Relation between the Sun's Altitude and Chemical Intensity. 57 



determinations of the chemical intensity of total daylight made in 

 the autumn of 1867 on the flat tableland on the southern side of 

 theTagus, about 8| miles to the south-east of Lisbon, under a cloud- 

 less sky, with the object of ascertaining the relation existing between 

 the solar altitude and the chemical intensity. The method of 

 measurement adopted was that described in a previous communi- 

 cation to the Society*, founded upon the exact estimation of the 

 tint which standard sensitive paper assumes when exposed for a given 

 time to the action of daylight. The experiments were made as fol- 

 lows : — 



1. The chemical action of total daylight was observed in the 

 ordinary manner. 



2. The chemical action of the diffused daylight was then observed 

 by throwing on to the exposed paper the shadow of a small blackened 

 brass ball, placed at such a distance that its apparent diameter, seen 

 from the position of the paper, was slightly larger than that of the 

 sun's disk. 



3. Observation No. 1 w T as repeated. 



4. Observation No. 2 was repeated. 



The means of observations 1 and 3 and of 2 and 4 were then taken. 

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

 immediately before and immediately after the observations of che- 

 mical intensity, the altitude at the time of observation being ascer- 

 tained by interpolation. 



It wasfirst shown that an accidental variation in the position of the 

 brass ball within limits of distance from the paper, varying from 140 

 millims. to 230 millims., was without any appreciable effect on the 

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

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

 viz. : — 



(1) Six hours from noon, (2) five hours from noon. (3) four hours 

 from noon, (4) three hours from noon, (5) two hours from noon, (6) 

 one hour from noon, (/) noon. 



Each of the first six of these groups contains two separate sets of 

 observations, — (1) those made before noon, (2) those made after 

 noon. It has already been pointed outf, 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 equidistant 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 in- 

 tensity 1 acting for 24 hours being taken as 1000. 



* Roscoe, Bakerian Lecture, 18G5. [Phil. Mag. S. 4. vol. xxix. p. 233.1 

 t Phil. Trans. 1867, p. 558. 



