Next message: Frank Reed: "Re: Timing Lunars with a Rock"
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
