( %\6% ) 
againft which it rubb'd, appeared like wife not only dif* 
colour'd, bat perfeftly fcorcht and burnt through : That 
altho the fa«!c Motion and the fame Attrition was given 
the Amber in the Open Air, yet very little Light did en- 
fue, in Comparifon to the Appearance of it in Vacuo; 
That the Travel or Velocity of the extreme parts of the 
Amber, was equal to fomething more than one third of a 
mile in a minute : Suppofing the large Wheel to make two 
Revolutions in a fecond of tinie^ whofe Diameter is 25 
Inch.^s 'y the Diameter of the fmall one mov*d by it, one 
Inch and a half ^ and the Diameter of the Wood and 
Amber on the fame Spindle with the fmall Wheel, four 
Inches and. a half. 
Experiment H. 
Shemng the Necefjltj of the Airs Prefe/tce ( at leaft fame de- 
gree of it) in the VroduBion of Fire^ upon the Attrition of 
flint and SteeL 
HAving provided a Steel Ring about 4 Inches Diame- 
ter, its Thicknefs about one eighth of an Inch^ 
"^hkh between two pieces of Wood of a lefs Diameter, I 
fixt on the Spindle with the Nutts, as mentioned in the 
foregoing Experiment^ its edge verging beyond the ex-^ 
treme of the Wood that held it about half an Inch. The 
Brafs Plate, which I made ule of for faftning the Woollen 
for the Attrition of Amber, ferv'd like wife to fix a piece of 
Flint ; An edge or corner of which ftood expos'd to thc"^ 
Steel, which Brafs Plate by its Spring would hold the Flint 
pretty ftrongly to it, notwithftanding (bme might be worn 
or chipt off by the Rapidity of the Motion given. In this 
manner it was Co ver'd with a Receiver, and Brafs Plate 
and Box, as the former. But before any Air was exhauft* 
ed, the Great Wheel was movM, which gave motion to 
the fmall one, and confcquently to the Included Steel 5 
which 
