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From: Dr Nick <3-nospam@temporary-address.org.uk>
Newsgroups: sci.logic,rec.puzzles,sci.math.num-analysis,comp.programming
Subject: Re: the hourglass problem
Date: Mon, 02 Jan 2012 16:17:55 +0000
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Patricia Shanahan <pats@acm.org> writes:

> I think you are ignoring two key features of a sand hourglass:
>
> 1. The sand flows at a more-or-less even rate if the top is not empty.
>
> 2. You can turn it over at any time, even if the top is not yet empty.
>
> Suppose you start a M-minute timer (M>1) with all the sand in the top,
> wait one minute, and then turn it over. The top now contains the amount
> of sand that takes one minute to flow to the bottom, and it will empty
> in one more minute.

While this is clearly the right answer to the puzzle I wonder how
accurate feature (1.) actually is.  Does anyone have - say - a three
minute egg timer to hand who'd try the actual experiment of running it
for one minute, reversing it and seeing how long it takes to return?

I'm not prepared to speculate on whether increased pressure might push
the sand through faster or make it pack together and flow less freely.
-- 
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