TRANSIT INSTRUMENT. 9 



18. In this part of the town the coral is covered by some four feet of 

 very light sandy soil. Although high-water mark was half a mile distant, 

 the tide percolated through the coral and rose many inches above its 

 surface, but in no way interfered with the stability of the piers, which were 

 set with cement. 



THE TRANSIT CLOCK. 



19. This was constructed by Messrs. B. Dent and Co., of London, in 1870, 

 and was numbered 1916. The compensation of the pendulum was effected 

 by their well-known cylindrical combination of zinc and steel. The clock, in 

 common with all the clocks employed on the Transit of Venus Expeditions, was 

 tested for a long time at the Royal Observatory, Greenwich, under very trying 

 variations of temperature, and its performance was excellent. At Honolulu 

 it was mounted on a massive tripod of mahogany and iron, which rested upon 

 three stakes driven 2 or 3 feet into the ground. It was wound every Sunday 

 at noon. The suspension spring of the pendulum was changed, 1874, 

 November 7, after which the rate was remarkably steady (Tab. V.). 



THE TRANSIT INSTRUMENT. 



20. Five transit instruments, precisely similar in all respects, were con- 

 structed for the Expeditions by Messrs. Troughton and Simms. One of these 

 was used at Honolulu. The object glass was very nearly 3 inches clear aperture 

 and 36^ inches focal length. The axis, consisting of central cube and cones, 

 was cast in one piece. Its length from shoulder to shoulder (that is, 

 exclusive of the pivots,) was 18 inches ; the pivots were 1| inches long and 

 1^ inches diameter. The cube was of 6 inches side ; the cones were 

 5^ inches in diameter at the cube and 2| inches at the shoulder. The two 

 tubes which, with the cube, formed the telescope, were 4^ inches in diameter 

 at their attachment (by flanges) with the cube. When the dew cap was on, 

 the instrument weighed 44 Ibs., and was perfectly balanced on its pivots. 



21. The system of webs, consisting of 5 vertical webs, at intervals of about 

 3^', and two horizontal webs, about 5' apart, was mounted on the plate driven 

 by the micrometer screw, so that the whole system moved when the screw 

 was turned. The head of the micrometer screw was divided on silver to 

 100 parts, and was provided with a movable brass cover to prevent it being 

 accidentally turned when not actually in use, as when observing time-stars. 

 The thread of the screw was kept bearing in one direction by a spiral spring 



B 



