288 Mr . Nicholson’s Experiments , &c. 
P. S. Since the above was written, the Journal de Phyjique 
for April, 1789, has arrived. It contains an excellent Paper 
of M. Van Marum, giving an account of fome very confi- 
derable amendments of the rubber, and of the manner of 
applying it to plate machines. The chief improvement con- 
lifts in fixing the filk to the pofterior part of the rubber, fo that 
it covers the whole face, and has the amalgam applied upon it. 
I cannot, however, avoid expreffing my furprize, that this im- 
provement, which has been in common ufe in England for 
upwards of twelve years paft, fhould now be offered as a 
difcovery by fo experienced a philofopher. With his new rub- 
bers M. Van Marum excites his plates of 33 inches diameter fo 
ftrongly as to produce pearly two* thirds of the former effedl 
of the Teylerian machine, though the rubbed furfaces of thefe 
machines are now as 691 to 2409. This power would charge 
the fingle jar by the friblion of 28.6 fquare feet, or the battery 
by rubbing 36.2 feet, inftead of the numbers 66.6 and 94.8, 
as given in par. 24. This is a vaft acquifition of intenfity; 
but {till little more than half that of the furface of a cylinder, 
as mentioned in the fame paragraph. But if par. 14. be ad- 
mitted to prove that plate machines gain nothing by the fridtion 
of the back furfaces, it will follow, that M. Van Marum ’s 
management, if applied to a cylinder, would do better than 
mine. 
( Vv ' • ' ' ' 
