Most shipmasters making a tortuous passage through a strait or among reefs and islands, after a long passage on one course in a vessel equipped only with magnetic compasses, are worried out of all proportion to the hazards of the manœuvre by reason of not knowing in all certainty what compass errors will pertain on widely different courses. Of course there is the Azimuth Record Book, but the last time similar courses were used could have been in the other hemisphere or under different conditions of trim, &c.

Given time, weather conditions and other suitable opportunity a prudent master will have conducted a swing to ascertain the errors before approaching the stretch of dangerous water. This may not be possible and, again, he knows in his mind that azimuths taken under these circumstances, with little time to waste, do not always conform to the later findings.

At the turn of the century I served in two four-masted barques under three captains hailing respectively from Nova Scotia, Liverpool and Glasgow. All three were rather elderly and their practice of navigation did not exhaust the theory even then included in the syllabus for second mate. However, it may be interesting to recall their methods.

The captains personally had to supply, in addition to sextants and nautical tables, chronometers, charts, ‘Pilots’, patent logs and nautical almanacs. I doubt if they bothered about tide tables. These were all kept in the captains' bedrooms. Each of these captains rented and insured two chronometers. In port, if convenient, these went ashore to a chronometer dealer and just before sailing were brought on board, each with a card showing its maker's name and number with error of instrument and its daily rate. If at sea it was possible to get a longitude by bearings of the land and also longitude by sights, they would be checked and error and rate corrected. Of course in sail it was impossible to maintain an even temperature in the chronometer compartment. It was generally assumed that, owing to effect of change of temperature on the lubricating oil, a chronometer ran slower in cold weather and faster in hot. I don't know when they were cleaned. Perhaps if a ship did return to where the chronometers' owner had his place of business they might be cleaned before being issued to some other ship.