The ships and sailors of old Salem

by Ralph Delahaye Paine

Prince Henry of Portugal resolved to collect and systematize all the knowledge of nautical affairs obtainable in the early part of the fifteenth century, preparatory to sending forth his intrepid seamen as explorers of the Atlantic, and established an observatory near Cape St. Vincent in order to obtain more accurate tables of the declination of the sun, by which the mariner obtained his latitude in clumsy and unreliable fashion. The sun’s “declination” is its angular distance from the celestial equator, or the angle that a line drawn to the sun from any point at sea or on the earth’s surface makes with the plane of the celestial equator. In other words, the most important early discovery in navigation, next to the use of the magnetic needle, was the use of an instrument by which these angles could be determined and then utilized by means of astronomical tables to find a ship’s distance north or south of the earth’s equator, in degrees and fractions thereof.

John II of Portugal, grand-nephew of this enlightened and ambitious Prince Henry, endeavored to make further advancement in the same field and employed a “Committee on Navigation” to collect new data and make more calculations to lessen the errors in the tables of the sun’s declination. They turned their attention also to the instrument then in use for taking observations at sea, the cross-staff, and recommended that the astrolabe should be employed instead. The shipmaster of Columbus’ time went to sea with a cross-staff or astrolabe, a compass, a table of the sun’s declination, a table for connecting the altitude of the pole star and occasionally a very incorrect chart. The first sea chart ever seen in England was carried there in 1489 by Bartholomew Columbus. The log-line had not been invented and it was not until 1607 that any means was known of measuring a ship’s course through the water.

The cross-staff, as used by Columbus and Vasco da Gamma, consisted of two light battens or strips of wood, joined in the shape of a cross, the observer taking his sights from the ends of the “cross” and the “staff,” on which the angles were marked in degrees. As a device for measuring altitudes, the cross-staff had been known to ancient astronomers, although unknown to their seamen. The astrolabe was a copper disk, suspended from above with a plumb line beneath, and was found to be more convenient for taking altitudes than the cross-staff, and gradually superseded it.

The problem of finding longitudes at sea was far more baffling than that of latitude. It was early discovered that the only accurate and satisfactory method must be by ascertaining the difference in time at two meridians at the same instant, but until the invention of the chronometer this could be done only by finding, at two different places, the apparent time of the same celestial phenomena. The most obvious phenomena occurring to the early navigators were the motions of the moon among the fixed stars, which was first suggested in 1514. Better instruments and a sounder theory of the moon’s course were needed before its motions could be predicted with accuracy and recorded beforehand in an almanac in order to give the mariner a basis of comparison with his own observations, and the very principal of such a theory was, of course, unknown until Newton’s great discoveries, after which the problem of lunar observations began to have a chief place in the history of navigation.

The cross-staff and astrolabe gave place in time to the quadrant, which was a much more accurate instrument for observation and was used by the mariners of the eighteenth century. It, in its turn, was discarded for the sextant during the nineteenth century, which instrument, as improved and perfected, is in universal use at sea to-day for helping to find a ship’s position by means of the measurement of angles with respect to the sun and stars.

The chronometer, for finding longitudes, has taken the place of lunar observations, and the story of the struggle to invent a time-keeping mechanism of requisite accuracy for use at sea is one of the romances of science. Watches were unknown until 1530, but before the end of that century efforts had been made to ascertain the difference in time between two places by means of two of these crude timepieces which, however, were too unreliable to be of any practical service to navigation. The study of the problem was stimulated by the offer of a reward of a thousand crowns by Philip III of Spain, in 1598, to him who should discover a safer and more accurate method of finding longitude at sea than those in use. The States-General of Holland followed this with the offer of ten thousand florins, and in 1674 England became actively interested in the problem and Greenwich Observatory was established for the benefit of navigation and especially to calculate the moon’s exact position with respect to the fixed stars a year in advance and so make the “lunar observation” method of determining longitude a safer guide for the seamen than was the case with the tables then existing.

The pressing need of such investigation was brought home to England by a series of great disasters to her naval force because of blundering navigation. Several men-of-war were wrecked off Plymouth in 1691 through a mistake in their landfall and Sir Cloudesley Shovel, one of Great Britain’s immortal admirals, was lost with his fleet of ships off the Scilly Islands in 1707 because of a mistake in reckoning position. The government became convinced that the whole theory and practice of navigation needed a radical overhauling, and in 1714 a “royal commission for the discovery of longitude at sea” was appointed and at the same time a series of splendid prizes was offered for the invention of an accurate chronometer; five thousand pounds for a chronometer that would enable a ship six months from home to find her longitude within sixty miles; seven thousand five hundred pounds if the limit of error were within forty miles; ten thousand pounds if the position were correct within thirty miles. Another clause of this bill as enacted by Parliament offered a “premium” of twenty thousand pounds for the invention of any method whatever by means of which longitude at sea could be determined within thirty miles. Two years later the Regent of France offered a hundred thousand francs for the same purpose with similar stipulations.

There lived in Yorkshire a young watchmaker, John Harrison, who learned to make better watches than anybody else in England, and he had followed with keen interest the experiments which attempted to find longitude by means of watches set to keep Greenwich Observatory time as nearly as possible. He determined to attack the problem in his way and to compete for these royal prizes if it meant the devotion of a lifetime to the art of making chronometers. He spent years in making one instrument after another until in 1736 he carried to Greenwich a “gridiron pendulum clock” which was placed on board a ship bound for Lisbon. It proved to be accurate enough to correct the ship’s reckoning of observations by several miles, and was a notable improvement on any other timepiece of the day.

The Royal Commission urged Harrison to drop all other work and make a business of competing for the prizes, and offered to supply him with funds. For twenty-four years John Harrison strove to make a chronometer that should win the twenty thousand pounds. He was sixty-eight years old when, in 1761, he wrote the Commission that he had a chronometer which he was willing to send on a trial voyage, and asked that his son William be allowed to go with it to take care of the precious instrument.

The Commission sent the chronometer out in a ship bound to Jamaica in order that its mechanism might be tested by extremes of climate and temperature. On arriving at Jamaica the chronometer had varied but four seconds from Greenwich time. When the ship returned to England after an absence of 147 days, the total variation was found to be less than two minutes, or eighteen miles of longitude. The Commission demanded that the chronometer be given another trial, and it was sent to Barbados on a voyage five months long, at the end of which it showed a variation of only sixteen seconds from Greenwich time, which meant that John Harrison’s chronometer had lost or gained an average of about two-thirds of a second a week.