140 
ON FLOW OF SOLIDS UNDER GREAT PRESSURES. 
of the forces controlling their flow and their mutual interfer¬ 
ences with one another; such an examination being impractic¬ 
able in the case of a liquid jet. It will be noticed that all the 
particles of the material under pressure have a tendency to 
move towards the orifice, for the purpose of yielding to the 
pressure, but only those particles that are actually opposite 
the orifice are able to flow directly outwards, whilst the 
others beyond that region and surrounding the orifice have 
first to move laterally in a radial direction towards the 
orifice, before they can get a chance of escaping. These 
lateral movements of the particles interfere with one another, 
and those from opposite sides of the orifice are directly 
opposed to each other, whilst the particles from higher levels 
move in oblique funnel-shaped directions towards the centre 
of the orifice. The result is that a general oblique direction 
is given to the particles in passing through the opening, 
forming a funnel-shaped centre to the hollow jet in the case 
where the total supply of material was not sufficient for 
maintaining a solid jet. This is exactly what occurs, as is 
constantly seen, in the case of a liquid flowing through a 
centre orifice (such as out of the plug opening at the bottom 
of a basin); when the depth of the liquid gets so low that the 
head or pressure is not sufficient to maintain a solid jet, a 
funnel-shaped hollow gets formed in the centre, as in Fig. 4, 
which increases in diameter as the depth and pressure become 
less. The liquid jet is also contracted round the outside, in 
a similar manner to the metal jet, and it cannot be got to 
fill up the entire diameter of the orifice through which it 
flows. 
This is a very interesting point, showing that a loss of 
•pressure takes place, whenever the matter is put in motion 
and caused to flow; and this loss of pressure may be carried 
so far under special circumstances of deficiency in supply of 
material for following up the flow, as to actually become 
negative pressure and produce a hollow. 
When the orifice in the lead jet experiment was not 
central, but eccentric, there was found to be a circular dis¬ 
placement added that gave a torsional movement to the jet; 
uniform equilibrium being disturbed by the preponderance 
of metal that had to flow towards the jet on the thick side. 
An analogous effect in water jets is shown by the spiral 
movement that is set up in water flowing out of an orifice in 
the bottom of a vessel, which has the central effect of its 
position upset by some lateral disturbance of the surface of 
the liquid. 
