376 Tlie Transit of Venus in 1874. [July, 



away on the darkened hemisphere at the moment of "retarded 

 egress," and vice versa. The point marked retarded ingress in Fig. 1 

 (Plate) has moved far upwards towards the centre of the illuminated 

 hemisphere at the epoch represented in Fig. 2 (Plate), that is, far 

 away from the point marked " accelerated egress." 



It was the consideration of these circumstances which led the 

 Astronomer Eoyal to pronounce the transit of 1874 altogether 

 unfit for the purpose of observing the durations of transit, as seen 

 from opposite parts of the earth's surface. And he suggested that 

 four sets of observers should be sent to watch each of the four 

 phenomena, — accelerated and retarded ingress and egress ; and that 

 by determining the exact longitudes of theii* stations, and so (with 

 the aid of exact chronometers) learning the exact Greenwich time 

 of each phenomenon, the transit might be rendered available 

 through the comparison of the results inter se. Clearly the 

 elements of difficulty and the probability of error are seriously 

 increased in this method as compared with one which 2)ractically 

 requires but the simple estimate of duration, and scarcely admits of 

 being affected by chronometer errors. However, let us note that 

 by this method an observed difference of at the outside 24 m. 

 might be obtained ; — not more, because the sun cannot be observed 

 when too close to the horizon, and because also of the difficulty of 

 findmg suitable stations. 



Now let us see what can be done towards the utilization of the 

 transit of 1874 by the simpler method : — 



Suppose the northernmost station taken in latitude CO^, that is, 

 along the uppermost parallel in the figui'e. As the whole duration 

 of transit is but about four hours, and day lasts about six hours in 

 this latitude on December 8, we may take a place two hours on the 

 left of the central meridian in Fig. 1 (Pkte), knowing that the same 

 place will be (at the end of transit) two hours to the right of the 

 central meridian in Fig. 2 (Plato) ; and at one epoch the sun will 

 be one hour risen, on the other one hour from setting. Doing this 

 we find that the station (which lies in Siberia, not far from Lake 

 Baikal) falls in Fig. 1 (Plate) on the sixth cross-line from the 

 centre, and in Fig. 2 (Plate) above the tenth cross-hne. In other 

 words, the transit as seen from this spot exceeds the mean by 

 (6 -f 10J-), or 161 minutes. 



Next for the southern station. Here we have a wide choice. 

 If we put our observer on Petra Island (a place probably very little 

 suited for astronomical observations) we get (from Fig. 1, Plate) 

 ingress retarded by 8 m., and (from Fig. 2, Plate) egress accele- 

 rated by 12 m., or in aU the duration of transit falls short of the 

 mean by 20 m. If we take the place marked out by the Astronomer 

 Koyal for observing the transit of 1882, a place near Repulse Bay, 

 in east longitude 105^, we get ingress retarded by 9 m. and egress 



