1879.] 



On the Transit of Venus. 



297 



spective of the position of the exciting helices, whether both were on 

 the magnet, both on the cores, or one on the magnet and one on a core. 



From the foregoing experiments it appears clear that the more near 

 the approach to a closed magnetic circnit, the stronger is the field of 

 force, and the longer is retained the magnetism of the mass or masses 

 of iron constituting the circuit. The same rule holds good with 

 regard to permanent magnets. In closed circuits the attractive force 

 is at its height, and diminishes in intensity as the magnetic field is 

 more extended. But the parallel goes beyond this, for the more open 

 the magnetic field, the more rapidly is the magnetic force itself dis- 

 sipated. 



These principles have guided us in the construction of a dynamo- 

 electric machine of whose magnetic circuits we here present a sketch, 

 and which we hope to describe more fully in a future paper. 



In the accompanying diagram six fixed electro-magnets are shown, 

 having alternate poles, opposite to which, and at a very short distance, 

 are placed three other electro-magnets so arranged with opposing poles 

 as to form three nearly closed circuits. Coils of wire are made to 

 revolve so as to cross the intervals between these opposing poles, and 

 the electric currents induced in the moving coils are made to pass 

 round the electro-magnets. 



IY. " Further particulars of tbe Transit of Venus across the 

 Sun ; December 9, 1874; observed on the Himalaya Moun- 

 tains, Mussoorie, at Mary- Villa Station, Lat. 30° 28' N., 

 Long. 78° 3' E., height above sea 6,765 feet, with the 

 Royal Society's 5-inch Equatoreal." Note III. By J. B. 

 N. Hennessey, F.R.S. Received October 4, 1879. 



1. The object of the present note is to add to Notes I and II* some 

 particulars of the transit not detailed in those notes. The latter con- 

 tained only sufficient extracts from my observatory notes in connexion 

 chiefly with the three contacts which I observed ; as, however, various 

 other facts, besides the contacts, were developed in course of the 

 transit, and elicited remarks from me at the time, it seems desirable 

 that a complete transcript of these observatory notes should also be 

 put on record; both in connexion with what hereafter follows, and 

 also to meet any possible future requirements of details, such as 

 expressed by Captain Tupman in his discussion of the mean solar 

 parallax.f 



* See " Proc. Roy. Soc.." Yol. xxiii, pp. 254, 379. 



f Royal Astronomical Society, "Monthly Notices." Yol. xxxviii, p. 452. 

 VOL. XXIX. Y 



