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Some comments about Herbert Prinz's posting-

>Borda reports (Verdun de la Crenne, Borda, Voyage fait par ordre du Roy en
>1771 et 1772) that the biggest effect on compass deviation is caused by the
>guns. The Endeavour apparently had some 20, I didn't check Cook's other
>ships.

The magnetic affect of a ship's gun, or any other iron object, falls very
quickly with distance, by a cube law if my memory is right. So it's not the
NUMBER of guns that will matter, it's a question of how near is the nearest
to the compass, which will dominate the effect of all the others, and how
big that gun is.

I'm interested in Borda's voyage, ordered by the King. Where did he go to?
Had he produced his "circle" by then, and how good were his longitudes ?

>When possible, observations for variation were also taken on land with a
>chance of better accuracy. For a land observation, I don't find a
>specification of minutes unreasonable. On a compass rose of 15cm diameter one
>can easily read 1/4 degrees, so that an average of 3 measurements would
>result in a nominal accuracy of 5'. At least, this was the reasoning.

Other factors come in, beside the accuracy of reading the dial;
particularly stiction at the pivot.

And even in the ideal situation of the observations being statistically
independent of each other, to reduce any random errors by a factor of three
requires nine observations, not 3, as Herbert must be well aware. However,
at that date, date navigators were not so aware, which is presumably why he
says "at least, that was the reasoning". As for such non-statistical errors
as inaccuracies in dial calibration, those remained, no matter how many
observations were taken.

So I think it's absurd for measured variations to pretend to be to the
nearest minute. The real reason for recording variation to the minute, it
seems to me, is that if you wanted to quote to any finer division than a
degree, then the minute was simply the next subdivision available.

I still think that Cook's quoted variation of 20 deg 59' should just as
well have been rounded off to 21 whole degrees.

George.



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