i92 



Pro/. F. T. Trout on on Viscous Substances. [March 11, 



In order to know the rate at which the material is being drawn 

 out, two things are required. The amount of material passing down 

 per second — this is easily found — and the slope of the surface to the 

 vertical at the place in question. We can observe this on reflecting 

 a beam of light from the surface of the column so as to pass out 

 horizontally, by measuring the angle between the incident beam and 

 the reflected one. So much for the rate of drawing out; now the 



other quantity required, the force of traction, is simply the weight of 

 the suspended column situated below the point. By cutting off the 

 column, collecting and weighing, we get its amount. It is true that 

 the value of this varies slightly, for if a stream is watched it will be 

 seen to grow longer and then break off when it gets too heavy to be 

 supported, somewhere higher up. However, this goes on only at the 

 end of the stream where it is extremely thin, and the alteration in 

 weight thus produced may be safely neglected. 



From the table it will be seen that the value of the viscosity so 

 obtained is, on the whole, in satisfactory agreement with that obtained 

 by other methods. It will be noticed that in the case of the thinner 

 liquid, the inertia term which was neglected has probably made itself 

 felt. ; 



[F. T. T.] 



