ther these requirements will be realized, _X, Y, Z_
must be balanced in a scale whose measuring standards are the five
geological weights and the general industrial outlook. The wise
engineer will put before his clients the scale, the weights, and
the conclusion arrived at. The shrewd investor will require to
know these of his adviser.

CHAPTER VII.

Development of Mines.

ENTRY TO THE MINE; TUNNELS; VERTICAL, INCLINED, AND COMBINED SHAFTS;
LOCATION AND NUMBER OF SHAFTS.

Development is conducted for two purposes: first, to search for
ore; and second, to open avenues for its extraction. Although both
objects are always more or less in view, the first predominates
in the early life of mines, the prospecting stage, and the second
in its later life, the producing stage. It is proposed to discuss
development designed to embrace extended production purposes first,
because development during the prospecting stage is governed by
the same principles, but is tempered by the greater degree of
uncertainty as to the future of the mine, and is, therefore, of
a more temporary character.

ENTRY TO THE MINE.

There are four methods of entry: by tunnel, vertical shaft, inclined
shaft, or by a combination of the last two, that is, by a shaft
initially vertical then turned to an incline. Combined shafts are
largely a development of the past few years to meet "deep level"
conditions, and have been rendered possible only by skip-winding. The
angle in such shafts (Fig. 2) is now generally made on a parabolic
curve, and the speed of winding is then less diminished by the
bend.

The engineering problems which present themselves under "entry"
may be divided into those of:--

1. Method.
2. Location.
3. Shape and size.

The resolution of these questions depends upon the:--

a. Degree of dip of the deposit.
b. Output of ore to be provided for.
c. Depth at which the deposit is to be attacked.
d. Boundaries of the property.
e. Surface topography.
f. Cost.
g. Operating efficiency.
h. Prospects of the mine.

[Illustration: Fig. 2.--Showing arrangement of the bend in combined
shafts.]

From the point of view of entrance, the cooeperation of a majority
of these factors permits the division of mines into certain broad
classes. The type of works demanded for moderate depths (say vertically
2,500 to 3,000 feet) is very different from that required for great
depths. To reach great depths, the size of shaft