93 
so that hght of the intensity 50, acting during time ¢, produces the 
same blackening effect as light of intensity ¢ acting during the time 
50. According to this method the chemical action of the total day- 
light was determined for Manchester, England, many times a day 
during 1864, and the total daily chemical intensity has been com- 
puted for the year August, 1863, to September, 1864. Very large 
changes in chemical intensity occur when the sky is cloudless and 
unchanged as far as the eye can perceive. The total intensity for an 
apparently cloudless day varies from 3.3 for December 21, 1863, to 
119, June 22, 1864. This last number, compared with the figure 50.9 
for June 20, and 26.6 for June 28, shows the enormous variations that 
take place in the chemical rays that reach the observer at Manchester 
on cloudless days. This variation is undoubtedly due in part to 
smoke and moisture, but possibly other unknown influences are also 
at work. 
In 1867 H. E. Roscoe communicated to the Royal Society the 
results of work done by his method at Kew, England, in 1865, 1866, 
and 1867; at Heidelberg, 1862 and 1863, and at Para, Brazil, 1866. 
The general results are that the chemical intensity attains its max- 
imum at noon and not, like the temperature, at some time after noon. 
Everywhere the intensity increases from hour to hour with the alti- 
tude of the sun, and is very closely proportional to it even when the 
sky is partially clouded, but of course the rate of increase varies with 
the season, the amount of cloud, and the degree of atmospheric opales- 
cence. The total chemical intensity for each month, as determined 
from numerous observations, 1s as follows for Kew: 
Total photochemical intensity of direct and diffuse light (Roscoe). 
’ 
Month. | 1865. | 1866. | 1867. Month. 1865. | 1866. | 1867. 
| | 
Mammary tic. 222.802) ecu 15 Bt Ful eee 8 Te Pore eee 
Hepnuaryees..-. ker seke alesse. Jkce 24 93 tll Aaioust st te a a: 8 89 942 Bose 2 
Rech ee pa yl sr 34| 81 || September. _......-.-.-- 108 | et eee 
sare labs ites Magis see 98 Foslcet | | Octaber_.--- Dayton) eee eae 
Niagra ets 2 ate Fes ee 118 | NO ee ee | November ]—) 2222.2 18 | Gil ees oe 
dibhe:) 2 eae a ee a 82 | 92 |, 24-) 52 | ecemiber 4752-2 4. (CS) ial el) yee ee 
Roscoe compares these figures with the cloudiness, and finds that 
the ratio between cloudiness, expressed on a scale of 10,and the chem- 
ical intensity is as 1 to 5 in some months and as | to $ in others. A 
similar irregularity of ratio is found when he considers the absolute 
moisture in the atmosphere; whence he concludes that the variations 
in chemical intensity, as between the spring and autumn, are not 
perfectly explained by either of these factors. He finds the high 
autumnal and low vernal intensity fairly well explained as due to the 
transparency or opalescence produced by finely divided solid particles 
or dust. 
