the effects of parallax, &c. of certain fixed stars. 337 
equalize the internal and external temperatures. Here the 
difference of temperatures is greatest after sunset in summer 
and autumn, except in extreme cold in winter ; and the equali- 
zation of the temperatures cannot be easily affected without 
too great an exposure of the instrument to the external air. 
Partial currents might derange it, and occasion more un- 
certainty than that arising from the difference of tempera- 
tures. The room in which my instrument is placed, containing 
also the transit instrument, is of considerable dimensions, 
being thirty-seven feet long, twenty-three feet broad, and 
twenty-one feet high. The instrument is several feet from 
the shutters, which may be supposed a favourable circum- 
stance. The apertures for observation are three feet wide. 
Having thus given a detailed account of observations that 
have been principally instituted with a view of obtaining an 
explanation of the source of the difference of the results of 
my former observations and of those of Mr. Pond, relative 
to parallax ; it is with concern I state, that it contains not a 
trace of any such explanation . I have been unable to obtain 
any result that is opposed to my former conclusions. 
It would be extremely important to ascertain the certainty 
of the results of an instrument, which, by its construction and 
principle of reversion, seems much better adapted to the pre- 
sent wants of astronomy than a mural circle. The advantage 
of referring each star to the apparent zenith point, and thus 
obtaining a knowledge of its motions without a reference to 
those of other stars, is easily appreciated. The advantage is 
also very great, of being able to observe a few minutes before 
the object arrives at the meridian, and, reversing the instru- 
ment, of then observing again. The zenith distance is thus 
