Royal Astronomical Society. 295 



ing within still smaller distances of the zenith. It is evident that 

 these, besides possessing in the highest excellence the astronomical 

 advantages ascribed to the second class, might be expected to enjoy 

 extraordinary freedom from the distortions produced by the weight 

 of the instrument, and might also give great facility for measures 

 of zenith distance by the simple use of the micrometer or the spirit- 

 level. This was the class of instruments which the Astronomer 

 Royal proposed as the subject of the present statement. 



The Astronomer Royal announced that his discussion would be 

 confined to a critical description of three instruments : the Green- 

 wich 2o-feet zenith tube, now dismounted; Struve's prime vertical 

 instrument, now in use ; and a construction of a reflex zenith tele- 

 scope, proposed by himself, but not yet actually constructed. The 

 second of these instruments, it is true, may be used for zenith di- 

 stances of several degrees, but its peculiar advantages are connected 

 with its use for very small zenith distances. These three instruments 

 would be found to embody, as essential parts of their construction, 

 the three different methods of referring to the zenith in use in 

 modern times ; namely, the plumb-line, the spirit-level, and the re- 

 flexion from the surface of quicksilver. 



I. The Greenwich zenith tube was planned and constructed by 

 Troughton, several years before Mr. Pond's direction of the Royal 

 Observatory ceased ; although, from some delays in regard to the 

 small but essential parts, it was not brought into use till within two 

 years of that time. It consisted of a telescope 25 feet long, revol- 

 ving in azimuth on a pin at the bottom and between guides near the 

 top, but absolutely confined to the vertical direction. Its range of 

 observation was therefore necessarily limited to that angle in which 

 the injurious efi^ect of obliquity of the pencil upon the image of the 

 star is insensible ; and practically it was confined to the single star 

 y Draconis, at two minutes only from the zenith. The aperture of 

 the object-glass was 5 inches ; the diameter of the tube at the top 

 6 inches, which increased at each successive step downwards (the 

 tube being made in five separate lengths), till it was 10 inches in 

 diameter at the bottom. About 6 inches below the object-glass, on 

 the outside of the tube, was a reel of silver wire ; the wire passed 

 upwards over a wheel admitting of a certain degree of end-motion, 

 and then passed through a hole in the side of the tube, and rested 

 between the threads of a screw, within the tube and 4 inches from 

 its top, whose axis was horizontal (a convenient arrangement for 

 small movements of the point of suspension), and then descended 

 vertically. Within the great tube was a small vertical tube, nearly 

 an inch in diameter ; the top of this tube was about 10 inches 

 below the object-glass, and here the small tube was in contact with 

 the side of the great tube ; the tube extended to the lower end, at 

 which part it was separated about 2 inches from the side of the 

 great tube. The plumb-line descended through this vertical tube, 

 and passed through a hole in the bottom plate of the telescope, and 

 there it carried the plumb-bob in a cup of water supported by 

 hooks upon the lower plate. Just above the top of the small tube. 



