158 Mr. H. E. Roscoe on a Method of measuring the [Feb. 5, 



only calling into use the fine tips of the third and fourth pairs of 

 thoracic limbs to hold fast its charge. When it fully retires into the 

 case, the claws of the two posterior pairs of legs are pressed backwards 

 against the lining membrane, so as still more effectually to secure its 

 hold on the approach of danger. 



II. "On a Self-recording Method of Measuring the Intensity of 

 the Chemical Action of Total Daylight." By H. E. ROSCOE, 

 F.R.S. Received November 27, 1873. 



(Abstract.) 



The object of the present communication is to describe an instrument 

 by which the varying intensity of the chemically active rays, as affecting 

 chloride of silver paper of constant sensitiveness, can be made self-record- 

 ing. The method described by the author in the Bakerian Lecture for 1865, 

 although it has been the means of bringing into notice many important 

 facts concerning the distribution of the sun's chemical activity throughout 

 the atmosphere, as well as in different situations on the earth's surface, has 

 not as yet been introduced as a portion of the regular work of meteoro- 

 logical observatories, owing to the fact that, in order to obtain a satisfac- 

 tory curve of daily chemical intensity, at least hourly observations need to 

 be made, and this involves the expenditure of more time and labour than 

 it has been found possible to give. In the present communication a 

 method is described, which, whilst preserving untouched the principles 

 and accuracy of the former method, 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, a constant sensitive paper is exposed by a self- 

 acting arrangement for accurately known times, at given intervals 

 throughout the day. 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 an 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 ordinary manner, the 

 hourly intensities which have been recorded on the paper during the 

 day. 



This self-recording arrangement, though apparently simple, involves 

 points which have rendered its successful completion a somewhat difficult 

 matter, owing, in the first place, to the great variations which octfur in 

 the chemical intensity of total daylight in different places, at different 

 times of the day, and in different periods of the year ; and secondly, 

 owing to the fact that, in order to be able to estimate the chemical 

 intensity, the coloration acquired by the paper must reach, but not much 



