Among the many artifacts observed on the Mardi Gras shipwreck site is a collection of navigation equipment, which includes a marine telescope, a compass (Figure 1), an octant, and three sand clocks (hour glasses). All of these artifacts are located within a few feet of each other, with the exception of one of the sand clocks, which has been identified under the weapons‘chest. By examining each of these instruments we hope to learn important clues about the vessel’s age, nationality, and perhaps even its identity.
Figure 1. A marine compass from the Mardi Gras Shipwreck site can be seen toward the front center of this image. A possible second compass, or perhaps part of the same compass can be seen in the back left of this image.
By the time the “Mardi Gras Shipwreck” was sailing (estimated between 1780 and 1820), navigation had reached a new level of accuracy with advances that were made in determining longitude in the late 18th century. Prior to this time, it was possible to determine latitude (location north/south) by measuring the altitude of certain stars in the night sky,but there was no accurate way to determine longitude (location east/west). Without a method for locating a vessel longitudinally, ships were forced to sail by dead reckoning. With the use of a compass and log line (below), an estimation of location could be calculated by tracking a vessel’s course and speed. However, given this method of positioning, it was often difficult to determine in which direction (east/west) a destination might be located when land was encountered.
In 1714 the British government established the Board of Longitude with passage of the Longitude Act, which offered a reward of £20,000 to anyone that could accurately calculate longitude. By the 1760’s, two concurrent methods for calculating longitude had been proposed. The first method, known as the lunar distance method, required taking measurements of the moon and nearby stars and comparing these observations with their known locations in Greenwich, England. The second method required the invention of an accurate way of telling time at sea, which led to the development of the marine chronometer (sea clock).
By the time the “Mardi Gras Shipwreck” was sailing, both of these methods had been well established, yet each was problematic. The marine chronometer method was highly accurate but very expensive and very sensitive, which could prove challenging when used in the rough conditions typically found at sea. Therefore, very few merchant vessels would have been able to employ this method early on. The lunar distance method, on the other hand, was easily affordable, yet far less accurate.
One of the most diagnostic pieces, based on current video documentation of the site, is the octant (Figures 2 and 3). Octants were used for celestial navigation to measure the altitude of the sun, moon, or stars above the horizon.
Figures 2 and 3. Octant from Mardi Gras shipwreck site (the two red dots are from a laser scale that is 5 inches apart).
John Hadley is credited with the invention of the octant in 1732, which is often referred to as “Hadley’s Quadrant.” Octants were officially adopted by the Royal Observatory of London in 1735.
Octant frames were typically constructed of hardwood (such as ebony) with brass fittings that held two mirrors. Many octants also had maker’s marks across the support frame. Octants received their name from the amount of arc of a circle that they were able to measure, one eighth. Initially octants were scaled by hand, but in 1771 a machine was invented that would mechanically engrave the degree scales on octants. This device, known as Ramden’s Machine (named after it inventor Jesse Ramsden), allowed for more accurate measurements to be taken; it also allowed for the size of octants to become smaller and more easily handled.
Octants were considered personal property, and would have belonged to the captain or his navigators. It was not uncommon for several people on board to possess one or more octants, therefore it is possible that additional octants may be found during our excavation.
A marine telescope (Figures 4 and 5) was identified during the last visit to the site, in July 2006. Telescopes would have been important tools on board a vessel like the Mardi Gras ship.
Figures 4 and 5. Marine telescope from the Mardi Gras Shipwreck site.
According to Waters (1958) telescopes were originally developed for warfare, but eventually came to play an important role in astronomy and navigation. According to Falconer’s Marine Dictionary (1815) the telescope was first developed by John Baptista Porta in 1560. There are two types of telescopes, refracting and reflecting. Refracting telescopes, such as the one identified on the Mardi Gras Shipwreck, contain a series of different lenses through which objects are seen, while reflecting telescopes contain a series of mirrors which reflect light to form an image.
Telescopes such as the one identified on the Mardi Gras shipwreck site would have been useful for general observations such as observing and communicating with other vessels at sea and for aiding in navigation by allowing the user to observe nearby coastlines.
Telescopes of this time period typically consisted of a wood barrel with a brass tube and fittings. Maker’s marks often appear on the brass tube of telescopes from this time period, which could assist in identifying the name of this ship, as well as its nationality.
The remains of at least one compass have been observed in the video footage collected from the Mardi Gras shipwreck site. At this point, it is not known how much of the compass remains but as Figure 6 shows, the brass gimble is clearly evident on the seafloor. It also appears that the magnetic needle may still be intact in the middle of the compass.
Figure 6. Brass gimble, and possible magnetic needle of compass from Mardi Gras Shipwreck.
A possible second compass, or possibly part of the same compass, has also been observed nearby. It was common for ships from this time period to carry more than one compass on board.
An early form of compass was first used in China in the 11th century. By the 13th century compasses were used on some European ships, however, according to Falconer’s Marine Dictionary (1815), the marine compass was invented in 1302 in the Kingdom of Naples by Flavio John de Gioja. So by the time the Mardi Gras ship was sailing, compasses would have been part of the standard navigation equipment, and were the primary means of determining a vessel’s direction.
Sand clocks (also commonly referred to as hour glasses) were a standard part of a ship’s navigation equipment. They were used not only to keep track of watch times, but also to determine vessel speed, and therefore used for dead reckoning.
Figure 7. Two sand clocks (hour glasses) from the Mardi Gras Shipwreck site, note the difference in size.
The use of sand clocks on board ships probably began around the same time as the use of compasses on ships. Both of these instruments were required for determining longitude through dead reckoning. The wooden frame that would have held these clocks are no longer visible, however, based on video documentation, it is evident that at least two different sizes exist at this site (Figure 7).
Figures 8 and 9. Sand clock underneath weapon’s chest.
Two of the sand clocks (Figure 7) are located near the compass, telescope, and octant. The third sand clock was identified underneath the weapon’s chest (Figures 8 and 9).
dead reckoning: determining a vessel’s relative location by tracking its speed and compass course from a known location.
lunar distance: determining relative longitude by measuring the altitude of the moon above the horizon. This method was commonly used by the late 1760s and required the use of an octant, a book of tables, and a Nautical Almanac.
Falconer, William, 1812 A New Universal Dictionary of the Marine. T. Cadell and W. Davies, London.
Lane, Frederic C., 1963 “The Economic Meaning of the Invention of the Compass.” The American Historical Review, Vol. 68, No. 3, pp. 605-617.
Solver, C. V. and G. J. Marcus, 1958 “Dead Reckoning and the Ocean Voyages of the Past.” The Mariner’s Mirror, vol. 44, no.1, pp. 18-34.
Waters, D. W., 1958 The Art of Navigation in England in Elizabethan and Early Stuart Times. Yale University Press, New Haven, CT.