NavList:

   

Reply

Bruce Stark wrote:
"The old navigation manuals suggested checking the  log line and half minute
glass occasionally. One way to check the glass was by  pendulum. As I recall,
the length of the pendulum, to the center of the musket  ball that formed the
weight, was sometimes given as 39 and 1/4 inches, and  sometimes as 39 and 1/8
[small typo corrected]. Count a second each time the  pendulum passed the
bottom. I suppose you had to give the pendulum a few moments  to settle the
length of its swing. "

For what it's worth, the difference  in length suggested (an eighth of an
inch) is comparable to the difference that  arises from the variation of g with
latitude. Personally, I don't think the  length of a crude pendulum can be
measured to better than an eighth of an  inch in any meaningful way. One problem
is that the length refers to the  center of mass of the whole pendulum. If the
plummet is a sphere (like the  musket ball you mentioned) that helps a lot.
Another problem is that the length  of the string will change.

For a two second pendulum period, I get a length of 99.6cm at the poles and
99.1 at th equator (cos(2*lat) variation in between).

-FER
42.0N  87.7W, or 41.4N 72.1W.
www.HistoricalAtlas.com/lunars

   

Reply