4: Velocity
A distinction is made between two parts of an action - the motion needed to perform the action and that required to reset the body to the position from which the motion is begun or repeated. Consider the act of driving a nail with a hammer - the hammer is brought down upon the nail, but then it is necessary to lift the hammer before striking it a second time. Or switch to driving a spike with a sledge hammer - the force of striking the spike is largely effected by gravity, and the greater human effort is in raising the hammer afterward.
Whereas strength is necessary to exert force on the hammer driving the nail and to raise the sledge for a second strike, a more significant factor is speed: the strength of a human being determines how quickly he can raise the hammer once, but over time the muscles tire of repetitive action and the speed at which the worker can strike (the number of times per minute he can swing the sledge) diminishes.
A man who is not bearing a load can walk up and down a flight of stairs faster than a man who is bearing a load. Or to remove strength from the situation entirely, a man cannot walk down a hill faster than a cart can roll down it - the wheel provides velocity greater than the human body.
Velocity is the most common comparison between man and machine: we measure the efficiency of a machine by the speed at which it produces. A mechanized loom creates more yards of cloth than does a worker with a manual loom - and the act of weaving requires little power to move the threads, but the machine accomplishes this simple task with greater velocity. Likewise, a spinning-wheel twists fibers of wool into thread faster than human hands. (EN: And again, more examples are given to illustrate the same basic principle.)
The value of velocity is increasing the work accomplished over a given amount of time. A man is certainly capable of making just as many needles as a machine - but the machine creates in an hour what a man might create in a week. (EN: This seems a bit oblique, the focus may be on time or volume depending on whether the need is perceived as "more now" or "faster completion.")
Aside of making more efficient time, velocity enables tasks to be completed that were previously impossible. To carry a pumpkin five hundred miles on foot would be a pointless task - it would rot before it arrived - but a steam locomotive is capable of accomplishing the task quicker, making it feasible to transport goods over greater distances. That is, velocity makes a task profitable and productive that would not be so if it were done manually.
He also mentions the blowing of glass as a task that is limited by human capacity. Because glass becomes rigid as it cools, the size of a bottle that can be blown (literally, by blowing into a tube with the mouth) is very limited - but if power is applied to blow air into the tube more rapidly, much larger glass vessels can be manufactured.
Velocity, like power, must be carefully controlled, largely to enable human reflexes to control a machine. It is possible for a weaving machine to run at an extremely fast speed, but it would not provide a human worker sufficient time to recognize that a bobbin is running out of thread and needs to be reloaded. (EN: Here is where computer sensors, ever more alert and observant than human beings, can enable machines to run at higher speeds.)
Finally, he mentions the speed of communication: a telegraph system enables information to be communicated quickly over vast distances, which facilitates the coordination of efforts. Babbage seems particularly amazed by this, having been present when an important message was received from hundreds of miles away in an instant, as well as having seen telegraphs used to communicate with ships in remote ports, which would previously have been impossible.