The idea that a fuel-burning engine could drive a vehicle over land, through the water, or even into the sky, was already old by the time of Watt’s triumphs. Leibiniz, Huygens, and Papin had all dreamed of vehicles powered by gunpowder or steam. The horseless carriage was the most popular concept; machinery to turn wheels was already commonplace, after all. John Robison had proposed a steam-driven carriage to Watt himself in 1759. But steam locomotion first became practical in the water, not on land. There were several reasons for this, but the most fundamental was size: early steam engines were large and heavy machines, built atop a large and heavy boiler which sat atop a large and heavy furnace (typically of brick). To float of all this mass on water was a far easier matter than to set it on wheels (though the crushing weight of its machinery sent many a steamboat to the bottom nonetheless). Moreover, a ship of moderate size, unlike a road-bound vehicle of any reasonable dimensions, could hold all the workings and still have plenty of leftover room for paying cargo – human or otherwise. To make a much more compact engine would require high-pressure steam, but that raised many technical problems that would take decades to work out.