1822.] Finite Extent of the Atmosphere. 253 
all space) an atmosphere* fully equal in density to our own, and 
consequently producing a refraction of more than one degree, in 
the passage of rays obliquely through it. 
If the mass of the sun be considered as 330,000 times that of 
the earth, the distance at which his force is equal to gravity will 
be / 330,000, or about 575 times the earth’s radius; and if his 
radius be 111,5 times that of the earth, then this distance will 
be a or 5°15 times the sun’s radius; and 15’ 49” x 5°15 = 
1° 21’ 29”, will be the apparent distance from the sun’s centre 
on the 23d of May, when the following observations were made. 
What deduction should be allowed for the effect of heat, it 
may be time to consider when we have learned the amount of 
apparent refraction at some given distance ; and we may then 
begin to conjecture, whether heat can counteract the increase 
of density that would occur in the approach of only 1-10th of a 
second towards his centre.+ 
As I had not any instrument in my possession that I consi- 
dered properly adapted for the purpose, I requested the assist- 
ance of several astronomical friends in watching the progress of 
Venus to the sun for some days preceding superior conjunction, 
and in recovering sight of her afterwards. But neither the 
Astronomer Royal at Greenwich, nor Prof. Brinkley of Dublin, 
nor Mr. South, with the admirable instruments they possess, 
were able to make any observation within the time required, not 
being furnished with the peculiar means adapted to this inquiry. 
Capt. Kater, however, who entered fully into my views, and 
engaged in the prosecution of them with all the ardour neces- 
sary for success, by using a reflecting telescope, was able to 
furnish me with a valuable set of observations, 34 days preced- 
ing conjunction, which, together with those in which I had the 
good fortune to succeed at nearly an equal interval subsequent to 
the passage, afford data quite sufficient to show that no refrac- 
tion is perceptible at the period of our observations ; and these 
come far within the specific distance above estimated. 
A selection from the series given to me by Capt. Kater is con- 
tained in the following table : 
* Such an atmosphere would, in fact, be of greater density on account of the far 
greater extent of the medium affected by the solar attraction, although of extreme rarity ; 
but the addition derived from this source, may be disregarded in the present estimate, 
without prejudice to the argument, which will not be found to turn upon any minute 
difference, 
+ If we attempt to reason upon what would be the progressive condensation of such an 
atmosphere downwards towards the surface of the sun, we are soon stopped by the limit 
of our experience as to the degree of condensation of which the atmosphere is suscepti- 
ble. If we could suppose the common law of condensation to extend as far as 46 miles 
in depth, the density corresponding to it would be about equal to that of quicksilver, 
from whence a refraction would occur exceeding all bounds of reasonable calculation. 
A a of 46 miles at the distance of the sun from us would subtend about one-tenth of 
# second, 
