Fifty years ago Hooke had caught sparks struck off of a steel, and put them under the microscope, and shown Daniel what they really were: pocked spheres of shiny metal, like iron planets. Daniel had supposed they were solid, until he saw some that had been blown open by internal pressure. For the sparks weren’t globules, but hollow bubbles, of molten steel that flailed, then froze, when they burst, leaving wild out-flung extremities that looked a little bit like clawing hands and a little bit like ancient tree-roots cast up on a beach. Some of Mr. Newcomen’s failed boilers look like those exploded sparks. Others have failed in ways not so obvious, and lie half-embedded in the earth, like meteors fallen out of the sky.