THE COMING TRANSIT OF VENUS. 27 



strained (in mere fealty to the cause of science) to sub 

 mit them to the examination of the scientific world. 



To begin with : Halley s method, of which, in 1857, 

 and again in 1864, and yet again in 1868, the Astro 

 nomer Eoyal had said that it is totally inapplicable in 

 1874, was found to be applicable under circumstances 

 altogether more favourable than those which will exist 

 in 1882. 1 It was found not only to be applicable with 



1 The origin of this mistake on the Astronomer Eoyal s part is 

 thus explained in an article in the Spectator for March 1, 1873: 

 Everyone is asking whether it is possible that an astronomer so emi 

 nent and so skilful as Sir George Airy for the time is past when 

 names need be concealed can have made any serious mistake in a matter 

 of this importance. And again, everyone is anxious to know precisely 

 what mistake is imputed, and how it arose (granting that a mistake has 

 been made). 



To this last question the reply is easy. It chanced unfortunately 

 that in 1857 the Astronomer Royal delivered a lecture on the subject of 

 the now approaching Transits. In that lecture his great mistake had 

 its origin. Intent on presenting the more striking and popular features 

 of his subject, and in a way which would be clear and convincing to 

 everyone, he was led to adopt a method of reasoning which on the face 

 of it seems convincing enough (and which, indeed, is sound in itself) ; 

 but the conclusions derived from which may be, and in the actual case 

 are, dependent on certain details into which the Astronomer Eoyal 

 neither then entered nor has ever entered since. It is the palpably con 

 vincing nature of the evidence at a first view which led to all the 

 mischief. We will endeavour to give a brief but sufficient sketch of 

 the line of argument. 



Let it be premised that, for applying Halley s method or the 

 English method, as it is often called with advantage, what is wanted 

 is that at some station the transit shall last as long as possible, while 

 at another it shall last as short a time as possible. It matters nothing 

 whether the increase or reduction of the time be obtained by a seeming 

 change in the length of the line traversed by Venus, or by a change in 

 the rate at which she seems to move during transit. So much premised, 

 let it be noted that in 1874 Venus will cross the sun s face on a line 

 placed somewhat as a line from the figure X to the figure I on a 

 clock-face. As seen from northern stations, the line of transit will be 



