XXViil. 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 very 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 
earty 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 was obtained showing Venus 
in transit across the Line. The distances of the planet from the edge 
of the sun were afterwards measured micrometrically and at leisure 
from the picture. The English astronomers, however, found the 
photographs unsatisfactory; though, on the other hand, the Americans 
met with considerable suecess in this mode of observation. 
