ON FLOW OF SOLIDS UNDER GREAT PRESSURES. 
139 
pened to be situated as regards the lateral orifice at the time 
that the pressure was first brought to bear upon them. The 
effect of the pressure showed most at the bottom edge of the 
orifice, and the several discs that were originally situated 
between the top and bottom of the orifice became at last 
reduced down to very thin layers at the bottom edge of the 
orifice, by the crowding of the upper discs as they were 
successively brought down and caused to flow out at the 
orifice. This action taking place only at the front, or the 
point of escape, those discs that are so excessively reduced 
there in thickness retain much more of their original thick¬ 
ness at the back of the cylinder, whilst on the contrary 
the uppermost discs have their back edges left thinner than 
at the front, by the compressing ram causing the top surface 
of the last disc to be kept level throughout. 
Another point of interest is that although the pressure at 
the bottom of the cylinder is uniform over the whole surface 
of the lowest discs that are below the level of the orifice, the 
effect produced upon them is modified by the circumstance of 
the discs above them being in a condition of flow towards 
the orifice, and the front edges of the lower discs are deflected 
upwards towards the general line of flow, in consequence of 
the pressure lessening gradually towards the point of escape. 
A remarkable modification of the result obtained in the 
first experiment, Fig. 1, with the central orifice in the bottom 
of the cylinder, was shown when the total supply of lead 
discs was reduced in number, as in Fig. 3, so far that the 
quantity of material present under compression in the 
cylinder was not sufficient to fill up the jet solid. The 
singular result was then obtained of a jet that neither filled 
up the orifice close on the outside, nor was solid in the inside; 
and in the flow of the lead through the orifice there was 
obtained a central hollow and a contracted jet on the outside. 
This bears a striking analogy to the well-known contracted 
vein that occurs when a liquid flows out through a similar 
central aperture, as in Fig. 4 ; and the flow of the jet of lead 
has to be looked upon as differing from the flow of a jet of 
liquid only in the degree of pressure that is required to 
produce this flow, for which gravity is sufficient in the case 
of a liquid, but a very high pressure is required in the case 
of the metal. 
From the circumstance of the metal jet being composed 
of a series of successive concentric layers, a valuable oppor¬ 
tunity is afforded by the separate examination of these layers 
for ascertaining the relative motions and lines of flow of the 
different particles composing the jet, and of studying the action 
