480 Royal Astronomical Society. 



concentric hour-circles (as they may be called) at the lower end of 

 the polar axis, gives the difference of the right ascensions. 



" If clock-work machinery be not in such perfect adjustment as 

 to keep the standard star, first observed, correctly in the centre of 

 the field of view, it may be brought to that point by a tangential 

 movement of the frame-work, to be made by hand, at the moment 

 of making the second observation, without in any respect vitiating 

 the integrity of the observation, for this small movement does no 

 more than compensate for any error in the goingpof the clock. 



" As it may not be always convenient to move the whole frame 

 which is attached to the clock-work, the upper speculum, at the 

 upper end of the axis, is so fitted as to be capable of being turned 

 round independently of the frame, to which it is fixed by a mode- 

 rately stiff collar. This movement, which may be made roughly by 

 hand, or more nicely by a tangent screw, enables the observer, 

 without stopping the clock-work machinery, to direct the speculum 

 to any given standard star ; and I may observe that only those of 

 the first and second magnitudes are named for this purpose by M. 

 Wettinger, as he fears the light lost by the three successive reflec- 

 tions might. render any less brilliant stars invisible. This, however, 

 does not affect stars viewed through the declination telescope, which 

 looks directly to its object, and is supposed to be capable of seeing 

 small stars as readily as large ones. 



" Observations for determining the differences of right ascension 

 and declination, in arc, between a standard star and any other, both 

 being above the horizon, may be repeated as often as required ; 

 and it does not appear how, supposing the machinery perfect, any 

 error can enter into these determinations, except what arises from 

 the false position in which refraction places celestial objects. In 

 the determination of right ascensions by an instrument placed in the 

 meridian, this source of error is avoided ; but it remains in full force 

 as to declinations. The question with respect to right ascensions, 

 therefore, resolves itself chiefly (if I rightly understand M. Wettinger) 

 into the fact of its being both easier and more exact to determine 

 the difference of right ascension in arc, by a leisurely and direct ob- 

 servation of the angle formed by the two meridians in which the 

 stars lie, than to infer that difference of arc by the uncertain agencv 

 of a clock, which is further vitiated, he thinks, by the uncertainty 

 of marking the exact moments when the stars respectively pass the 

 wires of the meridian instrument. To these sources of error he adds 

 that of the ear in appreciating the beats of the clock. 



" M. Wettinger is of opinion, that, although only experience can 

 determine the degree of accuracy with which such an instrument 

 could give the desired results, very fair estimates may be formed by 

 practical astronomers familiar with the difficulties and errors of the 

 existing methods, of the probable advantages of his invention. 

 Whether, for example, the effects of refraction on stars above a cer- 

 tain altitude, on their right ascensions and declinations, are not suf- 

 ficiently well known to admit of such exact corrections being applied 

 to the determinations made by his instrument, as would render their 



