Astronomical and Nautical Collections. 103 
mean value of the refraction,” is altogether inconsiderable. If the 
new table had been decidedly shown to exhibit results more ex- 
actly conformable with the actual determinations of astronomers 
than those of the Nautical Almanac, it would have become the 
duty of the editor to adopt them; but for the present there can be 
no question that such a change would at least be premature. 
iv. Note on Rurraction, addressed to Professor SCHUMACHER. 
My dear Sir, 
I was much surprised the other day to observe, that in copying 
the explanation of my Table of Refractions from the Nautical Al- 
manac, you had omitted, without assigning any reason, the words 
“¢ which would be more consistent with the theory;” an expression 
which I had employed in speaking of the use of the external ther- 
mometer, in preference to the interior. 
I am the more disposed to remonstrate with you on this occa- 
sion, because I observe that a great number of astronomers, and 
among them some who do not usually act without reflection, have 
inconsiderately taken it for granted that the correction ought to be 
made according to the height of the interior thermometer, as 
nearest to the place of observation. 
Now, with regard to the theory, it is perfectly obvious, that the 
computation extends only to such changes of density as take place 
between the different strata of the atmosphere considered as hori- 
zontal; and that its results must necessarily terminate where this 
regular constitution of the atmosphere ends; that is, owtside the 
observatory, or other building, containing the instruments; while 
the change of density between the external and internal air, taking 
place in general at surfaces more nearly vertical than horizontal, 
at least when the object is but little elevated, and certainly never 
at horizontal surfaces, will either have no effect at all in increasing 
the refraction, or as great an effect at higher as at lower altitudes; 
so that this little irregular addition or diminution can never require 
to be considered as a part proportional to the whole original mean 
refraction. 
With regard to practice and observation, I need only refer to 
