NavList:

I’m about 1/2 way through The Mapmakers by John Noble Wilford and there are so many goodies to mention! The one that finally got me to write was the saga of surveying India circa 1854 and the effects of the Himalayas on “level”. On pages 195 - 197 they talk about the “Indian Problem” of a 600 km survey across the top of India next to the Himalayas being off by 150 meters. Scientists and surveyors across the world pondered and discussed the discrepancy between triangulation and direct surface measurement (which I think was astronomical measurement). They state that 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”.”

Pause...
There was an earlier discussion on NavList of the 1984 movement of the Greenwich meridian by 335’ 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?)
Pause off.

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”.

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.”

If you can stand being addressed like a child, the Himalayas question is also addressed in:
https://youtu.be/Mn2_WIMza2I

[note from FER: this message has been edited slightly with permission from the author]