G. -H. KNIBBS. XXXIX, 
and in the other 1—7/3v2, that is to say, 47°64017 and 
25°9520°% respectively. It would appear from this that 48% 
is about the maximum porosity, and since the interstices 
ean be filled with still smaller grains, that sand may also 
be less porous than 25%), 
Professor Slichter solves for flow in triangular and 
other sections. This part of the work leaves something to 
be desired in the shape of rigour, but no doubt the solution 
is a highly approximate one. What Slichter has shewn to 
be very important is that the porosity (with which @ varies) 
has great infiuence on the flow. Any formula that does 
not take it into account is quite inadequate. 
6. The lines of fiow.—In applying Poiseuille’s,* and 
Darcy’s® law, viz.:— 
it is shewn that the constant k depends upon three things, 
a 
(a) The viscosity of the liquid, i.e. upon temperature. 
(b) The size of the grains of soil. 
(c) The porosity or ratio of the voids to the total volume. 
Since the flow is in an approximately horizontal stratum of 
porous rock lying between relatively impermeable strata, 
one may, in determining the lines of flow under given con- 
ditions, assume it to take place in two dimensions. The 
equations of motion are then simply 
W=O4L/d5 = Op OH... 55 dre.0s (8) 
v=dy/dt=k ap/ay..........+.(9) 
that for determining p being 
ey ee 
a? tay? Ore (0) 
an equation which remains true even if the velocity vary 
in such a form, as say v=kp-+ce, where ¢ is a constant. 
2 Poiseuille—Recherches sur le mouvement des liquides dans les tubes 
de trés petite diametres.—Mém. des sav. étrang. Acad. Sc. 1842, 1x., p. 433. 
2 H. Darcy—Les fontaines publiques de la ville de Dijon, Paris, 1856. 
