The Knowledge: How to Rebuild our World from Scratch by Lewis Dartnell

Author:Lewis Dartnell [Dartnell, Lewis]
Language: eng
Format: epub, mobi, azw3
Publisher: Random House
Published: 2014-04-02T23:00:00+00:00

Overshot waterwheel. The right-angle gear converts the vertical motion into horizontal rotation suitable for driving millstones to grind flour.

Although it can be dunked in the flow from practically any riverbank, or even mounted over the side of a milling boat anchored in the current, the undershot wheel is woefully inefficient, and in its simplest form has problems with varying river levels. Luckily it doesn’t take much technical know-how to build a far more capable and powerful waterwheel. The overshot wheel became widely exploited across Europe during the supposedly ignorant and stagnant Dark Ages following the fall of the Roman Empire and, despite similarities in overall appearance, functions on a completely different principle to the primitive undershot wheel.

Rather than being stuck into the flow, the bottom of the overshot wheel is held clear of the tailrace and water is delivered to the very top of the wheel by a chute. The overshot wheel derives its torque not from the impact of a current, but from the energy relinquished by the water as it falls. This design is far more efficient and can capture as much as three-quarters of the energy held in the head of water. Fit a sluice gate to the chute to control the flow on to the wheel, and if the stream is dammed to create a millpond, a reservoir of energy can be built up until it is required (something that wasn’t attempted until the sixth century AD, half a millennium after the first vertical waterwheels were used, but could be leapfrogged to during a reboot).

Harnessing wind is technically much trickier than tapping into water power, and consequently the technology arrived much later in our history of development (although boats with sails to catch the wind for propulsion date back to 3000 BC). Water is a far denser medium than air and so even a gentle flow carries a great deal of energy, making it an easy resource to exploit even with imperfectly designed elements and inefficient wooden gearing. A sluice gate can regulate the flow of water, but you have no control over the strength of the wind, so if it begins blowing too briskly the windmill blades or driven mechanisms may be damaged. Windmills therefore need a braking system and a method to control the effectiveness of the blades, such as by reefing canvas sails. The most fundamental challenge, however, is the constantly changing wind direction so a windmill needs to be quickly reorientated.

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