4 BULLETIN 660. U. S. DEPARTMENT OF AGRICULTURE. 
The expense of operation. — This includes the wages of operators 
and helpers and the cost of supplies during the periods of operation. 
Usually these are charged directly against the work done and not 
included in the plant rental. It is only necessary that they be 
charged in one place or the other, and it is important to specify 
what is included in rental when leasing equipment. 
The average cost of repairs. — There is a difference of opinion among 
cost accountants as to how repairs and renewals to plant should 
be charged. One view is that renewals may be of such a nature 
that the useful life of the machine has been increased and therefore 
the expense of such renewals should be looked upon as an offset to 
depreciation. Another view is that there is no difference between 
repairs and renewals, except hi degree, and that they all should be 
considered in the same light; i. e., independent of depreciation 
charges. It appears that the latter consideration permits simpler 
accounting and does not rely so much upon individual judgment as 
to whether the expenditure is for repairs or for renewals. 
After a machine has been rebuilt or repaired extensively with the 
intention of increasing its serviceable life, it should be considered 
as a piece of new equipment valued at its depreciated value, plus the 
cost of renewals. This necessitates the computing of a new rate 
of depreciation on the basis of the new value and assumed new 
useful life. 
The approximate average cost of repairs, including extraordinary 
repairs, often can be arrived at by casting up old accounts and 
finding what a similar piece of machinery used on similar work has 
cost for repairs over a term of years. 
Charges for time spent in idleness. — To arrive at a fair and equitable 
daily charge for rental some allowance must be made for time spent 
in idleness, because on these days the fixed charges still are con- 
tinuing and certain supplies are necessary even though the machine 
be not in operation. The usual way of arriving at the charge for 
lost time through idleness is to bring together all of the charges 
for a year and divide them by the number of days the machine 
actually was in use. By dividing the sum total of expense by the 
number of days the machine was available for use even though no 
work existed on which it could be used, the result would be a daily 
rental with no allowance for lost time. The difference between 
these two rentals will show what a considerable factor in the fixed 
charges this item of lost time may become. In all contracts or agree- 
ments on rental of equipment care should be taken to specify whether 
the rental is ''per day" or ''per day of service." 
Charges for depreciation. — Equipment is consumed in production 
just as truly as material. This loss is called natural depreciation. 
Depreciation maybe either natural or functional. ''All equipment 
