brownnsharp wrote:
(Beecrofter) wrote in message . com...

These gravity anomalies are called "vertical deflections" and are the
places where the gravity vector differs from perpendicular to the
earths surface.
Over large iron deposits you will find them to be a bit larger but
they really don't follow the geomagnetic fields to any degree.
I believe they used to test for them with pendulous gyroscopic
acellerometers but I bet the new stuff is a bit more refined.



I have a book on the history of maps and mapmaking. It seems that
back in the 1600's there was a big discussion about how big a degree
was on the earth and the way they chose to measure this was discussed
greatly in this book. They took pendulum clocks, adjusted them to
great precision in France. They then disassembled them carefully,
loaded them on ships and sent them down closer to the equator, to use
in astronomical observatories. They found that the rate of the clock
changed. They correctly assumed that the gravitational field was
different at different places on the earth. I believe that these
clocks qualify as the first precision gravity meters.

Principia Mathematica was published in 1687, so they would have had to
be pretty on the ball to think of this, if the date "in the 1600's" even
works out to be mathematically possible.