Comment by divbzero
13 days ago
> Primary sources:
> 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.
> Using the stars as a reference, Maskelyne’s team found that the plumb lines on either side of the mountain pointed just 0.0152 degrees apart.
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!
Maybe something similar to a vernier caliper.
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.
They had a vertical 'Zenith Telescope' that looked at the same star from two locations. They measured how far from vertical it shifted in the magnified field of view. https://en.wikipedia.org/wiki/Ramsden_surveying_instruments#... Similar instrumends measured the wobble of the Earth's axis. https://en.wikipedia.org/wiki/International_Latitude_Service
I'd love to know what a sufficiently high precision plumb bob is like. Is it very tall? How on Earth does one calibrate it?
Broadly speaking, you want it as tall as possible, usually we're talking a few stories high, so 20m or so.
Without the attracting masses on either side you can set it swinging and measure the period, which lets you compute the restoring force in the wire.