1824.J Proceedings of Philosophical Societies. 145 



Article XVII. 



Proceedings of Philosophical Societies. 



ASTRONOMICAL SOCIETY. 



June 11. — The following papers were read : — 



1st. On the Variation of the mean xMotion of the Comet of 

 Encke produced by the Resistance of an Ether ; by M. Mas- 

 sotti. This comet is well known to evince a diminution of its 

 periodic time at each revolution, and the object of this paper 

 was to demonstrate the cause of this effect. Encke himself 

 supposed it was occasioned by an ether diffused through space ; 

 but if so, how happens it that the planets also have not been 

 retarded ? This the author attempted to show might be the case, 

 although the phenomenon might pass unobserved. He adopts 

 with Encke, the hypothesis of Newton, that the density of this 

 ether diminishes in the inverse ratio of the square of the dist- 

 ance from the sun ; consequently that the planet Mercury would 

 be most likely to be affected by it ; and by a long series 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 3T2" 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 Differen- 

 tial Sextant ; by Benjamin Gompertz, Esq. FRS. — This paper 

 was a further and more particular description of the construc- 

 tion 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 celestial phenomena, occasioned 

 by those varying circumstances which produce small changes ; 

 such as refraction, parallax, aberration, &c. : and the paper 

 concluded with some appropriate hints as to the best manner of 

 applying the instrument to these purposes. 



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