i io Dr. Young's Experiments and Inquiries 



nates of the curves in Figs. 13— 23, were therefore taken recipro- 

 cally in the subduplicate ratio of the pressure marked by the 

 second gage to that indicated by the first, at the various dis- 

 tances 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 cur- 

 rent, 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 super- 

 ficial ones, of course, the smaller the aperture opposed to the 

 centre of the current, the greater the 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 dis- 

 tance 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 be- 

 tween 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 occa- 

 sioned by some resistance derived from the air returning between 

 the sides of the aperture and the current entering it. Where 



