58 Prof. J. Tyndall on the [Jan. 15, 



all the torsional effects are modified by the mechanical and magnetic 

 properties of that substance. 



Each current leaves a residuary magnetic effect in the bar, amounting 

 in iron to about one tenth of its original influence. The residuary mag- 

 netism of coil-currents is affected and sometimes reversed by axial ones ; 

 and that of axial currents is also removed by coil ones, and by a red heat. 

 The condition left by an axial current is smaller in degree and less stable, 

 in a vertical iron wire or one in the terrestrial magnetic meridian, than 

 that left by a coil one, partly because of the influence of terrestrial mag- 

 netism ; but in a position at right angles to that the effect is different. 



The torsion produced by a coil-current may be used as a test, and 

 partly as a measure, of the residuary effect of an axial one ; and that 

 produced by an axial current may be employed to detect, and to some 

 extent measure, ordinary magnetism in the bar. As an opposite coil- 

 current at once reverses the ordinary longitudinal magnetism of a bar of 

 iron, so also an opposite axial one at once reverses its transverse mag- 

 netism. 



Many instances have been met with in which the transverse and longi- 

 tudinal magnetic states produced by the two currents coexisted in the 

 same substance. The torsional influence of the excited helix is distri- 

 buted equally throughout its length ; so also is that of the current in the 

 bar. All the torsions are closely related to the well-known electric 

 sounds, and to particular positions and internal movements of the particles 

 of the iron. 



Signs of electrotorsion were obtained with a bar of nickel, but not 

 with wires of platinum, silver, copper, lead, tin, cadmium, zinc, mag- 

 nesium, aluminium, brass, or German-silver, nor with a thick rod of 

 zinc, or a cord of gutta percha. 



January 15, 1874. 

 JOSEPH DALTON HOOKER, C.B., President, in the Chair. 



The following Paper was read : 



I. " Preliminary Account of an Investigation on the Transmission 

 of Sound by the Atmosphere." By JOHN TYNDALL, D.C.L., 

 LL.D., F.R.S. Received January 1st, 1874. 



This notice embraces the scientific results of an inquiry on Fog- 

 signals, undertaken at the instance of the Elder Brethren of the Trinity 

 House, and communicated with their friendly concurrence to the Royal 

 Society. 



The investigation was begun on the 19th of May, 1873, and continued 

 till the 4th of July. It was resumed on the 8th of October, and con- 



