39 



the lines on which the present energies of the Observatory 

 are directed in normal times. 



Sir Frank Dyson's account of the Astronomical Observa- 

 tions of the year with which his Report is concerned, begins 

 inevitably with the work of the Transit Circle, the funda- 

 mental instrument of the establishment, that by means of 

 which " the rectifying the places of the fixed stars," mentioned 

 in the original warrant to John Flamsteed, is still carried out. 

 This Transit Circle, which was first brought into use in 1851, 

 was a great advance upon its predecessors, three in number, 

 the tubes of which are still preserved on the walls of the 

 transit room. 



The first of these was due to Halley, and was in use 

 down to 1743 ; but it was followed, six years later, by Bradley 's 

 transit, a much more efficient instrument, constructed by 

 JOHN BIRD (1709-1776), pupil of George Graham, and, after 

 his master's death, the most skilful astronomical instrument- 

 maker of his day, in 1749 and 1750, and was 8 feet in focal 

 length. Its original object-glass was i inches in diameter, 

 but after JOHN DOLLOND (1706-1761) had begun to make 

 achromatic object-glasses, one of these, 2 -7 inches in aperture, 

 was inserted and a power of 80 was used with it. Bradley 

 used a fixed eyepiece and generally only one wire, noting the 

 time to the nearest second. Maskelyne introduced a sliding 

 eyepiece, observed over five wires, and estimated the time 

 to a tenth of a second. 



The third transit instrument is known as " Troughton's," 

 inasmuch as it was made by the great instrument-maker, 

 EDWARD TROUGHTON (1753-1835). It is only right that it 

 should bear his name, for the great instrument-makers and 

 opticians, as likewise the great clockmakers and chronometer- 

 makers have played an indispensable part in the progress 

 of astronomy equally with the observers and the mathema- 

 ticians. This was Pond's chief instrument, and was fitted 

 with a 5-inch object-glass by Dollond of 5 feet focal length. It 

 carried seven fixed vertical wires, and a micrometer frame 

 carrying two parallel movable wires, and a magnifying power 

 of 170 was usually employed. Another fine instrument, also 

 by Troughton, was used in connection with this transit. This 

 was the mural circle, ordered by Maskelyne, but not delivered 

 until after his death. 



But the mural circle was open to the objection that, being 

 attached on one side only to its pier, the errors to which it 

 was exposed tended all to be of the same direction. But, 

 hi 1824, Pond was able to add a second mural circle to his 



