SUNS DISTANCE. 201 



if this piano is parallel to the Sun's limb at the point of ingress, the 

 disturbance of the apparent place of Venus will merely cause its place 

 to slide along the Sun's limb, and will not affect the time of ingress. 

 If the plane is perpendicular to the Sun's limb at the point of ingress 

 the disturbance will tend to throw Venus upon or off the Sun's disk 

 in the greatest possible degree, and therefore to accelerate or retard 

 the ingress in the greatest possible degree. But the observed time 

 of ingress must necessarily be expressed, in the first instance, in local 

 time; this can be converted into Greenwich time only by application 

 of the assumed longitude of the place, and, therefore, when avo com- 

 pare the Greenwich times of ingress as observed at two stations, the 

 result is necessarily affected by the possible errors of two longitudes. 

 The same remarks apply to the egress. 



We are now in a state to consider the applicability of the two 

 methods to the transits of Venus in 1S74 and 18S2. The calculations 

 of the places of the Earth and Venus, upon which the diagrams of 

 figure 4 and figure 5 are founded, have been made by Mr. Breen, 

 assistant to the Royal Observatory, and may be accepted as accurate. 

 At the commencement of this evening's meeting an independent set 

 of calculations was handed to the Astronomer Royal by Mr. Hind, 

 superintendent of the Nautical Almanac, which do not sensibly differ 

 from Mr. Breen' s. In the exhibitions of the illuminated side of the 

 Earth, the nearest integral hour of Greenwich mean time is taken, 

 because (as will be mentioned) there is yet a little uncertainty on the 

 exact time. 



First. On the application of the method of difference of duration 

 of transit to the transit of 1874. 



It has already been remarked that in this transit there is no possi- 

 bility of combining the effect of Earth's rotation with the effect of 

 difference of latitude of stations, so as to exaggerate the difference of 

 durations of transit depending on difference of latitude alone. And 

 if we consider the effect of difference of latitude only, we find that 

 circumstances are not very favorable. The most northerly stations 

 are to be found in Siberia, Tartary, and Thibet, (which will scarcely 

 be visited by astronomers in December,) on the coasts of China, and 

 in North British India. The most southerly stations will be Kergu- 

 elen's island, Van Dieman's Land, and New Zealand. But the 

 observable difference of duration will probably not be half of that 

 in 1882. 



Second. On the application of the method of the difference of abso- 

 lute times to the transit of 1874. 



For the ingress, favorable positions will be found at Owhvhee 

 (where the displacement tends to throw Venus upon the Sun's disk, 

 or to accelerate the ingress) and at Bourbon, Mauritius, and Kergue- 

 len's island, (where the displacement tends to throw Venus from the 

 Sun's limb, or to retard the ingress.) For the egress, Sicily, Italy,. 

 and portions of Europe Avest of the Black Sea, are so situate as to 

 throw Venus upon the Sun's disk, or to retard the egress; and New 

 Zealand, New Caledonia, Van Dieman's Land, and Eastern Australia, 

 are well situated for accelerating the egress. But it is doubtful. 



