1869.] The Transit of Venus in 1874. 371 



and in observing-skill, we cannot look upon the transit of 1874 as 

 otherwise than highly valuable. 



If we briefly consider the general nature of a transit, we shall 

 be the better able to define the circumstances on which the value 

 of any particular transit depends. 



Fig. 1. 



In Fig."l, let s represent the sun, v Yenus, and e the earth, 

 the j)lane of the paper representing the plane of the ecliptic, so 

 that the path of Venus (supposed to be near one of her nodes) must 

 be conceived as inclined rather more than 3° to the plane of the 

 pajjer. The arrows show the direction in which the planets move. 



Conceive the sun and Venus enveloped by two double cones 

 s q' Q s' and s e' e s', one having its vertex inside, the other 

 having its vertex outside, the orbit of Venus. These cones have a 

 common axis, namely, the hne joining the centres of Venus and the 

 sun. Now it is clear that as Venus with her more rapid motion 

 sweeps round the sun, the accompanying cones must overtake the 

 earth (situated as shown in the figure), and that as they sweep 

 onward, the earth will pass through them. As this takes place, 

 there will occur the following eight phenomena in the given order : 

 The forward part of the outer cone will reach (i) the nearer, then 

 (ii) the farther side of the earth's globe; the corresponding part of 

 the inner cone will reach (iii) the nearer, then (iv) the farther side 

 of the earth's globe ; next, the backward part of the inner cone will 

 reach (v) the nearer, and (vi) the farther side of the earth ; and 

 lastly, the corresponding part of the outer cone will reach (vii) the 

 nearer, and (viii) the farther part of the earth. And these several 

 events will readily be seen to correspond to the occurrence of : — 



(i) Most accelerated external contact at ingress. 



(ii) „ retarded „ „ 



(iii) Most accelerated internal contact „ 



(iv) „ retarded „ „ 



(v) Most accelerated „ at egress. 



(vi) „ retarded „ „ 



(vii) Most accelerated external contact „ 



(viii) „ retarded „ „ 



