512 Messrs. lloscoe and Thorpe on the Chemical [June 15, 



and the changes of area in the exposed portion of the sun's disk. The 

 attempt to establish this relation has already been made by one of us from 

 the results of observations carried out by Captain John Herschel, R.E., 

 F.R.S., at Jamkandi, in India, during the total eclipse of Aug. 18, 18C8. 

 Unfortunately the state of the weather at Jamkandi at the time of the 

 eclipse was very unfavourable, and the results were therefore not of so 

 definite a character as could be desired, and it appeared important to 

 verify them by further observation. The method of measurement adopted 

 is that described by one of us in the Bakerian Lecture for 1865 ; the ob- 

 servations were made in the Garden of the Benedictine Monastery of San 

 Nicola, at Catania, the position of which, according to the determination 

 of Mr. Schott of the United States' Coast Survey, is lat. 37° 30' 12" N., 

 long. l h 0' 18" E. In order to obtain data for determining the variation 

 in chemical intensity caused by the alteration in the sun's altitude during 

 the eclipse, observations were made on the three previous days, during 

 which the sky was perfectly cloudless. 



In the following Table the observations taken at about the same hours 

 are grouped together : — 



Table I. 



Mean Alti- 

 tude. 



No. of Ob- 

 servations. 



CI 



lemical Intensity. 



Diffused. 



Direct. 



Total. 



1 30 28 



1 



0-009 



0-000 



0-009 



9 28 10 



7 



0-044 



0-008 



052 



13 9 57 



7 



0-050 



0-014 



0-064 



19 57 49 



12 



0-072 



0-028 



0-100 



24 46 12 



7 



0-095 



0-049 



0-144 



28 24 10 



14 



0-108 



0-047 



0-155 



The above numbers confirm the conclusion formerly arrived at, viz. that 

 the relation between total chemical intensity and sun's altitude is repre- 

 sented by a straight line, or by the equation 



CI„=CI + const, xa, 



where CI„ signifies the chemical intensity at any altitude a in circular mea- 

 sure, CI the chemical intensity at 0°, and const, a a number derived from 

 the observations. 



The observations on the day of the eclipse (the 22nd) were commenced 

 about nine o'clock a.m., and up to the time of first contact were made re- 

 gularly at intervals of about an hour. The sky up to this point was 

 cloudless, and the measurements almost absolutely coincided with the 

 mean numbers of the preceding day's observations. As the eclipse pro- 

 gressed, and the temperature of the air fell, clouds were rapidly formed, 



