172 PHILOSOPHICAL TRANSACTIONS. [aNNO 1705. 



turned, gave a very swift attrition of the amber on the woollen, but no light at 

 first appeared; yet in a second or two of time it became visible enough: which 

 shows it requires some degree of heat to produce the phenomenon. I could 

 not observe that the continuance or increase of the motion, contributed any 

 thing to the increase of the light, after the first production of it: nor would 

 the light thus produced, remain on the amber to complete a circle, notwith- 

 standing the velocity of the motion given; but died as soon as it had deserted 

 the fricated woollen. However, the light continued without intermission, on 

 the woollen, during the motion, where the attrition of the amber was made, 

 and was discernible at 3 or 4 feet distance. What was further observable in 

 this experiment, was, that it strongly confirms those made on the production 

 of heat in vacuo by attrition: for the amber was not only moderately heated, 

 but appeared to be burnt and cracked by the intenseness of it. And the 

 woollen, against which it rubbed, appeared likewise not only discoloured, but 

 perfectly scorched and burnt through : that although the same motion and the 

 same attrition were given the amber in the open air, yet very little light ensued, 

 in comparison of its appearance in vacuo: that the velocity of the extreme 

 parts of the amber, was equal to something more than one third of a mile in a 

 minute: supposing the large wheel to make 2 revolutions in a second of time, 

 whose diameter is 23 inches; the diameter of the small one, moved by it, one 

 inch and a half; and the diameter of the wood and amber, on the same spindle 

 with the small wheel, 4 inches and a half. 



ExPER. II. Showing the Necessity of the Airs Presence, at least some De- 

 gree of it, in the Production of Fire, on the Attrition of Flint and Steel. — 

 Having provided a steel ring, about 4 inches diameter, and one-eighth of an 

 inch thick, which between two pieces of wood of a less diameter, I fixed on 

 the spindle with the nuts, as mentioned in the foregoing experiment; its edge 

 verging about half an inch beyond the extremity of the wood that held it. 

 The brass plate, which I made use of for fastening the woollen for the attrition 

 of amber, served likewise to fix a piece of flint, an edge of which stood ex- 

 posed to the steel, while the brass plate by its spring held the flint pretty strongly 

 to it, notwithstanding some might be worn or chipped off by the rapidity of 

 the motion. In this manner it was covered with a receiver, and a brass plate 

 and box, as the former. But before any air was exhausted, the great wheel 

 was moved, which gave motion to the small one, and consequently to the in- 

 cluded steel; which exhibited sparks of fire in a very plentiful manner. After 

 some air had been withdrawn, the great wheel was turned, as before, but the 

 number of sparks then produced did not only seem to be lessened, but a sen- 

 sible decay of their lustre and vigour was manifest. And so at every stop that 



