xxviii. New Zealand Institute. 



angular measure ; hence the problem can be solved. This, of course, 

 is a very general statement of the principle of the observation, and 

 takes no account of the many difficulties in giving effect to it, or of 

 the laborious and abstruse computations in the after reduction of the 

 observations. 



The methods of observation are known as Halley's and Delisle's. 

 In Halley's the duration of the transit is observed from two stations, 

 so selected that internal ingress and internal egress may be observed 

 at both ; also that, by the rotation of the earth, the interval between 

 ingress and egress may be increased at the one station and diminished 

 at the other. The duration of the transit gives the length of the 

 chords which Venus traces on the face of the sun, from which 

 the angle or parallax is deduced. Halley's method does not require 

 any great accuracy in the longitudes of stations, and in that respect 

 was very convenient at the time it was promulgated, the longitudes of 

 distant points not being then well known. But, as both ingress and 

 egress must be observed at both stations, there is always great risk of 

 failure from unfavourable weather. Previous to the transit of 1874 

 the utilization of this method was \erj carefully considered by Sir 

 George Airey and others. As it was found that full advantage of it 

 could only be obtained by establishing a station on the Antarctic Con- 

 tinent, the proposal was abandoned, and it was decided to rely on 

 Delisle's method. This has the advantage that two observations suffice 

 — one of internal ingress or egress at each of two stations. The 

 exact time-differences between the two observations is the essential 

 point in this method, consequently the absolute accuracy of the longi- 

 tudes of the observing stations, which is now obtained through the 

 electric telegraph, is of paramount importance. 



The transit of 1874, which I suppose all present here can recol- 

 lect, excited great interest. The observations on that occasion received 

 hearty and substantial assistance from Governments, and no less than a 

 quarter of a million sterling was expended in the aggregate by different 

 nations on this object. It was observed from numerous stations 

 throughout the globe, but, unfortunately, the observations in New 

 Zealand were obscured through bad weather. 



On this occasion, with the view of avoiding the errors incidental 

 to observations of contact, photography was employed in addition to 

 other means, and a series of pictures Avas obtained showing Venus 

 in transit across the Line. The distances of the planet from the edge 

 of the sun were afterwards measured micromctrically and at leisure 

 from the picture. The English astronomers, however, found the 

 photographs unsatisfactory; though, on the other hand, the Americans 

 met with considerable success in this mode of observation. 



