30 C. Abbe on the transparency of the Atmosphere. 
us a second determination of this constant. I was in hopes of 
securing a longer series of observations accompanied with a 
record of the barometer and ee — these seem to be 
the oar ones available for our present 
Mr. Clark, by diminishing the eareit jacueian of the sun 
until itis barely visible to the e eye, thus determines its brilliancy 
in units of the brightness of a faint sixth magnitude star. How- 
ever much his results may depend on the peculiarities of his 
method and the delicacy of his eye, yet we may fairly consider 
them as comparable among themselves. 
We have then 
1863, thes 2, Boston M. T, 
mis ee: o,.= i= 783100 times that of the 
peri a é27=1308000 § faintest visible star, 
Whence we a assuming the barometer and thermometer to 
have been at standard heights, 
pa) — 1847689 
0 = 7, 1680904 
The difference between the values B and C may be considered 
as due to the diurnal and other changes in the heights of the 
barometer and thermometer at Croisic and Cambridgeport; to 
the differences in the transparency of the atmosphere and ‘the 
annual and diurnal changes in the same; and to the fact that 
Bouguer observed the moon at night, and ‘Clark the sun by day, 
therefore, the different laws of variation of temperature in the 
strata of the atmosphere and the brilliancy of the atmosphere 
as illumined by these luminaries will introduce discordances. 
This latter is the most important source of error; the light 
which we must measure in observations of this nature being the 
sum of those rays which penetrate the rangeetbgh plus the light 
of the atmosphere as illumined by those rays which it absorbs, 
In respect to this point it is very important that observations be 
made on the stars, comets and moon, as well as the sun. Mr, 
Clark’s two observations at 18" 30™ and 10™ p. M. are interesting 
in this relation. They are as Sion Admitting a large cirele 
of the illuminated air which surrounds the sun he fin ds, 
April 27th 18h 30m =, =75° 2 = i, = 1055360 Hage that of faint 
3 28th 0 10 $,=28 20 i,=1574400 sible star. 
Treating these observations by equations (8) and (4) we find: 
I 1606041 
re encanta ~~ 1858843 sath leon (P) 
the rough agreement af which, with the values A, B, C, is due 
to the Teg ae influence of those rays of the sunlight 
=+ 0:8017, (C) 
none to make a — ret vio in- 
