John McAdam is born. Along the road of life, he’ll invent a new way to smooth the roads of our lives.
Road-building hadn’t improved much since Roman times. Your choices were between dirt (alternately dusty or muddy) and rock or cobble paving (hard on carriage wheels and passengers’ backs, and slippery when wet, which was frequently, thanks to bad drainage).
John Metcalfe introduced a three-layer system in the mid-seventeenth century to improve drainage: large stones, excavated earth, and gravel on top. Thomas Telford raised the center of his roads to let water drain off the convex surface toward the sides.
McAdam added a few ideas of his own: dispense with expensive, precisely cut foundation stones, elevate the road for drainage, and seal the top with successive levels of tightly packed small stones of varying sizes. You could obtain the stones by breaking rocks, then you would use sieves to sort the results by size. Road surveying became more expensive (to ensure good drainage), but you could actually build the road with low-paid unskilled labor instead of highly paid stonecutters.
His new system was such a success that from 1816 to 1818, he became consulting surveyor to thirty-four toll-road companies. Richard Edgeworth added an improvement that McAdam didn’t like but almost everyone else did: mixing stone dust with water to create a smoother “water-bound macadam.” McAdam’s patents were widely infringed. Parliament gave him a small payment. He was offered a knighthood, but, with his health declining, he declined the honor.
By the time McAdam died, in 1836, the word macadam was in common usage for a cambered, paved road. When the stone-dust sealer was replaced by asphalt or tar in the 1850s, people referred to it as tar-macadam, which was eventually shortened to tarmac.—RA