ALTAZIMUTH INSTRUMENT. 



19 



horizontal wires. For adjusting the verticality of the principal axis, two 

 sensitive levels were attached to the lower portion of the revolving body. A 

 sensitive striding level could be applied to the telescope axis, and the instrument 

 could be used as a portable transit by clamping it on both sides in the meridian. 

 The solid tripod base of the instrument stood upon three brasses let into a 

 thick slab of slate, which formed the head of the pier. Each instrument was 

 protected by a wooden hexagonal hut about eight feet in diameter, with a 

 revolving roof, slit, and shutter, and was accompanied by a secondary 

 sidereal clock with wooden pendulum rod, which was intended to be com- 

 pared with the transit-clock, before and after every observation, by the 

 intervention of a mean time half -seconds chronometer. A mercurial barometer, 

 an aneroid, and thermometers, were also supplied. At Honolulu the instru- 

 ment was supported by a solid pier of brick and concrete founded on the coral. 



47. The observer first opened the hut as much as possible to let the instru- 

 ment take up the temperature of the external air. He then compared a mean- 

 time half -seconds chronometer with the Transit-clock and Altazimuth-clock by 

 coincidence of beats, and recorded the barometer and external thermometer. 

 When the observations were concluded, the clocks were again compared, this 

 time commencing with the Altazimuth-clock ; and the barometer and thermo- 

 meter again recorded. 



48. When observing a vertical transit, the zenith-distance levels were read 

 before and after the transit, and the microscopes were read last. For a 

 latitude observation, in which the instrument is not, of course, moved in 

 azimuth, the levels were read immediately after the bisection. 



49. For determining the zenith-point and intervals of the horizontal wires 

 independently of the stars, collimating arrangements, similar to that described 

 with the transit instrument, were set up on the south and east sides, and the 

 same reversed telescope was used. This collimator was perfectly steady in 

 zenith distance, and by its means the intervals of the horizontal wires from 

 their mean were accurately obtained as follows : 



