10 
BULLETIN 1348, U. S. DEPARTMENT OF AGRICULTURE 
PRIMARY POWER AVAILABLE AND HORSEPOWER-HOURS 
UTILIZED ANNUALLY ON FARMS « 
Table II shows the estimated total primary horsepower available 
and Table III the horsepower-hours developed annually on farms by 
States. Data for horses, mules, tractors, and trucks have been 
worked out separately, but it has been necessary to base the figures 
for stationary power largely on the power required to do the work 
rather than on the amount of each kind of power developed, and for 
this reason stationary engines, windmills, and electric power have 
been grouped together. 
The figures for animal power are based on information contained 
in Table XXI. One 1,200-pound animal has been assumed to be 
capable of developing 1 primary horsepower. This rating is per- 
haps somewhat higher than it has been customary to use; but it is 
known that many horses of this weight develop a full horsepower 
Fig. 7.— Estimated distribution of tractors on farms in 1924. Each dot represents 250 tractors 
for a considerable period of time when doing heavy work, such as 
plowing, and in view of the results of recent tests with the Iowa 
horse dynamometer this figure is considered to be a reasonable basis 
for estimating the available primary power. The data for horse- 
power-hours per average work animal, given in Table XXI, were 
compiled from a large amount of information made available by 
• The most common unit used in the United States for measuring work is the foot-pound. This rep- 
resents the work done in lifting to a height of 1 foot a body weighing 1 pound, or moving an object 1 foot 
against a resistance of 1 pound. Power is the rate of doing work, and the usual unit for measuring power is the 
horsepower, which is equivalent to the power required to perform work at the rate of 33,(XX) foot-pounds per 
minute. (See Table XXIV of the Appendix for pounds pull exerted per horsepower at different rates of 
travel.) By primary power is meant the maximum load any power unit or series of units is capable of 
developing for a reasonable length of time. Some kinds of power, such as animal power, and most steam 
engines, have a considerable reserve capacity in addition to this that can be exerted for very short intervals. 
A horsepower-hour is equal to 1,980,000 foot-pounds (33,000X60), and is the most common unit used when 
determining quantity of work done or power developed. (See Tables lU and V and page 8 for quantity of 
power developed and amount required for various farm operations.) 
