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George H., you wrote:
"One thing that struck me was the rarity, in that office, of anyone with a 
direct practical interest, or any personal experience, in navigation. I would 
have thought that having a grizzled old retired seadog on-hand might have 
been rather useful, when choosing the bast layout for tables. I wonder 
whether that was the case, also, with their counterparts in the US."

I don't know if it was the same in the US. But it is my impression from 
reading various historical reviews that they got the input they needed from 
numerous interviews and surveys. I think an old seadog as authority might 
have been more trouble than good -- you might end up with a lot of strong 
personal opinions declared to be common practice 'throughout the seven seas'. 
It is worth noting that whatever process the two offices used to get input on 
the design and layout in mid-century (of the almanacs at least), it was very 
successful. The Nautical Almanac today is nearly identical to the first 
edition of the new almanac published in 1958. About the only component that 
could be counted as a failure (in my opinion) would be the short sight 
reduction tables which were added in the 1980s --as was discussed a few weeks 
ago, no one seems to use them.

Regarding Sadler's comments on his dip experiments, you asked:
"Why should the sea surface not be an equipotential near the shore? Is he
talking about gravitational effects from the adjacent land-mass? Or about
the effects of tidal inertia? Or what?"

The tides are only equipotential surfaces in static approximations. Just 
consider that it can be low tide at one port a few hundred miles from another 
where it's high tide. 

"Are these effects, whatever they are due to, likely to be big enough to affect dip observations?"

Maybe in some unusual areas with large tidal ranges (possibly inluding the 
seas around Britain) you might want to worry about that at least 
hypothetically. In order for this to make any difference, you would need a 
significant difference in the height of the tide between your observing 
location and the horizon. Suppose there's a five foot difference, which is 
about the most I can imagine. If your observing location is a hundred feet 
high, then the horizon is about ten nautical miles away. At that distance, 
five feet amounts to about 0.3 minutes or arc which would, in fact, be 
relevant to detailed observations of dip. In addition, the curve of the sea 
surface might be somewhat flattened or somewhat more rounded than the 
curvature of a sphere (not much though). But I suspect that Sadler's 
discussion of tides was just a speculation on his part --something that he 
hoped might explain away his unsuccessful experiments. Unfortunately, dip is 
variable, especially close to shore, and it's the atmosphere's temperature 
profile on a small scale that drives this, not the phenomena of tides.

Incidentally, you could also worry about the heights of hills and valleys on 
the ocean's surface. None of them that I have examined occur rapidly enough 
to affect dip. For example, near the margins of the Gulf Stream, the sea 
height changes by 4 to 6 inches in a distance which, in unusual cases, is as 
short as ten nautical miles. But half a foot seen from ten n.m. distance 
subtends an angle of less than two seconds of arc. So we don't have to worry 
about that.

-FER


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