==== WATER AND WIND ====

The first experiments with water power were probably
the horizontal waterwheels seen in China at early TL2. In
these, the wheel lay sideways in the water, pushed by the
stream, and rotated an axle attached to a grindstone above.
It resembled a wagon axle turned on its side. The design
was simple, requiring no gearing, but inefficient.

Undershot and overshot wheels were invented nearly
simultaneously around 200 B.C. These upright designs
were pushed by water either traveling under or pouring
down on top of the wheel. They were both more complex
and more expensive than a horizontal wheel (particularly
overshot wheels, which can require a long mill-race to
deliver water to the top of the wheel), but also more powerful
and more versatile.

The earliest windmills appeared in Persia around 600
A.D., and closely resembled the horizontal waterwheel.
Half of a shaft with paddles protruding from it was exposed
to the wind. The force of the wind against the paddles
turned the axle and powered machinery.

Around the 13th century, both China and northern
Europe developed vertical windmills. Sail-like vanes faced
the wind, engaging drag forces to make the mill much
more powerful. Many early windmills were in buildings
set on posts, so that they could be picked up and turned
when the wind shifted. By late TL4, windmills were built
with turrets that had rotating bearings, making them far
easier to readjust.

Most powered mills, regardless of type, provided power
equivalent to ST 20-40 – and horizontal mills rarely
exceeded that. The largest mills could generate the equivalent
of up to ST 125 at TL3 and ST 175 at TL4.

=== Rudimentary Steam Engines ===

The idea of steam power goes back at least to the Alexandrian philosopher Hero in the first century A.D.,
but came well in advance of the engineering know-how to make it meaningful. Hero’s aeolipile was a hollow
metal sphere on a pivoting mount. Nozzles at either pole shot out steam jets when water inside was heated
to boiling, making it rotate. Steam-based devices appeared sporadically thereafter: steam-driven pistons
opened doors in grand Roman temples, steam pipes made artificial birds flap their wings in Byzantine
palaces, and steam jets slowly turned an Ottoman philosopher’s roasting spit.

These devices were expensive toys. They leaked, wasted heat, and expended valuable fuel and metal to do jobs that any other energy source – human labor, draft animals, waterwheels – could do far more cheaply. A typical TL2-4 steam contraption consumes 30 lbs. of wood and 10 gallons of water per hour. It drives a single powered accessory that can perform any one repetitive action that a ST 10 man could perform; e.g., opening a door, sawing, or blowing a horn. Such a contrivance might be mounted on a ship or a heavy wagon for transport, but the engine isn’t powerful enough to be self-propelled. $20,000, 1,000 lbs.