146 



ANNUAL OF SCIENTIFIC DISCOVERT. \ 



The sun's altitude was determined by a sextant and artificial 

 horizon immediately before and immediately after the observa- 

 tions of chemical intensity, the altitude at the time of observation, 

 being ascertained b} 7 interpolation. 



One of the 134 sets of observations was made as nearly as 

 possible every hour, and they thus naturally fall into 7 groups, 

 viz. : 



(1) Six hours from noon, (2) 5 hours from noon, (3) 4 hours 

 from noon, (4) 3 hours from noon, (5) 2 hours from noon, (6) 

 1 hour from noon, (7) noon. Each of the first 6 of these groups 

 contains 2 separate sets of observations (1) those made before 

 noon, (2) those made after noon. It has before been pointed out, 

 from experiments made at Kew, that the mean chemical intensity 

 of total daylight for hours equidistant from noon is the same. 

 The results of the present series of experiments prove that this 

 conclusion holds good generally ; and a table is given showing 

 the close approximation of the numbers obtained at hours equi- 

 distant from noon. 



Curves are given showing the daily march of chemical intensity 

 at Lisbon in August, compared with that at Kew for the preceding 

 August, and at Para for the preceding April. The value of the 

 mean chemical intensity at Kew is represented by the number 

 94.5, that at Lisbon by 110, and that at Para by 313^.3 light of the 

 intensity 1 acting for 24 hours being taken as 1000. The follow- 

 ing table gives the results of observations arranged according to 

 the sun's altitude : 



No. of Observations. 



15 



18 



22 



22. 



19 



24 



11 



Mean Altitude. 

 / 



9 51 



19 41 



31 14 



42 13 



53 09 



61 08 



64 14 



Curves are given, showing the relation between the direct sun- 

 light (column 3) and diffuse daylight (column 4) in terms of the 

 altitude. The curve of direct sunlight cuts the base line at 10, 

 showing that the conclusion formerly arrived at by one of the 

 authors is correct, and that at altitudes below 10 the direct sun- 

 light is robbed of almost all its chemically active rays. The re- 

 lation between the total chemical intensity and the solar altitude 

 is shown to be represented graphically by a straight line for al- 

 titudes above 10, the position of the experimentally determined 

 points lying closely on the straight line. 



A similar relation has already been shown to exist (by a far less 

 complete series of experiments than the present) for Kew, Hei- 

 delberg, and Para ; so that although the chemical intensity for 

 the same altitude at different places and at different times of the 

 year varies according to the varying transparency of the atmos- 

 phere, yet the relation at the same place between altitude and in- 



