1 6 Eddies in the Air.—AJufical Tremor or Undulatiin. 



at the furface of the liquor after the funnel has been formed, be of fufficlent fize to cover the 

 whole cavity, it deftroys the funnel in the upper part, and fometimes alfo in the lower. The 

 reafon is, that the body itfelf cannot turn round its centre but according to the lawiy=:r; it 



therefore deftroys by friflion the law v=: in the parts of the fluid in conta<St with it, 



and confequently it deftroys the funnel itfelf. 



PROPOSITION XII. 



The lateral communication of motion takes place in the air as well as in wattr. 



The ftream of air which moves in the midft of a body of air at reft, produces undulations 

 and eddies round its current in the fame manner as in water. Thefe may be obferved in the 

 fmoice which rifes from a furnace, and produces a remarkable afpeft, when it iflues like a dark 

 tree from an agitated volcano. They may likewife be feen in the particles which float in an 

 obfcure chamber, when a ray of the fun fhines in, and the obferver blows through them. 



If a general wind comes, for exampl , from the fouth, it frequently happens that the north 

 fide of a mountain is at the fame time ftruck by a north wind. This partial and local wind is 

 nothing but the eddy produced by the mountain itfelf afting as an obftacle, againft the prin- 

 cipal wihd, from the fouth. It is probably from the fame reafon, that the wind fometimes 

 a6ts in the contrary direition on the fails of a veffel, when they are too obliquely prefented to 

 its ftream. 



The vapour of water which iflues from the eolipile carries the furrounding air with it, and 

 drives it againft the burning coals oppofite to the ftream of aqueous vapour. It muft not, 

 therefore, be concluded that the aqueous vapour is itfelf in this cafe decompofed to maintain 

 the combuftion of the charcoal. 



It is known that the flues of chlmnies aflift the rifing of fmoke by their figure; concerning 

 which, we have drawn fome indudtions, in the feventh propofition. 



In organ pipes, the air which ifl^ues out of the fide opening (lumiere) rubs laterally againft 

 the extremity of the column of the air included in the pipe. It rubs it on one fide in the longi- 

 tudinal direction, and is, as it were, an elaftic file, adting upon an elaftic furface. Though 

 the column of air be fluid, its parts are, however, fo far intermixed together, that the tremulous 

 motion excited at the place of fridiion, is foon communicated laterally through the whole thick- 

 nefs of the column, which receives vibrations of fuch a kind, that they are an equilibrium with 

 each other, and with the velocity of the ftream which affords the friftion. For this efFeft, it 

 is requifitc that the column fliould divide itfelf at difi^erent points or nodes diftributed through 

 the length of the tube*. It is by repeated actions that the wind which iflTues from the 

 fide aperture impreflTes at length upon the whole column contained in the pipe, a movement 

 of vibration greater than that which the laws of impulfe, and of the lateral communication, 

 would permit it to make by a fingle impulfe. In the hautboy, and other fimilar inftruments, 

 having a mouth -piece, or reed, the caufe which excites the tremulous motions, does not adt 

 fideways on the air contained in the pipe, but ftrikes the column diredlly in the middle: for 

 which reafon, it communicates its vibrations with fo much the more effcdt to the whole noafe. 



■' Memoircs de I'Acad. an. 1761, pnge 431. 



