68 Royal Astronomical Society. 



the approximate places of those stars, on which accurate and import- 

 ant comparisons depend. 



The Astronomer Royal has communicated, and doubtless will con- 

 tinue to communicate, the mean right ascensions of the stars em- 

 ployed at Greenwich, so that the Edinburgh places will harmonize 

 with those of Greenwich ; or, as Professor Smyth remarks, " it will 

 not work against but co-operate with Greenwich." 



There seems reason to complain of the smoke of the city, but pro- 

 bably this will not very materially injure the great mass of obser- 

 vations. I'he transit has a noble object-glass 6| inches aperture, 

 and the Professor proposes to use the circle with illuminated wires on 

 a dark field. 



The instability of the Edinburgh transit was suspected by Profes- 

 sor Henderson to arise from the effect of temperature on the founda- 

 tion. Professor Smyth has traced it to a much simpler cause, a de- 

 fective original construction of the Ys. He thus describes the con- 

 struction of the new Ys : — 



" They are large slabs of cast-iron, covering the whole area of 

 the top of the pier, and weighing several hundredweight ; there are 

 no adjusting-screws, but the sides of the angles in which the pivots 

 rest have been filed away, until the instrument is made to move as 

 nearly in the plane of the meridian as could, perhaps, have been 

 managed with screws. One good result has been certainly proved 

 to have followed, viz. that the reversing of the instrument to obtain 

 the error of coUimation does not now sensibly throw it out in azi- 

 muth, which Professor Henderson used to complain of with the old 

 Ys. Touching the fears that the new Ys might split the stone piers, 

 and the hopes that they might correct the temperature fluctuations, 

 there has not been sufficient time yet to settle that question through 

 the medium of the large transit ; it may, however, be considered to 

 be pretty well set at rest by the experiments on the 30-inch transit. 

 This was mounted on a similar huge block of cast-iron screwed and 

 cemented down to its pier ; on this it has now been sticking for a 

 year, as firmly but as innocently as could be desired. 



" The following is a list of azimuth errors of the 30-inch transit 

 in its cast-iron block, as determined by all the transits of a Ursae 

 Minoris, observed on all five wires, during the period which elapsed 

 from the final fihng of the Ys to a snapping of two of tlie wires, 

 which took place, it was supposed, from moisture : the clock-star 

 used on each occasion was 6' Ceti : — 



s 



1847. Nov. 1 -0-091 



2 -0003 



15 -0058 



16 -0040 

 30 +0043 



Dec. 14 -i-0006 



1848. Jan. 7 +0020 



13 —0024 

 " Now these apparent fluctuations of the Ys in azimuth, which 

 are very small, include the probable error with which each observation 



