Reel Lawn Mower

Hills claims a revolving cylinder of blades geared to the wheels that cuts grass by shearing it against a fixed bottom knife — a scissor, not a chop. Note the two edges.

Fix a straight strip of aluminium along the base of a frame, its edge horizontal just above the ground. This still edge is one half of the scissors.

Glue several thin blades along a dowel, each set at a slight spiral so they don't all strike at once. This cylinder is the moving edge.

Hang the reel in the frame so its blades sweep just past the fixed knife, and fit a wheel on each side.

Loop a wire or band from a wheel to the reel spindle so the wheel spins the reel several times faster than it rolls. Now pushing drives the cut.

Roll the mower forward. The reel spins fast from a slow push — the gearing multiplies wheel turns into many blade passes.

Stand strips of paper up in a row and mow over them. Each is sheared between the moving blade and the fixed knife — a clean cut, not a tear.

Open the reel-to-knife gap wide: the paper folds over uncut. Close it to a whisker: it shears cleanly. A scissor only cuts when its edges nearly touch.

Because the blades are spiralled, one is always crossing the knife — the cut is smooth and steady, not a series of jolts. Straight blades would all bang at once.

Run it over a tuft of real grass. It clips the tops evenly, throwing the cuttings forward — a close, level finish.

Raise or lower the whole reel-and-knife relative to the wheels. Higher leaves the grass long, lower cuts it short — the mower's height adjustment.

Try cutting the same grass by swinging a ruler like a scythe. It is uneven and needs skill and force. The mower gives anyone a level lawn by just walking. Note why that changed the suburbs.

The patent. US 73,807, "Improvement in Lawn-Mowers", granted 28 January 1868 to Amariah M. Hills of Hockanum, Connecticut. This is the first US lawn-mower patent, and Hills went on to found the Archimedean Lawn Mower Company. The reel mower itself, though, was English: Edwin Beard Budding patented it in 1830, having adapted the rotary shearing cylinder from a machine that trimmed the nap on woollen cloth. Hills' patent brought the type to American manufacture and improved the gearing and adjustment.

The physics is a rotary scissors. A scissor cuts because two edges pass each other with almost no gap, trapping and shearing whatever is between them (step 8). The reel mower makes one edge a fast-spinning cylinder of curved blades and the other a fixed bottom knife (the "bed knife"); each blade sweeping past the knife is one snip, and with the wheels geared to spin the reel several times per revolution, a walking-pace push delivers hundreds of clean snips a second. Setting the blades in a helix means only a small part of one blade meets the knife at any instant, so the cut is continuous and the effort smooth (step 9) rather than a hammering of simultaneous blows.

Why it changed the world outside. Before the mower, a smooth green lawn meant either grazing animals or an army of scythe-men, so it signalled wealth. The reel mower let one ordinary person keep a neat lawn in an hour — and with it came the suburban front lawn, the public park, and the modern sports field. Cricket, tennis, golf and football all depend on a closely-mown, level turf that simply could not be produced at scale by hand; the smooth pitch is a product of this machine. Your paper-shearing model is the whole invention: two edges, nearly touching, driven by the wheels you push.