1876.] On Electro-Magnetic Rotation. 403 



Tig. 2. Piece of the epiblast ; the cells and nuclei stained with carmine. Magnified 



about 370 diameters. 

 Fig. 3. Piece of the hypoblast ; similarly prepared. Magnified 370 diameters. 

 Fig. 4. Section across the germinal area of the developing ovum, stained with logwood. 

 Magnified 200 diameters. 

 z.f. zona pellucida. 

 e.p. epiblast. 

 h.jp. hypoblast. 

 m.l. Membrana limitans hypoblastica. 



VI. " Preliminary Report to the Hydrographer of the Admiralty 

 on some of the Results of the Cruise of H. M.S. ' Challenger 3 

 between Hawaii and Valparaiso.^ By Prof. Wyville Thom- 

 son, F.R.S., Director of the Civilian Staff on board. Received 

 February 26, 1876. 



[This Eeport will appear in a subsequent Number of Proceedings.] 



March 30, 1876. 

 Dr. J. DALTON HOOKER, C.B., President, in the Chair. 



The Presents received were laid on the table, and thanks ordered for 

 them. 



The following Papers were read : — 



I. " An Experiment on Electro-Magnetic Rotation." By W. 

 Spottiswoode, M.A., Treas. and V.P.R.S. Received February 

 24, 1876. 



The phenomena of the rotation of movable conductors, carrying cur- 

 rents, about lines of magnetic force are well known. One form of ex- 

 periment, commonly called the rotating spark, presents, beside the actual 

 rotation, some peculiar features which do not appear to have been noticed 

 in detail. The instrumental arrangements consist of a partially exhausted 

 chamber with a platinum point for one terminal, a ring for the other, and 

 the intervening air or other gas for the movable conductor. The 

 chamber is made in the form of a double cylinder, so that a magnet in- 

 serted through the ring may reach nearly to the point. The discharge 

 then passes between the point and the ring, and revolves about the 

 magnet according to Ampere's law. 



But beside the rotation, and even when, through weakening of the 

 magnet, rotation does not actually take place, the spark, when carefully 

 observed, is seeu to assume a spiral form ; and the spiral is right-handed 

 or left-handed according to both the direction of the current and the mag- 



