53i 



EXPERIMENTS AND INQUIRIES 



second cavity, with apertures equal in semi- 

 diameter to their initial ordinate, approach, 

 for a short distance, 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 so- 

 lid which would be formed by the revolution 

 of the internal curve, deduced from observ- 

 ations on a small aperture, which is seldom 

 inclined to the axis in an angle so great as 

 ten degrees. A similar conclusion may be 

 made, from observing the flame of a candle 

 subjected to the action of a blowpipe : there 

 is no divergency beyond the narrow limits 

 of the current ; the flame, on the contrary, 

 is every where forced by the ambient air to- 

 wards 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 toitie 

 motion here shown to take place in air ; and 

 these experiments fully justify him in reject- 

 ing 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, with respect to that of the particles in 

 thecontiguous strata, which, whatever mpy be 

 the supposed order of the single particles with 

 respect to each other, must naturally lead to 

 a communication of motion nearly in a pa- 

 rallel direction ; and this may properly be 

 termed friction. The lateral pressure which 

 urges the flame of a candle towards the 

 stream of air from a blowpipe, is probably 

 exactly similar to that pressure which causes 

 the inflection of a current of air near an ob- 

 stacle. Mark the dimple which a slender 

 stream of air makes on the surface of water ; 

 bring a convex body into contact with the 



side of the stream, and the place of the dim- 

 ple will inmiediately show that the current is 

 inflected towards the body ; and, if the body 

 be at liberty to move in every direction, it 

 will be urged towards the current, in the 

 same manner as, in Venturi's experiments, a 

 fluid was forced up a tube inserted into the 

 side of a pipe through which water was flow- 

 ing. A similar interposition of an obstacle, 

 in the course of the wind, is probably often 

 the cause of smoky chimneys. One circum- 

 stance was observed in these experiments, 

 which it is extremely difficult to explain, and 

 which yet leads to very important conse- 

 quences : it may be made distinctly per- 

 ceptible to the eye, by forcing a current of 

 smoke very gently through a fine tube. 

 When the velocity is as small as possible, 

 the stream proceeds for many inches with- 

 out any observable dilatation ; it then imme- 

 diately diverges at a considerable angle into 

 a cone, (Plate 3. Fig. 27) ; and, at the point 

 of divergency, there is an audible and even 

 visible vibration. The blowpipe also affords 

 a method of observing this phenomenon : as 

 far as can be judged from the motion of the 

 flame, the current seems to make something 

 like a revolution in the surface of the cone, 

 but this motion is too rapid to be distinctly 

 discerned. When the pressure is increased, 

 the apex of the cone approaches nearer to 

 the orifice of the tube (Fig. 28, 29) ; but no 

 degree of pressure seems materially to alter 

 its ultimate divergency. The distance of 

 the apex from the orifice is not proportional 

 to the diameter of the current; it rather ap- 

 pears to be the greater the smaller the cur- 

 rent, and is much better defined in a small 

 current than in a large one. Its distance in 

 one experiment is expressed in Table x. 

 from observations on the surface of a liquid ; 



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