jRoyal Astronomical Society. 299 



construction : — On the side opposite to the eyepiece, and attached 

 to the bottom plate of the telescope, a wooden box was carried out 

 horizontally, its bottom being nearly as low as the foot-plate, and 

 its top at the level of the bottom plate of the tube ; and upon the 

 end of this box was planted a wooden plumb-wire tube, connected 

 at its top with a frame attached to a higher part of the great tube 

 (upon which frame the wooden plumb-wire tube and the horizontal 

 box were, in fact, suspended). From the top of this wooden tube 

 was suspended (with screw-adjustment for moving it to or from the 

 principal plumb-line) a second plumb-line, 40 inches long, and di- 

 stant from the principal plumb-line about 12 inches. The two 

 plumb-lines supported in the horizontal box the two ends of a bar 

 12 inches long, and to this bar were attached one or two plumb- 

 bobs. A little consideration of the theory of parallel forces will 

 show that, if the distance between the plumb-lines at the top and 

 the bottom is the same (the criterion of which is, the distinctness of 

 the principal plumb-line in the field of view of the lower microscope, 

 whether the second plumb-line be used or not), the relative move- 

 ments of the principal plumb-line for varying inclinations of the 

 grand tube are the same as if the plumb-bob were immediately at- 

 tached to it. At the same time, the turning of the wire is effectu- 

 ally prevented. 



The instrument was used with this construction to the spring of 

 1848, when it was finally dismounted. The results of observation 

 were more accordant than they were before introducing the last 

 modification, but they were not superior to those derived from the 

 mural circle. There can be no doubt that the remaining errors 

 were greater than could be attributed to mere imperfection of obser- 

 vation, and that they must originate in some fault of the instrument. 

 Generally speaking, the greatest errors coincided with the greatest 

 irregularities in the readings of the lower plumb-line microsco])e. 

 It has already been pointed out that the fixation of that microscope 

 was not very firm, and the irregularities may have originated in this 

 weakness. Or the great tube may have twisted sensibly in the 

 reversion. Or the plumb-line may have been bound by spider- 

 threads, from which it was to a certain degree set free by the move- 

 ment of reversion (for it was found almost impossible to keep the 

 wire-plate free from spider-threads, the uniform temperature and 

 the constantly vertical position of the telescope being probably com- 

 fortable to those animals). Whatever the distinct cause might be, 

 the Astronomer Royal considered that the instrument had failed, 

 and that its failure was owing to the dependence on the plumb-line ; 

 and he expressed his hope that he might never again be compelled 

 to use an instrument relying for its verification upon a plumb-line. 



II. The applicability of a transit instrument in the prime vertical 

 to the determination of latitude of place from assumed polar distance 

 of the star, or vice versd (the polar distance of the star being greater 

 than the co-latitude of the place), has long been known ; but it seems 

 to have attracted more particular attention since it was used by 

 Struve in the determination of the diflference of latitudes at the ex- 



