REPORT ON ASTRONOMY. 
187 
discordancies. I think that astronomers would be glad to re¬ 
ceive simple directions for placing a thermometer so as to indi¬ 
cate correctly the temperature of the air at the place of observa¬ 
tion. I omit all discussion of the difference of the external and 
internal thermometer, as I think the only way of overcoming 
the difficulty is, to make the external and the internal tempera¬ 
ture as nearly as possible equal. 
2. In the theory of refraction, the following questions present 
themselves as only to be solved by experiment. What is the 
law of the decrease of temperature, or rather of density, in 
ascending ? How does this vary at different times ? Can any 
means be contrived for indicating practically at different times 
the modulus of variation ? (The last question is suggested by 
the remarks in Mr. Atkinson’s valuable paper, Mem. Ast. Soc. 
vol. 2.) Does the refractive power of air depend simply on its 
density, without regard to its temperature ? Is it well established 
that the effects of moisture are almost insensible ? From Car- 
lini’s Tables, as well as from general reasoning, it seems likely 
that refraction may be different in different azimuths, according 
to the form of the ground : can any rough rule be given for 
estimating its effect ? Finally, when the atmospheric dispersion 
is considerable, what part of the spectrum is it best that astro¬ 
nomers should agree to observe ? 
3. I have already stated that I think Lindenau’s constant of 
nutation has been adopted by the German astronomers on in¬ 
sufficient grounds. The value which I should certainly prefer 
is that determined by Dr. Brinkley, and which Mr. Baily, with 
his usual judgement, adopted for the catalogue of the Astrono¬ 
mical Society. The Greenwich circles have now been erected, 
and in a perfect state, long enough (or nearly so,) to determine 
this constant; and the mass of excellent observations which 
they have produced, applicable to this question, vastly exceeds 
any other that has been used for the same purpose. It is highly 
desirable that the coefficient of nutation should be investigated 
from the Greenwich circle-observations. 
4. Bradley’s observations of stars were nearly useless till 
Bessel undertook to reduce them. In like manner Bradley’s 
and Maskelyne’s observations of the sun and planets are still 
nearly useless. At different times observations of the sun 
have been reduced (by Delambre, by Burckhardt, and lately 
by Bessel or Schumacher), and probably much labour has 
been wasted. A reduction of these observations on a uni¬ 
form plan (adopting, for instance, Bessel’s Tabulce Regiomon- 
tance ,) would be invaluable in the application of the planetary 
theories. Many observations of the moon have been reduced 
