J 826.] Astronomical Society. 229' 



Telescope having only one Reflector ; " by Abram Robertson, 

 DD. FRS. Savilian Professor of Astronomy, Oxford. 



Also a paper, On the Constitution of the Atmosphere; by 

 John Dalton, Esq. FRS. ; of which we shall probably give an 

 account in our next number. 



ASTRONOMICAL SOCIETY. 



Jan. 13, 1826. — There was read a paper by Stephen Groom- 

 bridge, Esq. FRS. on the co-latitude of his observatory at 

 Blackheath, as determined from his own observations. The 

 author first describes a simple method of bringing the transit- 

 instrument into the meridian, by the observations of Polaris and 

 other circurapolar stars, and then by comparisons of high and 

 low stars. He next describes the method of ascertaining the 

 true zenith point, and thence the elevation of the pole, by obser- 

 vations of circumpolar stars in zenith-distance above and below 

 the pole, from which twice the co-latitude becomes known. 

 Employing his own constant of refraction, he obtains from obser- 

 vations of 32 circumpolar stars above and below the pole 

 77° 3' b5"-65 for the mean double co-latitude ; thence 

 38° 31' 57''- 82, and 51° 28' 2''-18 for the latitude; a result 

 which accords with his independent observations on the sol- 

 stices. 



Mr. Groombridge next proceeds to deduce from this, the 

 co-latitude of the Royal Observatory. He determines the differ- 

 ence of the zeniths of the two observatories at 35''-25, which 

 applied to the latitude of the Blackheath Observatory, by addi- 

 tion, gives 51° 28' 37"-43 for that of the Royal Observatory, 

 being less than Mr. Pond makes it by more than a second. Mr. 

 Groombridge imputes the difference to an erroneous constant of 

 refraction. The author concludes his paper, by presenting some 

 simple formulas for finding the position of a transit instrument, 

 from the observed transits of a high and low star, passing the 

 meridian to the south of the zenith ; or from the observed tran- 

 sit of a circumpolar star above and below the pole. 



There was next read, a communication, from Sir Thomas 

 Brisbane, dated Paramatta, 2d July, 1825. The contents were, 

 1st. Observations with a repeating circle for the winter solstice, 

 1825, extending from June 12th to July 1st inclusive. These 

 are not yet reduced. 2dly. Observations on the inferior con- 

 junction of Venus and the Sun, in May, 1825, with the mural 

 circle, from May 1st to 25th inclusive. 3dly. Observations 

 on the dip of the magnetic needle, March, 1825 ; — the mean of 

 the whole was 62° 41' 35". 4thly. Observations on the declina- 

 tion of the needle in March, April, and May, 1825;— the mean 

 of the whole is 8° 59' 48". Lastly. An abstract of the Meteo- 

 rological Journal kept at Paramatta, from April, 1824, to April, 

 1825* 



