556 MACHINES, WORK, AND ENERGY 



day, but while doing so he also transported his own body the 

 same distance. The stationary engine does no useless work 

 in moving its own mass through space. One of the reasons 

 why the efficiency of the stationary engine is greater than 

 that of the locomotive is that the locomotive does a large 

 amount of useless work in transporting its own mass. 



There are many kinds of work, however, which can be done 

 at a lower cost by the horse than by the steam motor. In 

 excavating for a house or in grading up around the house 

 after it is built, for most hauling about the farm, for the 

 delivery of grain to the local market, for the cultivating and 

 the harvesting of crops on the ordinary farm, the horse is 

 still often the most practical motor we have. 



661. Why We often Employ the Labor of Man instead of 

 the Horse or the Steam Motor. Just as we still employ the 

 horse instead of the steam engine to do many kinds of work, 

 so in spite of the high cost of his labor we still find it profitable 

 to employ man to do much of our work. In the construction 

 of buildings there will probably always be a demand for 

 laborers to handle the brick and mortar, and the wheel- 

 barrow and spade, as well as to handle the hammers, the saws, 

 the planes, and the trowels. In most lines of work there is 

 some rough work which might possibly be done by machines 

 but which can at present be more cheaply done by the cheap- 

 est of human labor. More and more, however, work of this 

 nature is being done by other motors and man is finding 

 that he must prepare himself for doing such work as requires 

 thoughtful, intelligent action. 



VI. SOME COMMON MECHANICAL MOTORS 



662. Common Mechanical Motors. In the last section we 

 saw that, while man and the horse were both more efficient 

 motors than the steam engine, it still is true that the steam 

 engine is a much less expensive motor to operate that the 

 most efficient motor is not necessarily the least expensive to 



