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Re: Metric Roof Slope
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 Subject: Re: Metric Roof Slope
 From: "John MacLean" <john_maclean(nospamat)pomeroy.bc.ca>
 Date: Mon, 24 Jul 2000 15:56:43 0700
Paul Ransom wrote:
<<x:10 or % (x:100)
degrees are handy for the designer but useless for the site.>> My experience has been that metric roof slopes are
specified as x:250 although in Canada we still mostly see metric jobs with roof
slope specified as x:12  go figure. I think the reason for using x:250 can be
traced back to the carpenter's framing square. The carpenter lays out his cuts
using x inches on one leg and 12 inches on the other or on a metric framing
square x mm on one leg and 250mm (about 10 inches) on the other. 250 mm is
about the right scale to do this sort of practical work. 100 would be too small
and 500 or 1000 would be too large.
I wonder how many people are aware that the metric
system started as an alternative way to specify latitude and longitude? In our
conventional coordinate system the angular distance from the equator to the
pole is 90 degrees with each minute of latitude equal to 1 nautical mile. The
linear distance from the equator to the pole is 60 min x 90 deg = 5400 nautical
miles. The original metric system, or so I understand, proposed a new unit of
angular measure called the grad where 100 grads = 90 degrees. The angular
distance from the equator to the pole would then be 100 grads. The linear
distance from the equator to the pole was to be 10,000 kilometres. Each grad of
latitude would be equal to 100 km along the surface of the earth. Cool, eh? I
believe the new "Greenwich Meridian" for this system was to run through Paris.
Speculating on the exact location within Paris would only get me in trouble.
Unfortunately the geodesics (18th century) were not quite right and it turns out
that the true linear distance from the equator to the pole is a few percent off
of 10,000 km. It's ironic that the only part of the metric system which hasn't
been adopted is the part that was the original basis for the
system.
On a separate topic, can anyone tell me how to send
an email into the list without generating all the "3 part mime"
stuff?
Cheers,
John MacLean
Pomeroy Engineering Ltd.

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