112 Dr. Young's Experiments and Inquiries 



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 Venturis 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 obstacle 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 diffi- 

 cult to explain, and which yet leads to very important conse- 

 quences : 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 im- 

 mediately diverges at a considerable angle into a cone, Plate IV. 

 Fig. 24 ; and, at the point of divergency, there is an audible and 

 even visible vibration. The blowpipe also affords a method of 

 observing this phsenomenon : 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, 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 ex- 

 periment is expressed in Table x, from observations on the sur- 

 face of a liquid ; in other experiments, its respective distances 

 were sometimes considerably less with the same degrees of pres- 

 sure. It maybe inferred, from the numbers of Tables vn and viii, 



