METEOROLOGY AND ALLIED SUBJECTS. 273 



S = 31.99 COS V + ^7.6 COS V - 248.7 cos V 



In this formula (p is the zenith distance of the sun, and the constants 

 themselves refer to a unit of light, of which each corresponds to the 

 development of 0.111 units of hydrochloric acid per minute per square 

 centimeter. 



The sum total H-f S represents the chemical intensity of all the day- 

 light which falls upon a horizontal element of the earth's surface, and 

 this, according to Eadau, can be fairly well represented by the formula : 



H + S = 3.1 + 0.635 X 7^ + 0.05775 h^ - 0.00048 )i^ 



where 7^ represents the altitude of the sun, or 90° — f. These results 

 of Bun sen and Roscoe may be compared with measures made by means 

 of photantitypimeter of Marchand, which instrument is especially 

 designed to give the normal changes of the chemical intensity of the 

 total daylight. Unfortunately the observations made by Marchand at 

 Fdcamp, on the coast of France, 49° 45' north latitude, suffered from 

 the frequency of cloud and haze. But they show that the maximum at 

 Fecamp occurred decidedly after midday, and that between 10 a. m. 

 and noon the chemical intensity experienced a decided diminution, while 

 the symmetry of the morning and afternoon observations is such that 

 the chemical intensity is decidedly greater in the afternoon than the 

 morning. In all these respects, therefore, Marchand's results are op- 

 posed to those given by Bunsen and Roscoe. But the differences may 

 be due, among other things, to the local peculiarities of stations. If 

 now we compare Bunsen and Eoscoe's results with those obtained by 

 means of the photographic actinometer, we find (1) that the latter show 

 the normal chemical intensity of total daylight to be a function of the 

 altitude of the sun, and represented by the equation J,, = Jo + «/*• 

 (2) The maximum occurs at midday and the same intensity prevails 

 for the same altitude before and after noon. (3) The constants in the 

 equation just given change for every locality and for the same locality 

 every day. These constants are functions of atmospheric moistiu-e and 

 whatever affects the clearness of the air. 



These are now to be compared with those deduced from observations 

 made by Eoscoe and Thorpe, in August, 1865, near Lisbon, Portugal. 

 During fifteen days of normal clearness their observations show that 

 the normal intensity at Lisbon during the course of the day can be fairly 

 represented by an equation whose form is J^ = Jo -f «/i, but the after- 

 noon shows a sensible diminution of the chemical intensity at about 2 

 p. m. ; were it not for this break at 2 p. m. it would be of the same form 

 as tor the morning, having, however, very different constants. 



Observations were also made by Eoscoe, in December, 1870, at Cat- 

 ania in Spain, but embraced only three complete days. From these it 

 results that the maximum of chemical intensity occurs at about 11 a. m. 

 S. :\ris. 109 18 



