May 20, 2009

One mile per pebble

SOME WHILE AGO I read in a magazine that the newest speed/distance log is totally electronic. Apparently, it measures speed by counting passing water molecules or something. It doesn’t sound like something I would trust. I prefer good solid mechanical logs. I can see how they work, and might even be able to fix one if it went wrong.

Old Vitruvius had the right idea way back in the year 20 BC. He describes it in his magnum opus, De Architectura.

His distance log consists of an axle carried through the side of the ship with a four-foot-diameter paddle wheel striking the water. The inboard end is attached to three drums, hundreds of cogs, and a store of smooth round pebbles.

“Thus,” said Vitruvius, “when a ship is moving, whether under oars or sail, the paddles on the wheel will strike the resisting water, and being driven forcibly backward will revolve the wheel, and the wheel as it revolves will turn the axle and the axle will turn the drum.

“The tooth of the first drum in every revolution strikes and moves one of the teeth in the second drum. And so, when, by the action of the paddles, the wheel has revolved four hundred times, it will, by the pressure of the cog at the side of the vertical drum, move the horizontal drum on one point.

“As often, therefore, as the horizontal drum in its course brings a pebble to an opening, it will let it drop through the pipe. Thus, by sound and by number (of the pebbles that have dropped) the length of the voyage in miles will be shown.”

As I write this, astronauts are walking in space, fixing the ailing Hubble telescope. It’s fascinating to think that 2,029 years ago, Vitruvius’s clunky distance log was one of the most advanced scientific instruments the world had ever known.

Today’s Thought
We have become a people unable to comprehend the technology we invent.
—Association of American Colleges, NY Times 11 Feb 85

“Was it very crowded at Fred’s stag party?”
“Not under my table.”

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