52 



Professor E. S. HeleShaiv 



[Feb. 10, 



speed of those in front was reduced. Applying this illustration to 

 the model, you will see that the impact of these particles in the 

 wider portion would necessarily involve a greater pressure in that 

 part. Turning next to the white balls, I imitate, by means of the 

 left-hand portion, the flow which will occur in a channel six times as 

 large as the original one, and you now see (Fig. 7) that as the par- 

 ticles have placed themselves six abreast, and the first and hist row are 

 3 inches apart instead of 18 inches, the speed in the wider portion 

 of the channel must have been one-sixth of that in the narrow portion. 

 Evidently, therefore, the velocity of the particles has been reduced 

 more rapidly than in the previous case, and the pressure must con- 

 sequently be correspondingly greater. 



Fig. 4. 



Fig. 5. 



We may now take it as perfectly clear and evident, that the pres- 

 sure is greater in the wider portion and less in the narrower portion 

 of the channel. Turning now to the two diagrams, we see that the 

 pressure is in each case greater in every row of particles as in the 

 wider portions of the channel, but that instead of being suddenly 

 increased, as in the model, it is gradually increased. The width of 

 the coloured bands, that is, rows of particles, or width apart of stream- 

 lines, is a measure of the increased pressure. Thus you will now re- 

 gard the width of the bands, or what is the same thing, the distance 

 apart of the stream-lines, as a direct indication of pressure, and the 

 narrowness or closeness of the stream-lines as a direct indication of 

 velocity. 



Next notice the great difference between the two diagrams. In 

 one diagram (Fig. 4), the change of width is uniform across the entire 



