Royal Astrofiomical Society. 65 



over the object-glass ; and let a basin of quicksilver be placed below 

 the object-glass, but in no mechanical connexion with It, at a di- 

 stance rather greater than half the focal length of the object-glass, 

 so that an image ot the star is formed on the wires after the rays are 

 reflected from the mercury. Such an instrument would at least be 

 free from all uncertainties of twist of plumb-line, viscosity of water, 

 attachment of upper plumb-line microscope, attachment of lower 

 plumb-line microscope, and the observations connected with them ; 

 and might be expected, as a result of this extreme simplicity, to give 

 accurate results." 



The Astronomer Royal was recommended by the Board of Visitors 

 to take the necessary steps for procuring a zenith instrument on the 

 principle described, and he has already printed and distributed an 

 account of the construction which he proposes to adopt, with expla- 

 natory drawings. There seems scarcely any limit to the power and 

 probable accuracy of such a zenith tube ; and as the mounting is 

 exceedingly cheap and simple, it will most likely come into general 

 use, especially for nice determinations of latitude. 



It will be remembered that an altitude and azimuth instrument, 

 made after Mr. Airy's designs by Messrs. Ransome and May, and 

 Messrs. Troughton and Simms, has recently been added to the Royal 

 Observatory, for the express purpose of observing the moon. Mr. 

 Airy says : — 



" The altitude and azimuth instrument having now been in use 

 for an entire year, I am able to give some account of its success or, 

 failure as a mechanical arrangement. The first subject for remark 

 is the steadiness of support of the upper pivot, which is held in its 

 place, as the Visitors will remember, by a frame of bars whose arrange- 

 ment in every part is triangular. The steadiness is perfect. In the 

 first observations, the levels were read before and after the telescopic 

 observation, but it was very soon found that this caution was entirely 

 unnecessary. The next point is the steadiness in the position of the 

 horizontal axis of the vertical circle relatively to the vertical revolving-: 

 fran:e, and generally the steadiness of the constants of instrumental 

 errors. For some time the constants were so unsteady as to give 

 me great trouble. Several observations of stars were absolutely re-, 

 jected. In the month of July, after careful consideration of the dis- 

 cordances, I came to the conclusion that there was a wandering of 

 the horizontal pivots in their Ys, caused probably by the counter- 

 poises : the counterpoises were therefore diminished by one-third 

 part, and since that time the constants of instrumental error have 

 been steady, and not a single observation has been rejected. The 

 next circumstance which gave me trouble was the uncertainty in the 

 scale-values of 3ome of the long levels. The singular good fortune 

 of having four parallel levels upon the instrument, which are always 

 read, enabled me to compare the proportion of the scale-values in 

 actual use to the scale-values determined before mounting. These 

 were very discordant. I became at length persuaded that this was 

 caused by the very defective construction usually adopted in the 

 mounting of English levels ; and in the autumn I applied to the two 

 Phil, Mag. S. 3. Vol. S4. No, 226. Jan. 1849. F 



