10 TRANSIT OF VENUS, 1874. HONOLULU. 



within the box, as with all Messrs. Troughton and Simms' micrometers. 

 There was no perceptible " loss of time " in any one of them. The eye-piece 

 was moved rapidly from wire to wire by a quick screw, the milled head of 

 which was opposite to the micrometer-screw-head. The illumination of the 

 wires was effected in the usual way by lamp-light passing through one of 

 the pivots on to a gilt reflector, regulated by a rod leading down to the 

 eye-piece. There was a polar distance setting circle 4^ inches diameter 

 on each side of the tube near the eye end. 



22. A solid pier of brick and cement, 6^ feet by 3^ feet at the top and 

 somewhat larger at the bottom, was built up from the coral (previously 

 levelled) to about the surface of the ground. On this was laid, levelled, 

 oriented, and cemented a great stone, 6 feet by 3 feet and 6 inches thick, 

 weighing 1,500 Ibs. After the cement had well set, the stone piers were 

 placed in position. These piers were 4 feet 11 inches high, 24 inches by 21 at 

 the base, and 11 inches square at the top, and weighed nearly 1,400 Ibs. each.* 

 The Y's were of massive construction, one having an adjustment for azimuth 

 the other for level. They were attached to the tops of the piers "by heavy 

 sockets cemented into the stone. The supporting faces of the Y's were 

 inclined 90, and were rounded so as to touch a very small surface of the 

 pivots. When the instrument was thus mounted, a heavy blow with the fist 

 administered to the top of one of the piers merely produced a momentary 

 tremor of the optical image of a distant object. 



23. The instrument was provided with three eye-pieces of powers 45, 75, 

 and 120. The power of 75 was always used at Honolulu. A small prism of 

 total reflection could be attached to the eye-piece for the more convenient 

 observation of stars near the zenith. The shape of the piers left plenty of 

 room for the observer to sit between them. 



24. For determining the level-error, the instrument was fitted with a 

 Bohnenberger eye-piece and mercury trough, and with a hanging level of 

 delicate construction graduated with about 12 divisions to each inch, 46 of 

 which were equivalent to one minute of arc. The divisions were numbered 

 from to 100, the readings increasing towards the cross-level. The glass 

 bubble itself was protected by a covering of plate glass. The value of the 

 graduations was re- determined by the makers just before the expedition 

 started, several years after the instruments were made. 



* Three great stones similar to these, forming the mounting of the transit instrument, and the 

 wooden observatory, 13 feet square, were sent out from England with each expedition. 



