VOL. XC.] PHILOSOPHICAL TRANSACTIONS. 607 



nication of motion nearly in a parallel 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 blow-pipe, is probably exactly similar to that pressure which causes 

 the inflection of a current of air near an obstacle. 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 dimple will immediately 

 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 flowing. A similar interposition of an ob- 

 stacle in the course of the wind, is probably often the cause of smoky chimneys. 

 One circumstance was observed in these experiments, which it is extremely difficult 

 to explain, and which yet leads to very important consequences: it may be made 

 distinctly perceptible 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 without any observable dilatation ; it then immediately diverges at a 

 considerable angle into a cone, fig. 24; and, at the point of divergency, there is an 

 audible and even visible vibration. The blow-pipe also affords a method of ob- 

 serving 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 in- 

 creased, the apex of the cone approaches nearer to the orifice of the tube, figs. 25, 

 26; but no degree of pressure seems materially to alter its divergency. The distance 

 of the apex from the orifice is not proportional to the diameter of the current; it 

 rather appears to be the greater the smaller the current, and is much better defined 

 in a small current than in a large one. Its distance in one experiment is expressed 

 in table 10, from observations on the surface of a liquid; in other experiments, its 

 respective distances were sometimes considerably less with the same degrees of 

 pressure. It may be inferred, from the numbers of tables 7 and 8, that in several 

 instances a greater height of the first gage produced a less height of the 2d: this 

 arose from the nearer approach of the apex of the cone to the orifice of the tube, 

 the stream losing a greater portion of its velocity by this divergence than it gained 

 by the increase of pressure. At first sight, the form of the current bears some 

 resemblance to the vena contracta of a jet of water: but Venturi has observed, 

 that in water an increase of pressure increases, instead of diminishing, the distance 

 of the contracted section from the orifice. Is it not possible, that the facility with 

 which some spiders are said to project their fine threads to a great distance, may 

 depend on the small degree of velocity with which they are thrown out, so that, 

 like a minute current, meeting with little interruption from the neighbouring air, 

 they easily continue their course for a considerable time? 



