yhlroiiomical Sociclj/. 293 



wliicii it approaclied within about 2' of the nucleus, at a quarter past 

 six solar time. As this star is in the catalogue of the Astr. Soc, 

 the right ascension and declination of the Comet at that time may be 

 nearly ascertained. On looking into the ephemeris of the Nautical 

 Almanac, I observed, that from the 28th to the 30th of October would 

 be a desirable time for determining the Comet's place ; but as my 

 large circle is now used only in the meridian, I was obliged to substi- 

 tute a small altitude and azimuth circle, which reads 10" in altitude 

 and 15'' in azimuth, with a telescope magnifying about 20 times, and 

 having an aperture of an inch and half diameter. Having placed this 

 small instrument in the meridian previously, in an open situation, 

 and made all the adjustments for my purpose, on the 29th 1 had 

 fortunately a view of the Comet between the south and west, though 

 it was frequently covered by passing clouds, and at last totally ob- 

 scured ; however, I succeeded in making two observations, while my 

 assistant marked tlie time by a sidereal clock, giving the right as- 

 cension of the mid -heaven : thus obtaining data for the accompanying 

 computations. A tnean of two results, corresponding to the mean of 

 the two times noted, it is presumed will not be far from the truth; 

 though not so correct as if the large circle had been used. 



" I send the computations to you, partly to show how a similar 

 portable instrument, with circles not exceeding seven inches in dia- 

 meter, may be used for this and similar purposes ; and, partly, be- 

 cause the observations were taken at the latest period that offered, 

 before the Comet's arriving at the perihelion of its orbit. Should the 

 determination now sent be found tolerably correct, you will perceive 

 that the right ascension on the 29th of October exceeded the maxi- 

 mum given in the Ephemeris, above referred to, by about thirty mi- 

 nutes." 



X. A letter from M. Mossotti to Mr. Baily. 



This letter contains computations and deductions relative to the 

 transit of Mercury over the sun's disc, on May .5, 1832, as recorded 

 in the Memoirs of this Society, vol. viii. p. 268. From fourteen 

 observations of the distance of the planet from the sun's centre he has 

 formed as many equations of condition, and resolving them by the 

 method of least squares he has deduced certain results, which are still 

 subject to a further correction, depending on the true time of con- 

 junction of the sun and planet at Greenwich ; which was not known 

 to M. Mossotti when he wrote the letter. M. Mossotti also trans- 

 mitted, at the same time, some further remarks on the solar eclipse 

 of January 20, 1833, recorded in the same volume of the Memoirs, 

 p. 224. In the computations relative to this subject, he has employed 

 an equation which he considers to possess an advantage (over all 

 those given by the common method) of presenting all the unknown 

 quantities, that we are in search of, in a linear form ; and is, in fact, 

 the very formula by means of which he deduced the results in the 

 equations employed in the transit of Mercury above mentioned. The 

 formula is a transformation of the general equation for an eclipse, or 

 occultation, inserted in Dr. Pearson's Introduction to Practical As- 

 tronomy, vol. ii. p.075,&c., but which is too long to be here inserted. 



