489 
Mr. Ivory on the astronomical refractions. 
Tables. I have not thought it necessary to insert all the 
observations communicated by Mr. Groombridge ; but such 
only, where the altitudes are less than five degrees. 
The correction for temperature is variously computed by 
astronomers. Some use the interior, and others the exterior 
thermometer ; and some prefer taking a mean between the 
two. But it may be affirmed with some degree of certainty, 
that the practice of computing by the exterior thermometer 
can be perfectly correct only when the temperature is the 
same within and without the Observatory. If we suppose 
that this is the case at first, and that afterwards the air within 
the Observatory is heated above, or cooled below, the ex- 
ternal temperature ; the consequence must certainly be, that 
the apparent place of a star will undergo some alteration. On 
the other hand, if the heat be equally distributed within the 
Observatory, and remain constant, while the temperature on 
the outside varies ; it is not clear whether any change at all 
would be observed in the place of a star, more especially if 
the change of temperature were small.* But this is a point 
that can be determined only by careful experiments ; and, 
until some light be thrown upon it, no great improvement 
can be expected in our knowledge of the astronomical re- 
fraction. -f 
i 
* See Dr. Brinkley’s Paper, Philosophical Transactions, 1821, p. 335. 
f N. B. In calculating the refractions, the temperature of the mercury in the 
barometer is estimated by the interior, that of the air by the exterior, thermometer. 
3 R 
MDCCCXXIII. 
