606 PHILOSOPHICAL TRANSACTIONS. [ANNO 1800. 



pressure would be required to stop its progress, and that the velocity of the current, 

 where it struck against the aperture, must be in the subduplicate ratio of the 

 pressure marked by the gage. The ordinates of the curves in figs. 1 3 to 23, were 

 therefore taken reciprocally in the subduplicate ratio of the pressure marked by the 

 2d gage to that indicated by the first, at the various distances represented by the 

 abscisses. Each figure represents a different degree of pressure in the first cavity. 

 The curve nearest the axis, is deduced from observations in which the aperture 

 opposed to the tube was not greater than that of the tube itself; and shows what 

 would be the diameter of the current, if the velocities of every one of its particles 

 in the same circular section, including those of the contiguous air, which must 

 have acquired as much motion as the current has lost, were equal among themselves. 

 As the central particles must be supposed to be less impeded in their motion than 

 the superficial ones, of course the smaller the aperture opposed to the centre of 

 the current, the greater velocity ought to come out, and the ordinate of the curve 

 the smaller; but where the aperture was not greater than that of the tube, the 

 difference of the velocities at the same distance was scarcely perceptible. When the 

 aperture was larger than that of the tube, if the distance was very small, of course 

 the average velocity came out much smaller than that which was inferred from a 

 smaller aperture; 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 some cases, a larger aperture seemed to indicate a 

 greater velocity: this might have arisen in some degree from the smaller aperture 

 not having been exactly in the centre of the current; but there is greater reason 

 to suppose, that it was occasioned by some resistance derived from the air returning 

 between the sides of the aperture and the current entering it. Where this took 

 place, the external curves, which are so constructed as that their ordinates are re- 

 ciprocally in the subduplicate ratio of the pressure observed in the 2d cavity, with 

 apertures equal in semidiameter to their initial ordinate, approach, for a short dis- 

 tance, nearer to the axis than the internal curve: after this, they continue their 

 course very near to this curve. Hence it appears, that no observable part of the 

 motion diverged beyond the limits of the solid which would be formed by the re- 

 volution of the internal curve, which is seldom inclined to the axis in an angle so 

 great as 10°. A similar conclusion may be made, from observing the flame of a 

 candle subjected to the action of a blow-pipe: there is no divergency beyond the 

 narrow limits of the current; the flame on the contrary, is every where forced by 

 the ambient air towards the current, to supply the place of that which it has 

 carried away by its friction. The lateral communication of motion, very ingeniously 

 and accurately observed in water by Professor Venturi, is exactly similar to the 

 motion here shown to take place in air; and these experiments fully justify him in 

 rejecting the tenacity of water as its cause: no doubt it arises from the relative 

 situation of the particles of the fluid, in the line of the current, to that of the 

 particles in the contiguous strata, which is such as naturally to lead to a commu- 



