272 Mr. Hopkins’s Abstract of his Memoir on PhysicalG eology. 
d?é ACTS aE is d'é , 
de satin gee “agit &e ’ 
d?y d?y d‘ 
ae Hs ae + Phy + Be. (2.) 
Se RE 94 hat tha 
de el dia? + Sy dx + &e. 
These equations show that the displacements in the three 
rectangular’ directions are, to the extent to which we have 
carried the approximation, independent of each other. 
We have supposed the masses of the molecules to be all 
equal; but if the medium be composed of two fluids uniformly 
mixed, and if the masses of the molecules of one fluid be all 
equal to m, and of the other all equal to m', the equations 
(2.), which we have just obtained, will still be of the same 
form ; because each of the sums = may then be divided into 
two parts, one of which parts, multiplied by m, will embrace 
the molecules of one of the fluids, while the other part mul- 
‘ tiplied by m/, will comprehend the molecules of the other 
fluid ; and the molecules of each fluid may be conceived to be 
arranged in the manner which we have supposed. In this way 
the equations may be extended to the case of any compound 
medium in which the elementary media are uniformly mingled. 
In another communication I propose to deduce the inte-:. 
grals of the equations (2.), and point out the extent of their 
application. I am, Gentlemen, yours, &c. 
Evesham, Feb. 9, 1836. Joun Tovey. 
P.S. There are three typographical errors in my last paper. 
At page 9, line 12, for z read Az; page 10, line 3, for dz 
12 
read A 24; and line 22, same page, for s” read ee 
LVI. An Abstract of a Memoir on Physical Geology ; with 
a further Exposition of certain Points connected with the 
Subject. By W. Horxins, Esq., M.A., F.G.S., of St. Peter’s 
College, Cambridge. 
[Continued from p. 236.] 
LE HAVING now reduced the determination of the hori- 
zontal directions of the fissures produced in the ele- 
vated mass to that of the fissure which would be produced 
in a plane lamina every point of which is subjected to known 
tensions, we may proceed with this latter problem. Our first. 
object is to determine the direction in which the tensions have 
