6 BULLETIN" 338, U. S. DEPARTMENT OF AGRICULTURE. 
By multiplying the average investment by the prevailing rate of 
interest in the locality where the equipment is located the average 
annual interest charge against the equipment is found. In con- 
nection with the tables which follow the replacement cost and the 
interest charge have been computed by the foregoing methods in 
terms of the year, the acre, and the day actually used. 
METHOD OF DETERMINING THE SERVICE OF FARM MACHINERY. 
The farm conditions which affect the annual service rendered by 
machinery are so many and various in the same locality, and so 
different between widely separated areas, that no general method of 
approximating the annual depreciation is feasible. Actual records 
on a sufficiently large number of machines extending through a 
period of years long enough to determine the life in use can not be 
secured on account of the expense and improbable continuity of the 
record keeping. A method was found, however, which yielded re- 
sults rapidly and gave averages based on large numbers of reports 
for each kind of implement, 1 this method being used in obtaining 
data in the limited area in western New York, which is presented in 
the tables that follow : 
EXPLANATION OF THE TABLES. 
In the tables the machines are, in most cases, grouped according 
to the amount of work done annually. In all cases the years of 
service decrease with increasing annual use, but not in the same 
proportion. This is due to the fact that machinery depreciates to 
some extent when not in use. The decrease in length of time with 
increased annual use would be greater than actually shown in the 
tables were it not for the fact that the machines used on the smaller 
acreages are smaller and less substantially built. The average size 
of the implements doing the respective amounts of annual work is 
shown in the third line in each table. 
The service of each implement is expressed in years and acres 
and in days of actual use. The performance in years was found by 
the method already explained; the service in acres was obtained by 
multiplying the service in years by the average work done annually; 
i Where the number of machines in a given area is stationary, the design and construction uniform and 
established, and the implement has been in use there for a period exceeding the average life of such imple- 
ment, the average years of service may be found by dividing the entire number by the number one year 
old. 
Or, under the conditions just mentioned, the average service may also be found approximately by adding 
the present age of the respective machines reported, dividing by the total number to obtain the average 
present age of the group, multiplying this average by 2 and subtracting 1. By the latter method not such 
large numbers are required to insure a reliable average. 
In computing the service in years for the tables in this bulletin both methods were used, and, while the 
results were practically the same in most cases by each method, they were combined and the average taken 
to offset to some extent the too great influence of the occasional very old individual on the average 
obtained by the second method. 
