828 
ME. W. HOPKINS’S EXPEEIMENTAH EESEAECHES 
ences of mean superficial temperatures in different latitudes, and for possible ’v ariations 
in the conductive po’wers at different points, it ’would still seem extremely improbable 
that the above law respecting the surfaces of equal temperature should not be approxi- 
mately true at depths beneath the surface at which such supei-ficial causes as are merely 
local cannot be supposed to exercise a very sensible influence. Let us suppose then 
that there is a surface of equal temperature concentric -with the earth’s surface at the 
depth, for example, of fifty miles; and let us also take a portion of the surface of 100 
miles in diameter, ’with one set of sedimentary strata, the thickness of which is small 
compared ’with the above depth ; and another smaller area -vvith another similar set of 
strata. If these portions of the earth’s crust be contiguous, the flow of heat near their 
common boundary will not be entirely vertical (as supposed in our investigations), but in 
some degree lateral also ; because the conductive powers being by h^q^othesis different, 
the temperatures at the same depth in the two sets of strata respectively ’will not be 
exactly the same, except at the lowest surface of each set, where they are here assumed 
to be the same. This difference of temperatures, however, at equal depths will be too 
small to produce any sensible lateral flow of heat except at points near the common 
junction of the two sets of strata, or therefore to affect sensibly the vertical flow of heat 
in all the more central parts of these two contiguous portions of the earth’s superficial 
crust. Also, it is manifest that the surfaces of the strata ha’vlng no more horizontal 
extent than here supposed, may be considered as plane instead of spherical. Hence, the 
hypothesis of the flow of heat being vertical will be very approximately true in the 
example now proposed. 
Again, on account of the great pressure to which the portions of the earth’s mass at 
considerable depths must be subjected, and the absence of stratification in the portion 
beneath the sedimentary deposits, it would seem impossible that the influence of discon- 
tinuity in the lower parts of the mass should not be considerably smaller than in the 
superincumbent stratified mass; and since it is small, as I have sho-um, in the latter, it 
must a fortiori be so in the former. It also follows fi-om the great thickness of the 
whole mass here assumed, and the general values of - determined in our o’wn experi- 
ments ^supposing them much the same as those of , that the value of 2 must 
be very large. Hence 1 + 2 may be neglected in comparison of ; and for 
the same reason 1 + 2 may be neglected in comparison ’sHth 2 • Also, if we 
suppose the two strata denoted by their conductive powers A’,, and Zv, to be any two 
sedimentary strata, their thicknesses and hi, must be very small compared uith the 
depth, which, for greater distinctness of conception, I have supposed to be fifty miles. 
Hence the equations (8, 9.) will be applicable to our present case. Moreover it seems 
extremely improbable that portions of the earth’s mass beneath the sedimentary beds, 
and for considerable depths, should vary much in their conductive powers. All the 
