458 Astronomical Society. 



of analytical investigation, assisted by Legendre's tables of 

 elliptic functions, arrives at the result, that this resistance 

 would not produce a greater change in the mean geocentric 

 longitude of Mercury, than 31 "-2 in the course of a century. 

 Hence he concludes that the comet may have such a resistance 

 from an ether, as will be sufficient to account for the difference 

 between the calculus and the observations, and yet that the 

 planets shall not hitherto have manifested the least effect of 

 such a medium. 



2d. On a new Astronomical Instrument called the Diffe- 

 rential Sextant; by Benjamin Gompertz, Esq. F.R.S. This 

 paper was a further and more particular description of the 

 construction and application of the instrument before invented 

 by Mr. Gompertz, and partially described in his paper on 

 Astronomical Instruments read before the Society on the 11th 

 of January 1822. In this instrument, the index reflector is 

 susceptible of motion on one end of the index as on a centre, 

 being the same as that on which the index itself turns, so that 

 the reflector may be set to make any angle at pleasure with 

 the index ; the whole being permitted to move, as a bent lever 

 about the centre. The horizon glass also is capable of being 

 adjusted and fixed at different angles to the fixed arm. The 

 object proposed by Mr. Gompertz in this contrivance is to 

 measure the difference of angular distances in any two ce- 

 lestial phaenomena, occasioned by those varying circumstances 

 which produce small changes ; such as refraction, parallax, 

 aberration, &c. : and the paper concluded with some appro- 

 priate hints as to the best manner of employing the instrument 

 to these purposes. 



3d. An Account of an Occultation of the Georginm Sidus 

 by the Moon, which will take place on the 6th of August 

 next, by Francis Baily, Esq. F.R.S. and V. Pres. Ast. Soc. — 

 Mr. Baily begged to call the attention of the Society to this 

 interesting phcenomenon which has never yet been seen, as no 

 occultation has occurred since the discovery of the planet. The 

 occultation will occur within a very few minutes after the 

 moon has passed the meridian ; insomuch that those persons 

 possessing a transit instrument will see the planet in the field 

 of view when the moon's centre is on the meridian. This 

 notice was accompanied by a diagram, showing that the planet 

 would enter the western or dark limb of the moon at about 

 half way between the moon's outer and the upper or northern 

 part of her disk. There will be sufficient time to observe the 

 occultation of the planet after the transit of the moon. This 

 interesting phaenomenon will no doubt attract the notice of 



every 



