NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Noell Wilson
Date: 2026 Jan 11, 09:01 -0800
Reading The Mapmakers, deflection of a plumb bob near the Himalyan Mountains is discussed a lot but it’s never said “towards or away”. As I wrote initially, it may seem “obvious” but it would have only taken a few words to say one way or another.
Later on in the book, Wilford, the author, describes an “away” situation along the coast of Peru.
I also found a video “2.14 Do mountains have roots” that talked about the lower density of mountains. As with much information today, the comments about mountains floating on molten magma ranged from religious admonitions to “So if I swim under it, what will I find?, to “Bull -!”
So I threw it out on NavList and Frank disagreed with my “plumb bob swings away from the mountain”.
Finding documentation today means wading through a lot of music, animation, and inexact statements.
Bottom line though is Frank is right. The best explanation is The Schiehallion Experiment (1774) .
The plumb bob problem with surveying was well known but measuring it was a big problem. After years of debate, “That won’t work”, “we can’t measure that precisely”, and “That’s too complex”, an experiment was proposed with Schiehallion Mountain in Scotland. It was relatively accessible, it was not near other mountains, it was symmetrical, and that simplified the detailed calculations to only two years
I came at this from the survey correction and also thinking about effects on CelNav observations that depended on “level”. The Schiehallion Experiment was mainly done for physics and astronomy. The mass and gravitational attraction between planets.
https://en.wikipedia.org/wiki/Schiehallion_experiment
The basic result was that, if the density of the mountain was the same as the earth, a plumb bob would be deflected towards Schiehallion Mountain by 20.9”. The test revealed that the plumb bob was pulled toward the mountain by only 11.6”.
Two years of calculations showed that the density of the mountain was 2500 kg/m3 and The Earth was 4500 kg/m3. (Today the earth density is known as 5515 kg/m3. Another physics deduction from all of this was that there was an extra mass. Probably towards the earth center. Probably metallic.)
So, yes. A plumb bob is pulled toward a mountain. But it is pulled only about half as much as it would be if the mountain’s density matched the earth. The debates about correction factors in the survey of India were how much to REDUCE the assumed attraction of the Himalayan Mountains.
