respecting Sound and Light. 1 09 



II. Of the Direction and Velocity of a Stream of Air. 



An apparatus was contrived for measuring, by means of a 

 water-gage communicating with a reservoir of air, the pressure 

 by which a current was forced from the reservoir through a 

 cylindrical tube ; and the gage was so sensible, that, a regular 

 blast being supplied from the lungs, it showed the slight varia- 

 tion produced by every pulsation of the heart. The current of 

 air issuing from the tube was directed downwards, upon a white 

 plate, on which a scale of equal parts was engraved, and which 

 was thinly covered with a coloured liquid ; the breadth of the 

 surface of the plate laid bare was observed at different distances 

 from the tube, and with different degrees of pressure, care being 

 taken that the liquid should be so shallow as to yield to the 

 slightest impression of air. The results are collected in Tables 

 v. and vi. and are exhibited to the eye in Plate III. Figs. 1 — 12. 

 In order to measure with greater certainty and precision, the 

 velocity of every part of the current, a second cavity, fur- 

 nished with a gage, was provided, and pieces perforated with 

 apertures of different sizes were adapted to its orifice : the axis 

 of the current was directed as accurately as possible to the 

 centres of these apertures, and the result of the experiments, 

 with various pressures and distances, are inserted in Tables vii. 

 vnr. and ix. The velocity of a stream being, both according 

 to the commonly received opinion and to the experiments 

 already related, nearly in the subduplicate ratio of the pressure 

 occasioning it, it was inferred, that an equal pressure would be 

 required to stop its progress, and that the velocity of the cur- 

 rent, where it struck against the aperture, must be in the sub- 

 duplicate ratio of the pressure marked by the gage. The ordi- 



