[ 655 ] 
XVIII. On a Self-recording Method of Measuring the Intensity of the Chemical Action 
of Total Daylight. By Henry E. Eoscoe, F.B.S. 
Received November 27, 1873. — Read February 5, 1874. 
Although the method of measuring the varying intensity of the chemically active rays, 
as affecting chloride-of-silver paper of constant sensitiveness described in the Bakerian 
Lecture for 1865, has been the means of pointing out many important facts* concern- 
ing the distribution of the sun’s chemical activity through the atmosphere as well as 
in different situations on the earth’s surface, it has not as yet been introduced as a 
regular portion of the work of meteorological observatories. Until this is done, and 
the measurements are regularly continued and made in many situations, we cannot 
hope to obtain any thing like a knowledge of the laws of distribution of these rays 
over the earth’s surface, or any information as to the yearly variation of the solar 
chemical activity. This non-adoption of the method has to be explained, not in any 
want of reliance in the process or in the results, but in the fact that, in order to obtain 
a satisfactory curve of daily chemical intensity, at least hourly observations need to be 
made ; this involves, however, the expenditure of so much time and labour that the 
permanent observatories, already too heavily weighted, have found it impossible to 
undertake the necessary work. In the present communication I have to describe a 
modification of the above-mentioned method, which, whilst preserving untouched the 
principles upon which it is based and the amount of exactitude of which it is sus- 
ceptible, reduces the personal attention needed for carrying out the measurements to a 
minimum, and thus renders its adoption in observatories possible. 
According to this plan, the constant sensitive paper is exposed to the action of total 
daylight at given intervals, say at every hour, during the day, by a self-acting arrange- 
ment, for accurately known times. The insolation-apparatus, stocked with sensitive 
paper, is placed in position either early in the morning of the day during which the 
measurements have to be made, or on the previous night ; and by means of electric 
communication with a properly arranged clock, the sensitive paper is exposed every 
hour during the day, so that, in the evening, the observer has only to read off in the 
* (1) Phil. Trans. 1867, p. 555, “ On the Chemical Intensity of Total Daylight at Kew and Para, 1865, 
1866, 1867,” by H. E. Roscoe, F.R.S. ; (2) Phil. Trans. 1870, p. 309, “ On the Relation between the Sun’s 
Altitude and the Chemical Intensity of Total Daylight in a Cloudless Sky,” by H. E. Roscoe, E.R.S., and T. 
E. Thorpe, Ph.D,, F.R.S.E. ; (3) Phil. Trans. 1871, p. 467, “ On the Measurement of the Chemical Intensity 
of Total Daylight made at Catania during the Total Eclipse of December 22, 1870,” by H. E. Roscoe, F.R.S., 
and T. E. Thorpe, F.R.S.E. 
4 T 
MDCCCLXXIV. 
