REGISTRATION OF THE CHEMICAL ACTION OE TOTAL DAYLIGHT. 
623 
and 6 h P.M., instead of about every fifteen minutes, the numbers for mean chemical 
intensity would have been — 
Date. 
Mean Chemical Intensity. 
From 26 observations. 
From 6 observations. 
] 863, August 26 
40-5 
43-0 
„ Sept. 4 
41-8 
42-7 
1864, April 20 
85-3 
96-3 
As examples of simultaneous determinations made in different localities, I give the 
results of observations made by myself in Heidelberg, lat. 49° 24' N., on July 4, 1864, 
and near Dingwall in Rossshire, lat. 57° 35' N, on September 27, 1864, compared with 
the results of observations made in Manchester, 53° 20' N. latitude, by my assistant. 
The curves for Heidelberg and Manchester are given in fig. 19, those of Dingwall and 
Manchester on fig. 20. The integral giving the mean action at Heidelberg on July 4 
is 160, that at Manchester on the same day being 51 - 2; so that the chemical action 
at Manchester and Heidelberg was on July 4 in the ratio of 1 to 3T2. The integral for 
Dingwall on September 27 is 66 - 4, whilst that of Manchester is 49 - 5 ; or the ratio of 
chemical action at Manchester and Dingwall on the day in question was 1 to T34. 
From these observations it would appear that the chemical action at Manchester is 
smaller than accords with the latitude of the place. This is easily accounted for by the 
absorptive action exerted by the atmosphere of coal smoke in which the whole of South 
Lancashire is constantly immersed. Indeed, from the frequent occurrence in Man- 
chester of dull or rainy days, and of fogs or mists, it would be difficult to choose a spot 
more unsuited to the prosecution of experiments on the chemical action of light. 
From the integrals of daily intensity giving the mean chemical action for each day, 
the mean monthly or yearly chemical intensity of the place of observation can, in like 
manner, be ascertained ; so that, should this method of measurement prove capable of 
general adoption, we may look forward to obtaining in this way a knowledge of the 
distribution of the chemically active rays over the surface of our planet analogous to 
that which we already possess respecting the heating rays. 
