[ 4^4 ] 
6. From thefe, nnd other like circumstances, I firft 
began to fufpeCt, that a fudden dip of the Sun into 
the horizontal vapours, might, fome how or other, 
be the caufe of a fudden apparent change of magni- 
tude, although the horizontal vapours had been dif- 
allowed to be able to produce any other than a re- 
fraction in a vertical direction ; and reducing things 
to calculation, found, that, from the time when the 
Sun is within a diameter or two of the horizon, to the 
time when he is a femi-diameter below the horizon, 
the Sun’s rays become paffable through fuch a length 
of medium, reckoning in the direction of the rays, 
that the total quantity of medium, (reckoning both 
depth and denfity) through which the rays pafs, being 
compared with the like total depth and denfity through 
which they pafs, at feveral elevations, it was propor- 
tionable to the difference of apparent magnitude, as 
appearing to the naked eye. 
7. This circumffance of fudden increafe and de- 
ereafe of apparent magnitude, and as fudden decreafe 
and increafe of light, (for they both go together) 
feejned to me no improbable caufe of the pheno- 
menon, although I could not then perceive how fuch 
vapours might contribute toward enlarging the dia- 
meter of the Sun apparently in a horizontal di- 
rection. 
8. I therefore examined the Sun’s difk again and 
again, by the naked eye, and by telefcopes, at dif- 
ferent altitudes, and, among feveral circumftances, 
found the folar maculae appear larger and plainer to 
the naked eye, and through a telefcope, the Sim 
being near the horizon, than they had appeared the 
fame 
