2 y 2 $dr. Nicholson’s Experiments and 
invention of the fiik flap upon the fame fuppofition. The 
former afferts, that the lines of light both at the entering and 
departing parts of the furface are abfolutely fimilar ; and thence 
infers, that the cufhion receives on the one fide, as it certainly 
does on the other. I find, however, that the fa£t is diredtly 
contrary to this affertion ; and that the oppofite inference 
ought to be made, as far as this indication can be reckoned con- 
clufive : for the entering furface exhibits many luminous per- 
pendiculars to the cufhion, and the departing furface exhibits a 
neat uniform line of light. This circumftance, together with 
the confideration that the line of light behind the filk in par. 
8. could not confift of returning eledfricity, fhewed the necef- 
fity of farther examination. I therefore applied the edge of 
the hand as a rubber, and by occafionally bringing forward 
the palm, I varied the quantity of eledfricity which pafled near 
the departing furface. When this was the greateft, the fparks 
at the eledfrometer were the moft numerous. But, as the ex- 
periment was liable to the objedtion that the rubbing furface 
was variable, I patted a piece of leather upon a thin flat piece 
of wood, then amalgamed its whole furface, and cut its extre- 
mity off in a neat right line clofe to the wood. This being 
applied by the conffant adfion of a fpring againfl; the cylinder, 
produced a weak excitation, and the line where the contact of 
the cylinder and leather ceafed (as abruptly as poffible) exhi- 
bited a very narrow fringe of light. Another piece of wood 
was prepared of the fame width as the rubber, but one quarter 
of an inch thick, with its edges rounded, and its whole fur- 
face covered with tin-foil. This was laid on the back of the 
rubber, and was there held by a fmall fpring, in fuch a man- 
ner as that it could be Hided onward, fo as occafionally to project 
beyond the rubber, and cover the departing and excited furface 
