Royal Astronomical Society. 523 



of the matter of the tail. The following distances were measured 

 with a reflecting instrument : — 



d h m s 



Feb. 28 3 2 15 p.m. Distance of sun's farthest limb from . „ 



nearest limb of nucleus 4 6 15 

 ... 3 6 20 ... ... sun's farthest limb from 



farthest limb of nucleus 4 7 30 

 ... 3 9 40 ... ... sun's farthest limb from 



extremity of tail 5 6 30 



Mr. Clarke supposes the first of these measures to be correct 

 within 15"; the other two are given as near approximations. Al- 

 lowance must, of course, be made for the motion of the two bodies 

 during the time of observation. When the sun was on the meridian, 

 the angle made by the line joining the centres of the sun and the 

 nucleus with the lower vertical, on the eastern side, was about 

 seventy-three degrees. 



X. On the Deducing of the Parallax of Mars, and hence that of 

 the Sun from the Geocentric Motion of the former when in opposi- 

 tion, and especially when near the Node of his Orbit. By S. M. 

 Drach, Esq. 



The author, after alluding to the method of determining the solar 

 parallax from observations of the transits of the inferior planets over 

 the sun's disc, states his method as follows : — 



" The counterpart of the above is the simultaneous observation at 

 different points of the earth's surface, of the time occupied by a su- 

 perior planet, when near opposition and near the node, in passing 

 through a certain interval of space, say about half a degree (the sun's 

 diameter) ; but as this happens at night, comparison stars are to be 

 used, and the interval assumed to be nearly equivalent to their di- 

 stance. Thus, e. g., if Mars be the object observed, and at Green- 

 wich x minutes are occupied by it in describing an arc which it re- 

 quires only y minutes to describe at the Cape of Good Hope, then 

 will the difference x— y, properly applied, give the parallax of Mars, 

 and hence that of the sun." 



XL A Letter from Sir J. F. W. Herschel, Bart., to Mr. Baily, 

 dated 6th Sept., 1842, on the Increase in Magnitude of the Star tj 

 CygnL 



"I beg to call your attention to the star ij Cygni (21 Cygni, 

 Fl.; Piaz. xix. 344), which appears to have increased in magnitude 

 very considerably since the date of Piazzi's observations. It is now 

 the principal star in the neck of the Swan, and of nearly the fourth 

 magnitude, — very conspicuous to the naked eye, and marking, in 

 fact, the only very distinctly seizable point between Albireo in the 

 beak, and the bright star y in the body. Now, Piazzi, from nineteen 

 observations in right ascension, and eleven in declination, sets it 

 down as of the 5*6 mag. It does not occur in the Astronomical 

 Society's Catalogue. The star b q Cygni, which does occur in that 

 Catalogue, is there set down as of the 5th mag., which is also what I 

 make it, or rather above than under ; but ij is now a much more di- 

 stinguished star. 



