[ 1 93 ] 
external contad at Chelfea was 8 h 34' 30" equal 
time, and 8 h 3 6 7 21" apparent time ; which makes 
8 h 37' 2 " apparent time at Greenwich. 
From the aforegoing circumftances, it appeared to 
me, that the external contact was more eafily to be de- 
termined than the internal one, which was contrary to 
what I had before expected ; and becaufe the point 
of contad muft have appeared through fuch a te- 
leicope as I obferved with, in its proper colour, dark 
or black, fooner than through a fmaller magnifying 
power of equal light, I concluded, that, through my 
telefcope, the internal contad was vilible, fooner than 
through a two foot refledor, ten or twelve feconds of 
time. 
And, confidering the aforefaid penumbra, or bor- 
der of partial light, furrounding Venus, as an atmo-, 
Iphere of that planet, with the time of its vanifhing, 
24 feconds of time ; and reducing this to the diameter 
of Venus, with due allowance for the oblique diredion 
over the fun’s limb, the atmofphere of Venus comes 
out 84 thirds of a degree, which is nearly about 
-?4- th P art of Venus’s diameter; which diameter being ' 
nearly equal to the earth’s diameter, the atmofphere 
of Venus comes out nearly 50 geographical miles. 
As thefe obfervations were made with care and at- 
tention, I have lain them before the Society only; 
and the more readily, as they reconcile a feeming con- 
tradidion in Mr. Short’s * numbers of the internal 
contad; 
* It having been 3 feconds of time from the inftant when the 
thread of light between Venus and the fun became fo indiftindl, 
as not to be properly termed light, to the ihftant when the black 
Vox. LII. C c cental 
