258 ■ 



REPORT 187G. 



away from the orifice (aa, for instance, by the insertion of a rigid obstruction) 

 ■will transmit scarcely the slightest effect back to the region of the orifice ; 

 or, in other words, that in a free-flowing jet spouting through the air, the 

 effects of obstructions fade away rapidly in the direction contrary to the 

 current, so as to become imperceptible at a very moderate distance taken 

 back from the obstacle in the direction against the flow — very moderate 

 relatively to the thickness of the jet. 



Even "without this appeal to experimental observation, we might almost 

 intuitively perceive, or might readily admit, that the introduction of more or 

 less pressure than any stated amount in the stream, at a place where it has 

 got well clear of the orifice, would be only very slightly influential on the 

 flow as to pressures and as to velocities and directions of motion within the 

 vessel and near the orifice and contracting vein. A reason for this is, that 

 while an obstruction in a free jet will require a great change in mode of 

 flow of the jet close in front of it, yet the jet approaching to that region 

 need have its outer filaments turned aside only very slightly indeed to allow 

 of aU parts moving forward without any of their stream-lines, whether me- 

 dial or at or near the surface, being subjected to almost any increase of 

 pressure, and consequently without the velocities of any of them being almost 

 at all retarded. This will readily be clearly understood by reference to 

 fig. 10, where the water is shown as spouting against a stone without being 



Fig. 10. 



made to thicken its stream sensibly in consequence of the obstruction, except 

 for a very short distance at G in front of the stone— that is to say, in the 

 back-stream direction from the stone. If we were to suppose that the stone 

 would have a tendency to produce, at such a place as K, any considerable 

 increase of pressure in the internal or central stream-filaments of the jet, we 

 would have to notice that the external stream-filaments next the atmosphere 

 would fail to resist this augmented pressure ; and, instead, they would, with 

 only a very slight change in their own velocities or pressures, yield a little 

 outwards, and so would not exert on the internal filaments the confining 

 action that would be requisite for the maintaining of more than an extremely 

 slight augmentation of pressure in those internal filaments. Then it is obvi- 

 ous that if the pressure is very little augmented, the velocity must be very 

 little abated ; and so, for this reason, the stream will not tend to thicken 

 itself except very slightly, because any considerable increase of cross-sectional 

 area of the stream would require an important abatement of velocity, which, 

 as said before, would require a great increase of pressure in the internal 



