386 On a Transit-Iiistrument and a Zenith Sector. [Feb. 28, 



a zenith sector by Messrs.. Trougbton and Simms) to the inspection of the 

 Koyal Society. Both instruments present pecuUarities. 



The D-feet Transit-Instrument. — This has a very powerful telescope of 

 5 inches clear aperture (a large diameter in proportion to its focal length). 

 The axis is of aluminium bronze, cast in one piece, hollow, and turned both 

 inside and out. The two halves of the telescope are easily separable from 

 the axis for portability. 



It is provided with four levels for rendering the axis horizontal ; these 

 are mounted on a plan suggested and devised by Mr. Cooke. He remarks 

 that the ordinary striding level usually applied to such instruments watches 

 the pivots only, whereas the observer wishes to be informed whether or not 

 the telescope itself describes a true plane. This it will not do if the flexure 

 of the axis differs, as it may do, in different altitudes. Mr. Cooke there- 

 fore attaches the levels to the telescope. His mode of doing this, and of 

 providing for their due adjustment, will, in the absence of drawings, be best 

 understood by inspecting the instrument. 



The means of adjusting the axis vertically and azimuthally are also pe- 

 culiar. The bearings on which the pivots turn are carried by strong three- 

 armed pieces, similar in form to the tribrach of an ordinary theodolite. On 

 one side the tribrach is raised or lowered by means of the three vertical 

 screws which form its feet, and the axis is thus made horizontal ; on the 

 other side the tribrach is pushed laterally by two horizontal screws, and 

 the telescope is thus brought into the meridian. Three principal objects 

 are sought in these arrangements — to exclude shake, to obviate strain, 

 and to cause the expansions to take place from the centre outwards. 

 I have been well satisfied with the trials I have made of them. I find 

 these adjustments to be exceedingly delicate in their action, and rery 

 stable. 



The Zenith Sector. — This is quite unlike any instrument of the same de- 

 nomination. My endeavour in designing it was to combine maximum 

 power with minimum weight. 



A solid steel vertical axis revolves within a hollow wide -based conical 

 cast-iron pillar. Across the vertical axis is placed a frame, in which are 

 formed bearings for the reception of a transverse horizontal axis. This 

 ■ axis carries outside the frame a telescope of 4 feet focal length and 4 inches 

 clear aperture, and a portion of a circle comprising two opposite sectors, 

 each containing about 45°. The telescope being vertical for the observa- 

 tion of stars near the zenith, the sectors are horizontal — that is, transverse 

 to the telescope. The frame which supports the horizontal axis carries 

 also four micrometer-microscopes for reading the sectors. These micro- 

 scopes are arranged conically, so that all four are illuminated by a single 

 light, in the manner adopted by the Astronomer Royal for the Great Green- 

 wich Transit-Circle. The telescope and sectors revolve together, the micro- 

 scopes being fixed. When packed for carriage, the telescope and sectors 

 can be made to lie in the same direction, and so take up much less room 



