"Heat can also be produced by the impact of imperfectly elastic bodies as well as by friction. This is the case, for instance, when we produce fire by striking flint against steel, or when an iron bar is worked for some time by powerful blows of the hammer"

This quote by Hermann von Helmholtz is describing the production of heat through mechanical ways. He discusses that heat can be produced by the impact of imperfectly flexible bodies, such as when flint is struck versus steel to produce fire. He likewise points out that heat can be produced by friction, such as when an iron bar is worked for some time by powerful blows of the hammer. This is an example of power being converted into thermal energy. This process is known as mechanical work, and it is a form of energy transfer. The energy from the hammer is transferred to the iron bar, which then produces heat. This quote highlights the value of comprehending the various methods which heat can be produced, and how power can be converted into thermal energy.