484 PHILOSOPHICAL TRANSACTIONS. [aNNO J 747-8. 



if you reverse the experiment, by electrifying the bottom plate, and suspending 

 the other over it. 



. If the upper plate be electrified when the leaf-silver is brought near, it will be 

 attracted upwards by it, and thus become electrical , and as long as it continues 

 to be electrical, it will be attracted downwards by the non-electric plate. When- 

 ever, therefore this last attraction added to the gravity of the silver, which acts 

 in the same direction, is equal to the contrary attraction upwards, the leaf-silver 

 will, by means of these 2 opposite forces, be kept suspended between the plates, 

 and will continue to be so as long as the equality of these forces is preserved. 



That the leaf-silver is always nearer to the non-electrical than to the electri- 

 fied plate, is owing to its receiving its supply of effluvia from the atmosphere 

 surrounding the electrified plate : for as the plate is more strongly electrified than 

 the silver, its atmosphere of effluvia will be denser to a greater distance than that 

 surrounding the leaf-silver, and therefore can supply an equal quantity at a greater 

 distance than what tlie lower plate can receive from the silver, whose atmosphere 

 is rarer ; and therefore, as the silver will always be suspended in that part where 

 the 1 currents are equal, without which the proportion would be destroyed, it 

 will consequently be always nearer to the non-electrical than to the electrified 

 plate. If the experiment be reversed, by electrifying the under plate, and mak- 

 ing the upper one the non-electric, the only diflference will be, that the 

 gravity of the silver must then be added to the attraction of the electrified 

 plate, and will therefore cause the silver either to be nearer the non-electrical 

 one, or the plates to be moved a little farther asunder, or perhaps both. 



A Brief Account of a Roman Tessera. By Mr, John Ward, F.R.S., a7id Prof. 

 Rhetor. Gresh. N" 486, p. '224. '' 



TTie brass plate, which accompanies this paper, was dug up, some time since 

 at Market-street in Bedfordshire ; which lies in the Roman road called Watling- 

 street, about 5 miles on this side Dunstable. The inscription engraven on the 2 

 sides is, tes. del mar 



SEDIABVM 



which words may, as he apprehends, be read at length in the following manner:. 



Tessera Dei Martis Sediarum. 



Of a very learned Divine, who was born with Two Tongues. By Cromwell 

 Mortimer, M.D., and Sec. R.S. N° 486, p. 232. 



[The supposed 2d tongue here mentioned was probably nothing more than an 

 enlargement of the sublingual gland, which in process of time contracted to its 

 natural dimensions.] 



On the Sounds and Hearing of Fishes. By Jac. Theod. Klein, R. P. Gedan. 

 F,R.S. Or some Account of a Treatise, entitled, " An Inquiry into the Rea- 



