93 
finite extent ofi the atmosphere. 
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 
of a second towards his centre.* 
As I had not any instrument in my possession that I con- 
sidered properly adapted for the purpose, I requested the 
assistance of several astronomical friends in watching the 
progress of Venus to the sun for some days preceding supe- 
rior conjunction, and in recovering sight of her afterwards. 
But neither the Astronomer Royal at Greenwich, nor Pro- 
fessor Brinkley of Dublin, nor Mr. South, with the admira- 
ble instruments they possess, were able to make any obser- 
vation within the time required, not being furnished with the 
peculiar means adapted to this inquiry. 
Captain Kater, however, who entered fully into my views, 
and engaged in the prosecution of them with all the ardour 
necessary for success, by using a reflecting telescope, was 
able to furnish me with a valuable set of observations, 
days preceding 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 suffi- 
cient to show, that no refraction is perceptible at the period 
* 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 susceptible. If we could suppose the common law of condensation to extend as 
far as forty-six 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 space of forty-six miles at the distance of the sun 
from us, would subtend about T V of a second. 
