Experlftuntt and Inquiries refpBing Sound and Light, y r 



flame, on the contrary, is every where forced by the ambient air towards the current, to 

 fupply the place of that which it has carried away by its fridlion. The lateral communr- 

 cation of motion, very ingenioufly and accurately obferved in water by Profeflbr Ventuti, is 

 exadly fimilar to the motion here Ihown to take place in air ; and thefe experimetits fully 

 juftify him in rejefting the tenacity of water as its caufe : no doubt it arifes from the rela- 

 tive fituation of the particles of the fluid, in the line of the current, to that of the particles 

 in the contiguous ftrata, which is fuch as naturally lead to a communication of motion 

 nearly in a parallel direftion ; and this may properly be termed friction. The lateral pref- 

 furc~which urges the flame of a candle towards the ftream of air from a blowpipe, is pro- 

 bably exaftly fimilar to that preflure which caufes the inflection of a current of air near an 

 obftacle. Mark the dimple which a flender ftream of air makes on the furface of water, 

 bring a convex body into contaft with the fide of the ftream, and the place of the dimpld 

 will immediately fliow that the current is infledled towards the body ; and, if the body be 

 at liberty to move in every diredion, it will be urged towards the current, in the fame 

 manner as, in Venturi's experiments, a fluid was forced up a tube inferted into the fide of 

 a pipe through which water was flowing. A fimilar interpofition of an obftacle in the 

 courfe of the wind, is probably often the caufe of fmoky chimneys. .One circumftaWCff waS 

 obferved in thefe experiments, which it is extremely difficult to explain, and which yet 

 leads to very Important confequences : it may be made diftinftly perceptible to the eycj by 

 forcing a current of fmoke very gently through a fine tube. When the velocity is as fmall 

 as poflTible, the ftream proceeds for many inches without any obfervable dilatation j it then 

 immediately diverges at a confiderable angle into a cone, Plate IV. Fig. 24 ; and, at the 

 point of divergency, there is an audible and even vifible vibration. The blowpipe alfo 

 afibrds a method of obferving this phenomenon : as far as can be judged from the motion 

 of the flame, the current feems to make fomething like a revolution in the furface of the 

 tone, but this motion Is too rapid to be dlftlnflly difcerned. When the prefigure is" in- 

 creafed, the apex of the cone approaches nearer to the orifice of the tube, Figs. 25, 26 ; 

 but no degree of preflure feems materially to alter its divergency. The diftance of the apex 

 from the orifice. Is not proportional to the diameter of the current ; it rather appears to bfe 

 the greater, the fmaller the current, and Is much better defined in a fmall current than in a 

 large one. Its diftance in one experiment Is exprefled in Table x, from obfervations on 

 the furface of a liquid ; in other experiments, its refpefllve diftances were fometimes con- 

 fiderably lefs with the fame degrees of preflure. It may be Inferred, from the numbers of 

 Tables vii and viii, that in feveral Inftances a greater height of the firft gage produced a 

 lefs height of the fecond : this arofe from the nearer approach of the apex of the cone to 

 the orifice of the tube, the ftream lofing a greater portion of its velocity by this divergence 

 than it gained by the increafe of preflure. At firft fight, the form of the current bears 

 Ibme refemblancc to the vena contr'aHa of a jet of water: but Venturi has obferved, that in 

 water an increafe of preflTure increafes, inftead of dlmlnifhing, the diftance of the contrafted 



L 2 fe^lon 



