74 Expey'tinefits and Inquiries nf^eBing Sound and LighU 



in Tables v. and vi, and are exhibited to the eye in Plate IV. Figs, i— 12. In order to 

 meafure with greater certainty and precifion, the velocity of every part of the current, a 

 fecond cavity, furnifhed with a gage, was provided, and pieces perforated with apertures 

 of different fizes were adapted to its orifice: the axis of the current was direfted as ac- 

 curately as poffible to the centres of thefe apertures, and the refult of the experiments, 

 with various preffures and diftances, are iuferted in Tables vii. viii. and ix. The velo-. 

 city of a ftream being, both according to the commonly received opinion and to the expe- 

 riments already related, nearly in the fubduplicate ratio of th? preffure occafioning ic> it 

 was inferred, that an equal preffure would be required to ftop its progrefs, and that the 

 velocity of the current, where it ftruck againft the aperture, muft be in the fubduplicate 

 ratio of the preffure marked by the gage. The ordinates of the curves in Figs. 13 — 23, 

 were therefore taken reciprocally \t\ the fubduplicate ratio of the preffure marked by the 

 fecond gage to that indicated by the firft, at the various diftances reprefented by the ab- 

 fciffes. Each figure reprefents a different degree of preffure in the fitfl. cavity. The curve 

 neareft the axis, is deduced from obfervations in which the aperture oppofed to the tube 

 was not greater than that of the tube itfelf ; and (hows what would be the diameter of the 

 current, if the velocities of every one of its particles in the fame circular fedlion, including 

 thofe of the contiguous air, which muft have acquired as much motion as the current hag 

 loft, were equal among themfelves. As the central particles muft be fuppofcd to be, lefa 

 impeded in their motion than the fuperficial ones, of courfe, the fmaller the aperture op- 

 pofed to the centre of the current, the greater the velocity ought to come out, and the 

 ordinate of the curve the fmaller ; but, where the aperture was not greater than that of the 

 tube, the difference of the velocities at the fame diftance was fcarcely perceptible. When 

 the aperture was larger than that of thp tube, if the diftance was very fmall, of courfe, the 

 average velocity came out much fmaller than that which was inferred from a fmaller aper- 

 ture; but, where the ordinate of the internal curve became nearly equal to this aperture, 

 there was but little difference between the velocities indicated with different apertures. 

 Indeed, in fome cafes, a larger aperture feemed to indicate a greater velocity : this might 

 have arifen in fome degree from the fmaller aperture not having been exadly in the centre 

 of the current ; but there is greater reafon to fuppofe, that it was occafioned by fome re- 

 fiftance derived from the air returning between the fides of the aperture and the current 

 entering it. Where this took place, the external curves, which are fo conftrufted as that 

 their ordinates are reciprocally in the fubduplicate ratio of the preffure obferved in the fe- 

 cond cavity, with apertures equaLin fcmidiameter to their initial ordinate, approach, for -jd 

 fhort diftance, nearer to the axis than the internal cyrve : after this, they continue their 

 courfe very near to this curve. Hence it appears, that no obfervable part of the motion 

 diverged beyond the limits of the folid which would be formed by the revolution of the 

 internal curve, which is feldom inclined to the axis in an angle fo great as ten degrees. A 

 fimilar conclufion may be made, from obferving the flame of a candle fubjedied to the ac- 

 tion of a blowpipe: there is uo divergency beyond the narrow limits of the current; the 

 4 Same, 



