NavList:

Noell W, you wrote:
"I’m about 1/2 way through The Mapmakers by John Noble Wilford and there are so many goodies to mention!"

Did you know he died just a week ago? I did not know this myself until I looked up his Wikipedia biography a few minutes ago, and I assume that the timing of your post is just a coincidence --a remarkable one.

There are plenty of "goodies" in "The Mapmakers", as you say, but Wilford's own occasional misunderstandings are mixed in as well. Considering it was written over four decades ago, it's worth remembering that he did not even have the benefit of basic internet searches to clear up his sometimes distorted models of the science underlying the story.

Wilford was a talented New York Times science reporter who brought lasting credit to the famous NYT Science section. He was also famous for helping to "kill Einstein" --in the sense that he and other NYT science journalists were always eager to write articles that announced apparent contradictions to Einstein's work in relativity and gravitation (always wrong). I see the same problem in "The Mapmakers". Wilford was smart, and he wrote great prose, but he leapt to conclusions and included his own misconceptions without clarifying the difference between his journalism and his opinion.

Regarding the Trigonometric Survey of India, you noted:
"they “assumed that the mass of the Himalayas would be sufficient” to deflect the plumb line by 15”. - - - it turned out that the deflection was only 5”. "

Yes, the "naive" calculation based on modeling the Himalayas as a giant block of rock resting on atop the Earth's crust implied a different rate of change. The DOV (deflection of the vertical) should have jumped up much more quickly. Instead it rises at a more leisurely pace, as if the mountains are made of "cheese" (or something less dense than solid rock at least!). But it should also be added that the DOV eventually catches up and far exceeds the 15" naive estimate. The DOV is over 60" further up into the mountain plateau.

You added:
"If you are like me, and a bunch of people on the internet, you are visualizing the plumb bob being pulled towards those 25,000’ high Himalayan mountains. That’s just obvious, huh? Back to the Himalayas and deflected plumb bobs - to focus this, I’ll say now that the plumb bob swings away from the Himalayas! By 5”."

The deflection is towards the mountains, though there are some modest local variations, of course. In general, all around the world, the DOV is towards mountains and away from mass "deficits". The deficits that can lead to seemingly reversed deflection are enormous: the great ocean trenches, the subduction zones of "plate tectonics". These are in some ways more interesting for navigation since one can sail above a mass deficit blissfully unaware that the local vertical (the direction to the zenith which is so essential to celestial navigation!) is swinging significantly despite the fact there is no major landmass visible to do the work.

You added:
"There’s a lot of discussing in The Mapmakers about less dense mountains and the presence under the mountains of less dense “roots” that extend below the mantle floating on the more dense Magma below. The book never says “The plumb bob swings away Himalayan Mountians.” On p197 it does say “Along the shores of South America - a pendulum swings away from The Andes and toward the abyssal floor of The Pacific.” "

Could I ask, then, how did you get the impression that the deflection is away from the Himalayas, if not from Wilford? I'm just curious if you found some source suggesting that...

The South America example Wilford provides is an excellent case of a mountain range immediately adjacent to a deep ocean trench. You get deflection towards the Andes on one side and deflection away from the trench on the other side. In between there is a "stripe" of zero deflection many miles wide paralleling the coast. The subduction zone and the Andes are all one active tectonic complex. That's why Peru and Chile have dangerous volcanoes and sometimes colossal earthquakes. It's the southeastern edge of the Ring of Fire. By contrast, the Himalayas have been uplifted by a collision of two "continental" plates without a major subduction "trench".

Parenthetically in your post you mentioned the modern prime meridian:
"There was an earlier discussion on NavList of the 1984 movement of the Greenwich meridian by 335 [feet] to the east of the original 1884 location. There was a NavList discussion about this ten years ago, the last entry was Frank’s on 2015 Aug 19. The subject got around to extra local mass that had deflected the 1884 Greenwich plumb bob. I don’t think I, or anyone else at the time, speculated on where that mass might have been. (ie: Did Greenwich zero move towards, or away from, the different mass?)"

This is a measure of the difference between astronomical and geodetic coordinates. The DOV at Greeenwich is only a couple of seconds of arc. The meridian line at the transit instrument in Greenwich still has zero astronomical longitude. Want more? Start with the summary article from AGI which is the unlikely "home" for this research. A key point in the article is that this was about time-keeping more than geographic coordinates. The prime meridian could have been "forced" to the line of the old transit instrument at Greenwich, but moving it slightly kept the definition of UTC stable.

Frank Reed
Clockwork Mapping / ReedNavigation.com
Conanicut Island USA