of Total Daylight during the Total Eclipse of 1870. 383 

 Table I. 



Mean Alti- 

 tude. 



No. of Ob- 

 servations. 



Chemical Intensity. 



Diffused. 



Direct. 



Total. 



°1 30 28 

 9 28 10 

 13 9 57 

 19 57 49 

 24 46 12 

 28 24 10 



1 



7 

 7 



12 

 7 



14 



0-009 

 0044 

 0-050 

 0-072 

 0-095 

 0-108 



o-ooo 



0-008 

 0-014 

 0-028 

 0-049 

 0047 



0-009 

 0052 

 0-064 



o-ioo 



0-144 

 0-155 



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

 that the relation between total chemical intensity and suu's altitude 

 is represented by a straight line, or by the equation 



CI a =CI -f const. X «, 



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

 measure, CI the chemical intensity at 0°, and const, a a number de- 

 rived from the observations. 



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

 menced about nine o'clock a.m., and up to the time of first contact 

 were made regularly 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 observa- 

 tions. As the eclipse progressed, and the temperature of the air 

 fell, clouds were rapidly formed, and from l h 40' up to the time of 

 totality it was impossible to make any observations, as the sun was 

 never unclouded for more than a few seconds at a time. As the illu- 

 minated portion of the solar disk gradually increased after totality, 

 the clouds rapidly disappeared, the amount falling from 9 (overcast 

 = 10) to 3 in about fifteen minutes. The observations were then 

 regularly continued to within a few minutes of last contact. 



Although the disk and by far the largest portion of the heavens 

 were completely obscured by clouds during totality, rendering any 

 determination of the photochemical action perfectly valueless for our 

 special object, it was yet thought worth while to attempt to estimate 

 the chemical intensity of the feebly diffused light at this time, which 

 certainly is capable of producing photographic action. 



Immediately after the supposed commencement of totality the slit 

 was opened, and the sensitive paper exposed for ninety-five seconds. 

 Not the slightest action, however, could be detected on the paper ; 

 and we therefore believe that we are correct in estimating the inten- 

 sity of the chemically active light present at certainly not more than 

 0*003 of the unit which we adopt, and probably much less. 



The Table containing the experimental numbers and the graphic 

 representation of them are given in the memoir. By a graphical 



