266 The PhilosojMcal Society of Glasgow . 



Is this new planetary matter, like the other till discovered by its in- 

 fluence, unseen ? Surely, on the contrary, it is it that we see as the 

 Zodiacal Light, long before conjectured to consist of a cloud of cor- 

 puscules circulating round the Sun, and, in the dynamical theory of the 

 Sun's radiation, supposed to contain the reserve of force from which this 

 Earth, as long as it continues a fit habitation for man as at present con- 

 stituted, is to have its fresh supplies of heat and hght. 



On Photographed Images of Electric Spa/rks. 

 By Peopessoe Wm. Thomson. 



Professor W. Thomson exhibited photographed images of electric 

 sparks reflected from a revolving mirror, which a few days since he had 

 received from Mr. Feddersen of Leipsic, and which aSbrded a remark- 

 able illustration of the " oscillatory discharge" indicated by dynamical 

 theory as occm-riug when a Leyden phial of not too great electrostatic 

 capacity is discharged, by a sufficiently easy conducting train, through a 

 channel presenting sufficient "induction on itself" or " electro -dynamic 

 capacity." The occm-reuce of an oscillatory discharge, under certain 

 conditions, had been first anticipated by Helmholz, in his Erhaltung 

 der Kraft (Berlin, 1847.) The law of discharge, when the discharg- 

 ing train possesses no sensible electrostatic capacity, had been fully 

 investigated, and the conditions under which it takes place with oscilla- 

 tions, discriminated from those under which it takes place with conti- 

 nuous subsidence, had been determined, in a mathematical paper com- 

 municated to this Society about seven years ago " On Transient Electric 

 Currents" — Proceedings of Glasgoio Philosophical Society, January, 1853, 

 and Philosojjhical Magazine, June of same year. 



At that time the numerical relation between electrostatic and elec- 

 trodynamic units had not been determined, and therefore a certain 

 co-efiicient in the mathematical formula was left for experimental 

 investigation. The want has been since supplied by W. Weber, 

 who has continued the system of absolute measurement inaugurated 

 by himself and Gauss for terrestrial magnetism, and has extended it 

 with the greatest advantage into every department of electric and 

 magnetic science. The consequence is that the mathematical formula 

 for an electric discharge can now be fully reduced to numbers, the 

 criterion as to whether it is oscillatory or continuous appHed, and, 

 when it is oscillatory, the time of an osciUation determined for any 

 stated dimensions and form of apparatus. Professor Thomson further 



