\Horse" pow`er\
1. The power which a horse exerts.
2. (Mach.) A unit of power, used in stating the power
   required to drive machinery, and in estimating the
   capabilities of animals or steam engines and other prime
   movers for doing work. It is the power required for the
   performance of work at the rate of 33,000 English units of
   work per minute; hence, it is the power that must be
   exerted in lifting 33,000 pounds at the rate of one foot
   per minute, or 550 pounds at the rate of one foot per
   second, or 55 pounds at the rate of ten feet per second,
   etc.
Note: The power of a draught horse, of average strength,
      working eight hours per day, is about four fifths of a
      standard horse power.
{Brake horse power}, the net effective power of a prime
   mover, as a steam engine, water wheel, etc., in horse
   powers, as shown by a friction brake. See {Friction
   brake}, under {Friction}.
{Indicated horse power}, the power exerted in the cylinder of
   an engine, stated in horse powers, estimated from the
   diameter and speed of the piston, and the mean effective
   pressure upon it as shown by an indicator. See
   {Indicator}.
{Nominal horse power} (Steam Engine), a term still sometimes
   used in England to express certain proportions of
   cylinder, but having no value as a standard of
   measurement.
3. A machine worked by a horse, for driving other machinery;
   a horse motor.