nd as such charged to working costs,
and thus into revenue account in proportion to the tonnage extracted.
This matter is further elaborated in some mines, in that winzes
and rises are written off at one rate, levels and crosscuts at
another, and shafts at one still lower, on the theory that they
lost their usefulness in this progression as the ore is extracted.
This course, however, is a refinement hardly warranted.

Plant and equipment constitute another "suspense" account even
harder to charge up logically to tonnage costs, for it is in many
items dependent upon the life of the mine, which is an unknown
factor. Most managers debit repairs and maintenance directly to
the revenue account and leave the reduction of the construction
outlay to an annual depreciation on the final balance sheet, on the
theory that the plant is maintained out of costs to its original
value. This subject will be discussed further on.

INHERENT LIMITATIONS IN ACCURACY OF WORKING COSTS.--There are three
types of such limitations which arise in the determination of costs
and render too detailed dissection of such costs hopeless of accuracy
and of little value for comparative purposes. They are, first, the
difficulty of determining all of even direct expenditure on any
particular crosscut, stope, haulage, etc.; second, the leveling effect
of distributing the "spread" expenditures, such as power, repairs,
etc.; and third, the difficulties arising out of the borderland
of various departments.

Of the first of these limitations the instance may be cited that
foremen and timekeepers can indicate very closely the destination of
labor expense, and also that of some of the large items of supply,
such as timber and explosives, but the distribution of minor supplies,
such as candles, drills, picks, and shovels, is impossible of accurate
knowledge without an expense wholly unwarranted by the information
gained. To determine at a particular crosscut the exact amount of
steel, and of tools consumed, and the cost of sharpening them,
would entail their separate and special delivery to the same place
of attack and a final weighing-up to learn the consumption.

Of the second sort of limitations, the effect of "spread" expenditure,
the instance may be given that the repairs and maintenance are done by
many men at work on timbers, tracks, machinery, etc. It is hopeless
to try and tell how much of their work should be charged specifically
to detailed points. I