18 TRANSIT OF VENUS, 1874. HONOLULU. 



as the rate. The clock-error proper to apply to the transit of the Moon's 

 limb is explained on page 23. 



THE ALTAZIMUTH INSTRUMENT AND OBSERVATIONS OF ZENITH DISTANCE. 



45. Three instruments for zenith distance, rotating round vertical axes 

 (frequently called Altazimuths, though possessing no accurate graduation in 

 azimuth,) precisely alike in every particular, were constructed for the expe- 

 ditions to Honolulu, Mokattam, and Rodriguez, by Messrs. Troughton and 

 Simms. Bach of these instruments had two vertical circles attached to the 

 horizontal axis which carried the telescope. They were 14 inches in diameter, 

 and were divided on silver to 5' spaces. One circle was read by four 

 micrometer-microscopes, supported by radial arms cast in one with the 

 rotating body of the instrument. One revolution of each micrometer-screw 

 was intended to be 60". The movable micrometer frames carried two parallel 

 wires of spider's web instead of the usual cross. The probable error of 

 bisecting a division of the circle with one microscope was something under 

 1". The other circle was read by the pointer on the other side of the 

 instrument, where also was the illuminating lamp. Two zenith-distance 

 levels, each divided approximately to 2", were attached to the arms which 

 carried the microscopes. The value of the divisions engraved on the levels 

 were re-determined by the makers just before the Expeditions started. 



46. The horizontal axis, telescope, and circles, were built together like a 

 transit-circle. The object-glass had a clear aperture of 2*00 inches and focal 

 length of 20 inches. The reticule consisted of five horizontal and six vertical 

 webs, but the latter were only employed to define the middle points of the 

 horizontal webs. The eye-piece used for all observations was a four-glass 

 diagonal, power 33. The pivots were three-quarters of an inch in diameter, 

 and were both pierced. The circles were interchangeable, and could be 

 turned on their axes; but their positions were not altered during these 

 observations. 



Circles of plate glass, parallel to and concentric with the graduated circles, 

 were placed outside the framework to protect the levels from the heat of the 

 observer's body and reading-lamp. The illuminating-lamp was on the side 

 opposite to the microscopes. The horizontal circle (which was only used for 

 setting in azimuth) was 12 inches in diameter, and was read by two verniers. 

 There were clamps in zenith-distance and azimuth, the former with a fine 

 slow-motion screw, the latter with a rather quick screw, which was turned 

 during a vertical transit to keep the object at the middle part of the 



