NavList:

Lars,

I've been trying to phrase this carefully so that I don't sound like I'm disagreeing with you on facts. There's a difference in emphasis, however, because I was describing chronometer rating in the context of an imagined explanation for the high-power inverting scope among late 19th/early 20th century navigation sources. I'm not advocating that specific model of chronometer rating.

You wrote:
"First I would dare to say that no one took his chronometer to a clock shop for any other reason than repair, or regular service. Moving a chronometer will likely affect its rate and is thus to be avoided."

Some recommendations about chronometer care can create the impression that even breathing on a chronometer will do it terrible harm. Breathe on it!? Don't even look at it!! But in the real world, this extra-special care was "cultural". It was an attitude. Some navigators were obsessive-compulsive. Some... not so much. And yes, navigators did take chronometers to "clock shops" for rating. It wasn't a rare thing.

You added:
"Instead a portable watch could be used, compared with the chronometer before being taken ashore, then compared with the correct time, and at last again compared with the chronometer when back on board; after as short time ashore as possible."

This isn't really a significant difference when it comes to error determination, and I agree that this a safer and more practical procedure in most cases (of course it is). But what you have just described, by narrow definitions, is not the determination of the rate, but the cumulative error. This distinction between checking a chronometer's error and checking its rate seems small to us today, and arguably it is small, in point of fact. But the reason I brought it up was to discuss the odd excuse for the high-power inverting telescope that seems to have emerged by co-optation later in the 19th century. After lunars became superfluous, the putative reason for that scope's existence shifted, necessary or not.

"Chronometer ratings at sea were performed whenever a good opportunity arised."

Again, you're talking about getting the cumulative chronometer error, which implies some information about the rate, but isn't quite the same thing. And like the case you describe this was easily done by ordinary sextant sights when close to any land with a confidently-known longitude [I emphasize "ordinary" sights here because this sort of test did not provide any justification for a more powerful sextant scope]. In the Atlantic, on a passage from high northern latitudes down to the southern end of the ocean, there were multiple locations where this could be done. I have called this the "Atlantic slalom course" occasionally because, like in slalom skiing, these various tiny or otherwise uninteresting islands were directly targeted as waypoints on many voyages, and it seems clear that they did this as a convenient check on the longitude. In the era when chronometers were not yet widely available or not regarded as reliable, these were counted as direct checks on longitude. In later decades, they were tests on the chronometer error. Two of the locations you mention, Fernando Noronha and Staten Island turn up in logbooks regularly. Another pair off Brazil, Trindade and Martim Vaz, are in the same category. Without this "slalom course" for longitude, no one would have paid any of these islands much attention. But knowing them and their more famous larger cousins (Azores, Madeira, Canaries, Cape Verde, Ascension, St. Helena), it was possible to work down the Atlantic with a rather mediocre knowledge of Greenwich Time.

Why fuss over error versus rate? Let's consider some scenarios (100 to 150 years ago). Suppose I arrive at some island with a known longitude after 20 days at sea and discover that my chronometer's error amounts to ten seconds slow, then that implies that its rate is 0.5 seconds per day. Easy, right? But we should note that this is the average "rate" of the chronometer during the previous twenty days. After we leave the island on the next leg of our journey, we want the best estimate of the rate going forward. We want the chronometer's error, yes, to reset the GMT (not an actual physical reset, of course, but an arithmetic reset), and we want to know how the chronometer will behave as best as possible in the next 10 or 20 days. We want its instantaneous error and its instantaneous rate (rate of change of error) to put the matter in "calculus" terms.

Wouldn't the rate derived from the cumulative error give us a good enough estimate of the rate, and wouldn't that be the best we could do in any case? As far as estimating the current rate from the average rate over the voyage up to that point (over the course of the last leg), yes, that would certainly be an acceptable estimate and probably all we need if --and this is an important condition-- if the chronometer has been subjected to stable conditions on that entire leg. But let's go ahead and make them significantly unstable. Suppose we encounter a major storm two days before arriving at our island. The chronometer has been bounced around a bit, and it would be quite possible that its rate has changed. Then the rate implied by the current error would be a less reliable estimate of the rate going forward. And better yet, instead of arriving at an island with a confidently-known longitude, let's suppose the storm causes enough trouble for us that we must land at an island with only crudely known longitude.

How can we get the chronometer's rate so that we can sail confidently on the next leg of our voyage if we're not all that sure of our current landfall's longitude, and we have reason to believe that the rate has changed? This is where a specific method for determining rate using an artificial horizon and a high-power scope and lots of attention to the detail of a sight can make a real difference. We do a common time sight on the first morning after we arrive at our "Isla Incognita". We record the exact time and the implied GMT based on some assumed longitude (any best guess will be good enough). Then we come back again two or three days later. If we use high magnification and great care, we can time our sights to perhaps a quarter of a second and from this and the change in the implied GMT over a few days, we can get a good (not great!) rate for the chronometer going forward.

That's the theory, the imagined scenario, that was supposed to provide a good use for a high-power sextant scope in that period before radio distributed accurate time globally. Even without a known longitude to determine the error of the chronometer with certainty, the rate could be assessed by careful sights at a fixed location.

I hope I've explained this properly and sensibly. :)

Frank Reed