



According to this plan, a constant sensitive paper is exposed by 

 a self-acting arrangement for accurately known times, at given 

 intervals throughout the day. The insolation apparatus stocked 

 with sensitive paper is placed in position either early in the morn- 

 ing of the day during which the measurements have to be made, or 

 on the previous night, and by means of an electric communication 

 with a properly arranged clock, the sensitive paper is exposed every 

 hour during the day, so that, in the evening, the observer has only 

 to read off, in the ordinary manner, the hourly intensities which 

 have been recorded on the paper during the day. 



This self-recording arrangement, though apparently simple, in- 

 volves points which have rendered its successful completion a some- 

 what difficult matter, owing, in the first place, to the great varia- 

 tions which occur in the chemical intensity of total daylight in 

 different places, at different times of the day, and in different pe- 

 riods of the year ; and secondly, owing to the fact that, in order 

 to be able to estimate the chemical intensity, the coloration ac- 

 quired by the paper must reach, but not much exceed, a given tint. 

 It becomes necessary therefore that on each occasion when an ob- 

 servation is needed, the sensitive paper should be exposed me- 



