he edge of
the diamond-iferous area and enlarged until the pillar between
crushes.

In the caving methods, between 40 and 50% of the ore is removed
by the preliminary openings, and as they are all headings of some
sort, the average cost per ton of this particular ore is higher
than by ordinary stoping methods. On the other hand, the remaining
50 to 60% of the ore costs nothing to break, and the average cost
is often remarkably low. As said, the system implies bodies of large
horizontal area. They must start near enough to the surface that
the whole superincumbent mass may cave and give crushing weight,
or the immediately overhanging roof must easily cave. All of these
are conditions not often met with in mines of the character under
review.

CHAPTER XII.

Mechanical Equipment.

CONDITIONS BEARING ON MINE EQUIPMENT; WINDING APPLIANCES; HAULAGE
EQUIPMENT IN SHAFTS; LATERAL UNDERGROUND TRANSPORT; TRANSPORT IN
STOPES.

There is no type of mechanical engineering which presents such
complexities in determination of the best equipment as does that of
mining. Not only does the economic side dominate over pure mechanics,
but machines must be installed and operated under difficulties which
arise from the most exceptional and conflicting conditions, none of
which can be entirely satisfied. Compromise between capital outlay,
operating efficiency, and conflicting demands is the key-note of
the work.

These compromises are brought about by influences which lie outside
the questions of mechanics of individual machines, and are mainly
as follows:--

1. Continuous change in horizon of operations.
2. Uncertain life of the enterprise.
3. Care and preservation of human life.
4. Unequal adaptability of power transmission mediums.
5. Origin of power.

_First._--The depth to be served and the volume of ore and water
to be handled, are not only unknown at the initial equipment, but
they are bound to change continuously in quantity, location, and
horizon with the extension of the workings.

_Second._--From the mine manager's point of view, which must embrace
that of the mechanical engineer, further difficulty presents itself
because the life of the enterprise is usually unknown, and therefore
a manifest necessity arises for an economic balance of capital
outlay and of operating efficiency commensurate with the prospects
of the mine. Moreover, the initial capital is often limited, and
makeshifts for this reason a