185 



the outer and inner thermometer throughout the whole course of 

 observation. It is shown that temperature cannot have vitiated 

 these results by its effect on the instrument, since they are the same 

 whether we employ the readings of two, or of six microscopes. 



The absolute parallax of a Lyrse is investigated by the method of 

 reflection, which the author had successfully introduced into other 

 astronomical researches. Although the period of observation em- 

 braces only half the period of the double parallax, yet that apparent 

 disadvantage is more than compensated, in the author's opinion, by 

 an uniformity of the temperature obtained, such as can never be ex- 

 pected between the extreme seasons. Here, again, from the season 

 chosen, from the pains taken to equalize the temperature, and from 

 the concordance of the results obtained with two and six micro- 

 scopes, the author believes that no errors of sensible amount have 

 been introduced by change of temperature. These observations in- 

 dicate that the absolute parallax of a Lyra does not exceed a small 

 fraction of a second. 



The argument that has been advanced by Dr. Brinkley in favour 

 of parallax, and on which the greatest reliance has been placed, is 

 next adverted to ; namely, that founded on the disengagement of 

 the solar nutation after allowing for parallax, from the observations 

 made with the Dublin instrument. This reasoning is considered 

 strictly logical, as proving the disengagement of two equations, hav- 

 ing each a regular period ; but by no means so, as establishing that 

 the larger equation results from parallax ; since those stars in which 

 the Dublin instrument discovers parallax, are at some distance from 

 the zenith ; and the more so as their parallax appears to be greater ; 

 and moreover, since those in which the greatest parallax is found, 

 are stars whose maximum and mininum of parallax fall in the ex- 

 treme seasons ; the author thinks it probable that the discordances 

 observed are owing to changes of temperature, which either alter the 

 form of the instrument, or modify the refractions of the ray intro- 

 duced within the observatory , since, on this supposition, we shall 

 account for the want of parallax in zenith stars, and in those whose 

 greatest and least parallax would happen at the mean seasons, and 

 also for the regularity of the period that the discordances have been 

 found to observe. 



Observations on the Heights of Places in the Trigonometrical Survey 

 of Great Britain, and upon the Latitude of Arbury Hill. By B. 

 Bevan, Esq. Communicated by Sir H. Davy, Bart. P.R.S. Read 

 May 23, 1822. ^Phil. Trans. 1823, p. 73.] 



The result of the trigonometrical survey, relative to the different 

 sections of the meridian in this country, having disappointed public 

 expectation, Mr. Bevan lately examined with some care the calcu- 

 lations affected by the observations made at Arbury Hill, with the 

 hope of reconciling the anomaly in that part of the meridian. The 



