304 Royal Astronomical Society. 



regard to the reversion, by a forked apparatus rising from the floor, 

 there is no difficulty ; the only difficulty is in the application of the 

 level while the instrument is in the position of observation. There 

 appears to be no valid reason prohibiting the use of one of the 

 following constructions. The level-frame might consist of two 

 bars extending from the pivot-forks towards the telescope, there in- 

 terrupted in their straight course, and united by a large oval ring, 

 through which the telescope could play ; each bar must then carry 

 a short spirit-level. Or it might consist of a parallelogrammic 

 frame, strengthened in the middle by a ring through which the 

 telescope could play, the two short sides being attached to the 

 pivot-forks, and the two long sides carrying two long levels. But a 

 different form of instrument may be suggested, allowing of the ap- 

 plication of the ordinary single level at all times, and apparently 

 embodying the conveniences of Struve's form, while it does not 

 abandon the strength of the usual form. The transit-instrument 

 may be made in the form of the letter T (the horizontal line of the 

 T representing the axis of rotation), the object-glass of the tele- 

 scope scarcely rising above the thick part of the axis. As the weight 

 of the telescope is entirely on one side of the axis, it must be ba- 

 lanced by counterpoises carried by a very large fork ; the stalk of 

 the fork being within the telescope-tube, the two arms of the fork 

 being in the axis of the transit, and resting within the pivots for a 

 fulcrum, but projecting out beyond the pivots ; and the two prongs 

 of the fork projecting towards the object viewed by the telescope, 

 and being loaded with the counterpoises at their ends. 



If, however, we rely upon our transit taking the same bearing in 

 the Y's after reversion, a very much simpler principle may be used, 

 dispensing entirely with the level. It is only necessary, after having 

 miade the observations on the north side of the east prime vertical 

 (as already described), to reverse the instrument and to observe on 

 the south side of the east prime vertical by reflexion in a trough of 

 quicksilver ; then to reverse and observe on the south side of the 

 west prime vertical by reflexion ; then to reverse again, and to 

 observe, by direct vision, on the north side of the west prime vertical. 



The Astronomer Royal expressed himself confident that in some 

 of these ways the advantages of Struve's construction might be 

 secured, with the additional guarantee for the goodness of the results, 

 that they are obtained with an instrument of firm mechanical con- 

 struction. 



The Astronomer Royal then explained that his attention had been 

 directed to these constructions by the necessity created by the pre- 

 sent condition of astronomy for a few accurate observations, at what- 

 ever trouble obtained, of stars near the zenith. Struve's instrument 

 is now employed on three stars only, and M. Struve is satisfied if 

 of each of these stars he can obtain eight observations in a year. At 

 Greenwich there is special need of observations of one star, namely 

 y Draconis, a star that may with propriety be considered as the 

 birth-star of English Astronomy. Unfortunately no instrument on 

 the prime vertical principle is applicable to this star, because it passes 



