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 Continent, 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 longitudes 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 recollect, 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 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 success in this mode of observation.
