372 



As his instrument had the advantage of being fixed upon stone 

 piers, which are not liable to partial expansion, and as the size of 

 the instrument itself seemed to him better adapted to determining 

 the real quantity of atmospherical refraction than any which had 

 been before employed for the same purpose, he extended the range 

 of his observations as low down towards his north horizon as his 

 situation would permit. For this purpose he selected fifty stars of 

 different polar distances, and of these he made, upon the whole, up- 

 wards of 1000 observations. 



The observed zenith distances being first corrected by the usual 

 equations, so as to reduce them all to the same period, January 1, 1807, 

 a correction is next made for refraction, according to Dr. Maskelyne's 

 last precepts, in which the refraction at 45 is estimated at 56^", 

 with due allowance, as usual, for the states of the barometer and 

 thermometer, as noted at the time of observation. 



Since the co-latitude is equal to half the sum of the real zenith 

 distances of any one star that has been observed, both above and be- 

 neath the pole, it is evident that the same result should be obtained 

 from stars near the pole, as from those which are more distant, after 

 all the requisite corrections have been rightly made. But since, by 

 the author's observations, his co-latitude deduced from distant stars, 

 which are subject to greater refraction, was found to be about 2^" 

 greater than from stars near the pole, he presumed that the allowance 

 of 5 65" for mean refraction at 45 was too small. For if both the 

 greater and less refraction be increased in the same ratio, the cor- 

 rections thus made will be unequal, and their difference may be made 

 to remove the inequality of the co-latitudes, as deduced from the 



From the mean of 13 stars, which do not pass lower than 56 from 

 the zenith, compared with the mean of 21 stars, between 60 and 78 

 zenith distance, Mr. Groombridge infers that the mean refraction is 

 really as much as 58" and a small fraction ; and accordingly, in his 

 table of observations, he gives corrections computed according to this 

 supposition, whereby his column of co-latitudes is rendered uniform, 

 without departing from the law of refraction at different altitudes 

 laid down by Dr. Bradley. 



The deductions thus made from observations on the fixed stars, 

 are next compared with those obtained from the meridian alti- 

 tudes of the sun at the solstices, which he thinks afford satisfac- 

 tory proof of their correctness ; as the latitude of his observatory, by 

 the former method, was found to be 51 28' 2"'l, and by the latter 

 51 28' 2"-35. 



The author proceeds to ascertain the difference of latitude between 

 the Royal Observatory at Greenwich and his own, by comparison of 

 his observations of the zenith distance of y Draconis, with some of 

 the same star communicated to him by Dr. Maskelyne ; and by simi- 

 lar comparison of zenith distances of other stars observed at the 

 Royal Observatory by Colonel Mudge with the zenith sector. 



