_Answer._ They are used to produce _motion_, which is opposed by some _resistance_. Thus, if an engine is employed to raise grain from a railroad car to the top of a warehouse, it must produce motion, which is resisted by the weight of the grain; if it is used to saw wood, it must give motion to the saw, which is resisted by the fibres of the wood; a locomotive engine must produce motion of a train of cars, which is resisted by the air, the friction of the journals and the rolling of the wheels on the track; if the locomotive is employed on a grade or incline, besides the frictional resistance referred to it must overcome that due to its own weight and that of the train, which is gradually lifted as it ascends the incline. In producing motion opposed by some resistance an engine is said to be doing “_work_.” QUESTION 29. _Can this work be accurately measured?_ _Answer._ Yes; but in order to measure anything we must first establish some accurate standard or unit of measurement. Thus we say a bar of iron is so many inches long, or a road is so many miles long. In like manner we speak of so many seconds, or minutes, or hours, or days, or years, when we speak of time. So it is necessary, in order to estimate or measure “_work_” in a strictly scientific manner, for us to fix upon some accurate standard or unit. In this country and in Great Britain the unit agreed upon for this purpose is the amount of power required to raise ONE POUND ONE FOOT, and is called a _foot-pound_. If we raise one pound two feet we do two foot-pounds of work; if three feet, three foot-pounds, and so on. Again, if we raise a weight of two pounds one foot high, we likewise do two foot-pounds of work; or if we raise it two feet high, we do four foot-pounds, and so on. In order to determine the amount of work done, we must MULTIPLY THE MOTION PRODUCED (_in feet_) BY THE RESISTANCE (_in pounds_), AND THE RESULT WILL BE THE WORK DONE IN FOOT-POUNDS. QUESTION 30. _How many foot-pounds of work are performed in a pile-driving machine in raising a weight of 1,200 lbs. 24 feet?_ _Answer._ 1,200 × 24 = 28,800 foot-pounds. QUESTION 31. _When this weight is raised, is the force which was exerted in raising it annihilated or lost?_