14 BULLETIN 423, U. S. DEPARTMENT OF AGRICULTURE. 
DEPRECIATION. 
It is not possible, in view of the short time the mechanical milker 
has been in general use, to secure accurate data on the rate of de- 
preciation of such machines. Some of the parts, such as the vacuum 
tank and the iron piping, should last almost indefinitely. The 
pails, if heavily nickeled or made of nickel alloy or other noncor- 
rosive metal, should last for a number of years. The parts most 
subject to depreciation, the rubber tubes and teat cup rubbers, are 
normally subject to such frequent renewal that the cost of keeping 
them up has been included under repair costs. The parts always 
subject to a certain amount of wear are the engine, pump, and pul- 
sator. Eeliable data is at hand to justify the assumption that the 
depreciation of the small stationary engine averages not more than 
10 per cent per year. Since the pump is much less complicated 
than the engine and is not subject, as is the engine, to deterioration 
due to excessive heat, it may be assumed that the life of the pump 
is somewhat longer than that of the engine. This leaves the pul- 
sator, the cost of which is not more than 25 per cent of the total cost 
of installation, as the only wholly unknown quantity. All things 
considered, it has been thought safe to assume that the life of the 
outfit as a whole, not counting rubber parts, is slightly longer than 
the known life of the engine. Hence 8 per cent per year has been 
taken as the rate of depreciation of milking-machine outfits in this 
discussion. It should be borne in mind, however, that this figure 
is a somewhat arbitrary one, used in this connection only for lack 
of any more definite figure and that it applies to the entire outfit, 
exclusive of rubber parts. It was felt that the use of even this rough 
approximation would tend to make the results of this study more 
reliable than they would have been had depreciation been ignored 
altogether. 
The depreciation charge, like the investment charge, is affected 
by the initial cost of the milking outfit. "When an arbitrary depre- 
ciation figure is applied to all outfits the more expensive ones re- 
ceive a higher depreciation charge. In actual use, however, the de- 
preciation is determined by the initial cost and the length of life 
of the machine, with the last factor the more important. 
COMPARATIVE COST OF MILKING BY HAND AND BY MACHINE. 
As previously stated, the cost of hand milking is determined by the 
time taken to milk and the wages paid to the milkers. In figuring 
the cost of milking with the mechanical milker the following items 
were considered in addition to the labor cost : The interest on the 
investment, the fuel expense or cost of power, and the repairs and 
