Cavendish was a peculiar fellow.
> At his death, Cavendish was the largest depositor in the Bank of England. He was a shy man who was uncomfortable in society and avoided it when he could. He could speak to only one person at a time, and only if the person were known to him and male. He conversed little, always dressed in an old-fashioned suit, and developed no known deep personal attachments outside his family. Cavendish was taciturn and solitary and regarded by many as eccentric. He communicated with his female servants only by notes. By one account, Cavendish had a back staircase added to his house to avoid encountering his housekeeper, because he was especially shy of women.
A few years later, the gravitational deflection of the Himalayas on a plumb line by Airy proved less than expected, which suggested that mountains have 'roots' that extend below them, displacing more dense rock--like icebergs more or less.
I used the gravitational force of the Longmenshan range to calculate the perturbations in the elastic stress field of the Earth's crust in Sichuan province, China, to estimate the tectonic forces in the region, which caused the 2008 Wenchuan earthquake: https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/201...
> Maskelyne’s notes: https://doi.org/10.1098/rstl.1775.0050
> Hutton’s notes: https://doi.org/10.1098/rstl.1778.0034
> Cavendish’s notes on his own experiment: https://doi.org/10.1098/rstl.1798.0022
I got to reproduce Cavendish’s experiment when I was a student. Love that we can easily read the primary source today, archived and indexed by DOI.
I'm really interested in knowing how they could get such a precise measurement (even accounting for errors), especially in the field (outdoor). There's no figure depicting the apparatus they used, I wonder how it looked like.
Sometimes, I just ponder at how ignorant I am. If I was tasked with the same assignment, I'd definitely fail and this was performed 250 ago!
From Wikipedia:
> The first caliper with a secondary scale, which contributed extra precision, was invented in 1631 by the French mathematician Pierre Vernier (1580–1637).[1] Its use was described in detail in English in Navigatio Britannica (1750) by mathematician and historian John Barrow.[2] While calipers are the most typical use of vernier scales today, they were originally developed for angle-measuring instruments such as astronomical quadrants.
https://en.wikipedia.org/wiki/Vernier_scale
So it would have been a contemporaneous technique with that initial angle measurement, and the use of a Vernier scale for angular measurements would have itself been common.
IIRC, it was partly the Mason Dixon line that inspired this experiment. They noticed syatematic errors in the line because their plumb bobs were deflected by gravitational pull from local terrain. At the time they speculated it was because of the Alleghenies, though it was probably more localized variations in gravity.
or maybe that upcoming space laser interferometer (LISA) since it has to figure precisely how all mass is affecting its position?
I love the history of figuring the circumference of the earth, imagine getting it right within 2% in 240 BC
(then Columbus effing it up by 25%)
https://en.wikipedia.org/wiki/Earth%27s_circumference#Histor...
https://en.wikipedia.org/wiki/GRACE_and_GRACE-FO
The primary challenge in determining the mass of Earth is actually measuring the gravitational constant, G, itself. Everything else involved is known at much higher precision. The product of G and Earth's mass is known to two parts in a billion, but the uncertainty in G is ~22 parts per million.
https://en.wikipedia.org/wiki/Gravitational_constant
LISA is primarily sensitive to time-varying gravitational gradients on timescales of a fraction of a minute to a few hours and won't be terribly useful for determining the orbits of objects in our solar system. (but it is very, very cool).