374 The Transit of Venus in 1874. [July, 



corresponding epochs at egress. It must be remarked further that 

 Figs. 1 and 2 (Plate) will be found not to correspond to the hours 

 of Greenwich mean solar time, which are affixed to them, but to 

 the corresponding hours of apparent solar time.* It is clear that 

 the conditions of a transit have nothing to do with our horological 

 arrangements, but only with the actual aspect of the earth as sup- 

 posed to be seen from the sun. 



Fig. 1 (Plate), then, represents the earth as she would appear 

 from the sun when in the positions marked 2, 3, or 4, m Fig. 2 

 (Woodcut), or in any intermediate position. In a similar way 

 Fig. 2 (Plate) represents the earth as she would appear when in 

 the positions 5, 6, or 7, or in any intermediate position. 



The hues which lie side by side across the earth's face in 

 Fig. 1 (Plate) represent the actual position of the edge of the large 

 circle (a b of Fig. 2), at intervals of one minute, counted with 

 reference to the moment of central passage. We notice that while 

 the large circle takes but about 12 m. in sweeping across the north- 

 eastern hemisphere, it occupies more than 13 m. in sweeping across 

 the south-western. This obviously agrees with the facts exhibited 

 in Fig. 2 ; since it is clear that the nearer the earth approaches 

 towards the middle of the hne marked 1874, the slower is her rate 

 of approach towards 0. 



Considerations of precisely the same sort apply to Fig. 2 

 (Plate), which will be at once understood when examined with 

 reference to the three positions of the earth marked 5, 6, and 7, in 

 Fig. 2 (Woodcut). 



It must be here remarked that as respects all this part of the 

 inquiry there is no doubt or difficulty whatever, the calculations 

 involved being of the most elementary character. But we may 

 stay a moment to inquire how far the results exhibited in Figs. 1 

 and 2 (Plate) agree with those which have been obtained by the 

 Astronomer Pioyal ; because, although there is no difficulty in the 

 work thus far, a different method has been pursued by him in 

 obtaining corresponding information. In obtaining those points 

 of the earth marked " accelerated ingress," " retarded ingress," 

 "accelerated egress," and "retarded egress" in Figs. 1 and 2 

 (Plate), the Astronomer Koyal has employed the terrestrial globe 

 adjusted with reference to the epochs of the passage of Venus' 

 centre across the centre of the sun, the phenomena being supposed 

 to be seen from the earth's centre. This would correspond to the 

 passage of the centre of the small circles, marked 1, 2, 3, &c., in Fig. 2 

 (Woodcut), across a large circle midway between the circles a b 

 and a b. He has assumed the position-angles at ingress and egress 

 to be equal to NOl and N08. Neither the method nor the 

 assumptions are strictly exact (as Mr. Airy has himself pointed out), 



* The equation of time on December 8th is nearly Sm. additive to mean time. 



