As you might expect from its name, the "Difference Engine" is a strangely difficult object to describe. You might start by imagining the side of a large crib with uprights ringed by small metal wheels ...

But it wasn’t an engraving. It was a sheet of finely woven silk – a thousand threads to the inch – so subtle and detailed that it deceived many who saw it. It also held the secret to Babbage’s ...