( 370 ) [July, 



IV. THE TEANSIT OF VENUS IN 1874. 



By KiCH. A. Pkoctor, B.A., F.E.A.S. 



On account of the important bearing of the transits of Venus upon 

 the problem of the sun's distance, men of science are looking 

 anxiously forward to the two transits which occur in the present 

 century. Although the later of the two will not take place for 

 thirteen years, its circumstances have already been examined. 

 Indeed both transits were subjected to careful examination by the 

 Astronomer Eoyal so far back as 1857; and since then he has con- 

 tinued to put forward from time to time the considerations which 

 have suggested themselves to him as his examination of the subject 

 proceeded. Early in the inquiry he expressed the opinion that 

 the method founded on the observed dilferences of the transit's 

 duration, as seen from opposite points of the earth's surface — which 

 method had been the sole one employed in the treatment of the 

 transit of 1769 — is whoUy inapplicable to the transit of 1874 ; and 

 he suggested another method of utilizing that transit, — a method 

 less perfect in itself, more difficult (astronomically) to carry out, and 

 involving processes of preparation essentially difterent from those 

 which would be required under the other method. To the prepa- 

 rations thus called for, astronomers and geographers have hitherto, 

 I believe, solely confined themselves. 



Having had occasion to examine the reasoning of the Astronomer 

 Eoyal, and to test the conclusions he had arrived at, I have been 

 led to form a somewhat different opinion of the value of the transit 

 of 1874, so far as the simpler method of observation is concerned. 

 I have found that, if consideration be made of internal contacts — 

 the only phenomena on which estimates of the sun's distance have 

 ever been founded — the actual diflerence of duration which can be 

 made available in 1874, is about 35m. or 36 m., as against an out- 

 side value of 28 m. in 1882, and an actual observed maximum of 

 difference of 23im. in 1769, 



I am sensible that mere magnitude of observed difference is not 

 the sole point on which the value of a transit depends. The rate at 

 which the planet crosses the sun's limb is an almost equally impor- 

 tant subject of consideration. I shall be able to show that when 

 this point is dealt with in the manner most unfavourable to my 

 case, the value of the transit of 1874 yet remains superior to that 

 of the famous transit of 17()9 (as actually utilized), and scarcely 

 inferior to the most favourable estimate which can be formed of the 

 transit of 1882. Therefore, remembering the importance which 

 lias been always attached to the observations made in 17(59, and the 

 immense advances since made in the construction of instruments 



