5: Stamina

Another considerable limitation of the human body is its stamina: the ability to continue to perform a repetitive task over a long period of time. Human workers must have frequent rest breaks, and cannot work more than a certain number of hours in a given day without becoming fatigued.

The same is not entirely true of a machine - it can continue to operate at the same speed indefinitely. However it is not "entirely" true because the stresses on a machine can cause its parts to wear out or break more quickly if it is run at too great a speed for too long a time without maintenance. But so long as the force and speed are not so great as to damage the parts of the machine, it can operate around the clock without becoming "tired" and be productive so long as it is fueled and supplied with material.

He provides a few examples of machines that require rest periods - a clock must be stopped in order for the spring to be wound, and any instrument that uses or generates heat must be given cool-off time to avoid melting or warping its parts.

There is also the general rule that simple machines break down less frequently than more complex ones - as it is difficult to engineer a complex system in such a way that all the parts are moving at a speed that can be maintained indefinitely.