_Answer._ No; because the weight has the capacity of doing an equal amount of work when it falls, from the momentum[8] it acquires in falling. This _power of doing work_ which it acquires in falling is called _energy_. Now, although the weight has no motion-producing power when it is raised to the top of the machine, yet obviously such action is then _possible_ which when it rested on the earth was not possible. It has no energy as it hangs there dead and motionless; but energy is possible to it, and we might fairly use the term _possible energy_ to express this power of motion which the weight possesses,[9] and which is therefore called _potential energy_. As soon as the weight is allowed to fall it acquires a greater velocity the farther it falls, and its potential energy thus becomes and is called _actual energy_. [8] Momentum is not a very exact term, but is used here because it ordinarily conveys the idea we wish to express. [9] Tyndall’s “Heat Considered as a Mode of Motion.” QUESTION 32. _How do we explain such phenomena as the heating of a car-axle while turning under a car, the heating of brake-blocks when the brakes are applied to car-wheels, the heating of an iron rod by hammering, and of a turning tool when cutting a piece of metal?_ _Answer._ All of these phenomena are due to the fact that the _actual energy_ of motion is converted into heat, as has been repeatedly proved by many able and ingenious investigators and experiments. QUESTION 33. _When the weight of the pile-driver falls, is its energy also converted into heat?_ _Answer._ A part is expended in compressing the material into which the pile is driven and in overcoming the friction of the earth against the pile, each of which efforts develops heat, and another portion is converted into heat by the impact or blow of the falling weight on the head of the pile. QUESTION 34. _Is all energy convertible into heat and heat into energy?_ _Answer._ Yes. Science has demonstrated very clearly that they are mutually convertible. QUESTION 35. _Has it been ascertained how much heat is equivalent to one foot-pound of work?_ _Answer._ Yes; it has been found, from the most carefully-made experiments that the amount of heat which is required to raise the temperature of one pound of liquid water by one degree of Fahrenheit[10] is equivalent to 772 foot-pounds of work. It must be remembered that this is the theoretical equivalent of heat, and that only a very small proportion of this amount of work is ever realized from the heat developed by the combustion of fuel. [10] Thermometers are divided into different scales. The one called the Fahrenheit scale, after its originator, is the one ordinarily used in this country. QUESTION 36. _If, then, heat is convertible into work and work into heat, can the transmutation of the heat of the steam in the cylinder of an engine into work, and the reverse process, be explained?_ [Illustration: _Fig. 9._ Scale ³⁄₈ in. = 1 foot.]