416 TRANSIT OF VENUS, 1874. KERGUELEN ISLAND. 



portion of each, wire, the zenith distance slow-motion-screw being turned as 

 requisite, and the times being recorded by the Altazimuth-clock. The 

 microscopes of the horizontal circle were then read, and the striding level 

 applied. The Altazimuth-clock was made by E. Dent, of London ; the 

 pendulum-rod was of wood. The clock was compared with the Transit-clock 

 by the intervention of a solar half-seconds chronometer at instants when 

 the beats coincided. 



For imperfect transits over the five vertical wires a correction has been 

 applied to the circle-reading, equal to the distance of the mean of the wires 

 observed from the mean of the five, multiplied by the cosecant of the 

 apparent zenith distance ; and the same factor corrects the collimation. 

 The correction to the circle-reading for level-error is the level-error 

 multiplied by the cotangent of the zenith distance. 



The Greenwich mean solar time, corresponding to the local sidereal 

 time of each, observation (obtained by applying the Altazimuth-clock 

 correction to the clock-time of transit) has been computed on the two 

 assumptions of east longitude 4 h . 39 m . O s> and 4 h . 49 m . O'O. For each of 

 these Greenwich mean times the Moon's Geocentric Eight Ascension, North 

 Polar Distance, Equatorial Horizontal Parallax, and Semidiameter, have been 

 interpolated with second differences from the hourly ephemeris in the 

 Nautical Altnatiac. 



The tabular azimuth of the Moon's limb has then been computed in the 

 following manner : 



The logarithm of the distance from the center of the earth to the point 

 where the normal of Observatory Bay intersects the axis of the earth, called 

 the axial distance of the normal-center, has been taken as 7'7046. 



The correction to be applied to the Geocentric S.P.D. to obtain the 

 Normal- centric S.P.D. has been computed from the formula 



Correction (always negative) = seconds of equatorial horizontal 



parallax 



X sine Geocentric S.P.D. of center 

 X axial distance of normal center. 



The tabular azimuth of the limb, and the tabular zenith distance, are thus 

 computed : 



If a great circle passing through the center of the Moon intersect the 

 meridian at right angles, and the arc of the meridian between the points of 



