Sunday, 25 October 2009

The Blue Cogwheel of Death


Last week I had the great good fortune to be invited to the Computer History Museum's Annual Fellow Awards event. I first visited the Museum when it was in Boston, shortly after it opened there, and I again find myself living close by. The collection is truly amazing - if you haven't been and you find yourself nearby, you should definitely go.

And of all the amazing items, the most outstanding is the Babbage Difference Engine, courtesy of Nathan Myhrvold, who paid the London Science Museum to build him one and then lent it to the Museum. It weighs four tons, and is eight feet high by about twelve feet long (I didn't have a tape measure with me so this may not be accurate). In principle it's extremely simple - repeated addition can be used to calculate any polynomial whose degree does not exceed the number of stages in the machine, and hence to approximate a great many interesting functions. However given that it is built with 18th century technology, the reality is anything but simple. To anyone who admires mechanics and machinery, it is truly a thing of beauty - four tons of gleaming brass and steel and cast iron, thousands of cams, levers, wires and pulleys. To sit down and design it from scratch now would be an achievement; that Babbage did so over two centuries ago is almost beyond belief.

The machine is operated by a large crank handle, which is turned slowly and evenly by a trained operator. Sadly though, when I was there to see it, a short way round the first turn the handle jammed solid. No amount of gentle persuasion would shift it. The docents (guides, for any British readers I may have) told us that this happens occasionally, and when it does, the machine needs to be reset. As you may imagine, this involves a lot more than pressing a button and waiting a couple of minutes. Screws have to be loosened, cams carefully reset to their starting position, and goodness only knows what. In any case it takes way too long to do in front of an audience.

I feel quite honored to have seen the first computer ever built, crash.