By Jon Cooper

Navigation technology was vital to securing imperial power at sea. On naval and merchant ships, astronomers taking celestial measurements using instruments such as astrolabes, backstaffs, octants, and sextants played a central role in discovering, charting, and claiming new markets and territories for empire. Writing in 1576, the English mathematician Thomas Digges noted that there was ‘one great imperfection yet’ hindering the art of maritime astronomy – ‘and that is the wante of exact rules to know the Longitude … without the which they cannot truly give the place or situation of any coaste, Harborough, Roade, or Town’. Almost two hundred years later, surprisingly little had changed. By the mid-eighteenth century, the backstaff and octant were readily available as instruments for celestial navigation, with which astronomers could easily determine latitude by measuring the altitude of the sun or pole star above the horizon. However, the apparent positions of these celestial guideposts do not change as the observer moves east or west. To determine their longitude, navigators had to calculate both local time (ascertained from latitude) and the simultaneous time at the Prime Meridian reference line in Greenwich, London.

Without a workable solution to calculating longitude at sea, navigators had to proceed by dead reckoning, a notoriously unreliable method that involved charting a course by compass and judging currents by experience. The longitude problem became ever more pressing from the early eighteenth century, as Britain came to define its national identity through public science, imperial commerce, and naval strength, with outposts spanning vast expanses of the Atlantic and Indian Oceans. In 1714, Parliament passed the Longitude Act, offering £20,000 (equivalent to over £4 million today) for a method that could determine longitude at sea within 30 minutes. The Act inspired two solutions to emerge as contenders. The first was John Harrison’s marine chronometer, a clock that could keep reference time at Greenwich. The second was Jesse Ramsden’s engine-divided sextant, a more affordable instrument, which with a nautical almanac could also determine longitude in the field.

John Harrison’s H4 marine chronometer – described by Simon Schaffer as among the ‘charismatic megafauna’ of scientific instrument collections – was a state-of-the-art solution to the longitude problem and awarded the most prize money. Unlike pendulum clocks, its complex machinery could keep time at the reference point in Greenwich for long periods with remarkable accuracy, withstanding rough seas and variations in temperature, pressure, and humidity. The H4 was heralded as a mechanical marvel and has since been celebrated in popular histories. But Harrison’s marine timekeeper did not solve the problem of determining longitude at sea in practice. Chronometers were expensive, and their fragility meant that ships had to carry several together. As late as 1804, Andrew Mackay’s The Complete Navigator held that much ‘confidence cannot be placed in time-keepers, as their rate of going is so liable to be altered from the least accidental injury.’  

Like many navigators, Mackay preferred the celestial solution, which involved using sextants to calculate longitude by comparing lunar distances (the measured angle between the Moon and another celestial object) with data from astronomical data reference tables (the nautical almanac was compiled from 1767 under the royal astronomer at the observatory in Greenwich). Where backstaffs and octants sufficed for measuring altitudes above the horizon, sextants enabled the measurement of lunar distance in two ways. First, their arcs measured one sixth of the circumference of a circle, permitting the measurement of wider angles. Since sextants reflected images from an index mirror onto a horizon mirror, the 60-degree arc could be used to read angles of 120 degrees. Second, their arcs had graduated scales precise enough to measure lunar distances down to a sixtieth of a degree. John Bird produced the first sextant accurate enough to measure lunar distances in 1759. But the process of engraving scales with a beam compass was arduous, requiring weeks of skilled labor.

For the celestial solution to the longitude problem to be practicable, it would be necessary to reduce the cost of production. And when Jesse Ramsden completed his dividing engine for cheaply manufacturing sextants in the 1770s, he did exactly this. The dividing engine’s wheel, cut precisely with 2160 gear teeth, enabled unskilled workers to engrave an accurate measuring scale onto a blank sextant frame within minutes. As Allan Chapman explains, ‘Ramsden turned the scientific instrument … into a cost-effective industrial artefact, … where high quality was made to cost less in real terms than ever before.’

I was already familiar with this story when, preparing a final paper for Paula Findlen’s class on material culture in early modern Europe, I discovered such an instrument in the David Rumsey Map Center at Stanford University. As the catalogue description states, this ‘9-3/8” Radius lattice frame sextant’ (pictured) had a scale ‘almost certainly cut on a Ramsden engine’. The Board of Longitude had awarded Ramsden £615 for his engine, on the condition that he disclosed its workings to other instrument makers. This instrument’s maker, George Adams, probably had it cut on an engine made by Ramsden, Troughton, or Spencer, Browning & Rust. Handling and inspecting this carefully preserved sextant on a quiet table, it was easy to forget the bitter winds and rocking decks that confronted marine astronomers trying to use such instruments to take celestial measurements at sea. But sitting from this privileged position was perhaps befitting, since what struck me most about this instrument was that it appears never to have been used. As I probed further, I began to suspect that this sextant hadn’t been purchased for its ostensibly intended use, but rather as a showpiece.


George Adams’s shop at 60 Fleet Street was called Tycho Brahe’s Head – a reference to the Danish astronomer and nobleman whose observations using altazimuth quadrants introduced a new degree of accuracy to the astronomical sciences. Adams apprenticed under his father, who specialized in providing navigation instruments for the Board of Ordnance. Their products were traded across Britain, its provinces and colonies, and across Europe and the world. The Adams workshop functioned as a household, a ‘combination of apprentices, journeymen, employees, and family members,’ enmeshed in networks of patrons and subcontractors such as opticians, metalworkers, and engravers. The Adams name was associated with luxury, precise instruments. For George III’s collection, they manufactured spectacular orreries, vacuum pumps, and the famous ‘philosopher’s table’. Otherwise, they were best-known for their microscopes and globes.

Sextant in George Adams, Description, Use, and Method of Adjusting Hadley’s Quadrant and Sextant (1789).

Adams’s Description, use, and method of adjusting Hadley’s quadrant and sextant (1789) celebrated the improvement of ‘the art of navigation by the present method of finding the longitude, which enables the mariner to ascertain with certainty his situation on the unvaried face of the ocean’. He not only explained how to use his sextant, but also included a detailed diagram and an advertisement for the various navigational instruments available at his shop. This brass sextant was the priciest available, with a maximum cost of £15 15s (around £2,500 today) – over seven times that of an octant in mahogany.

Why would anyone have purchased such an expensive sextant? The instruments made by a fashionable workshop like Adams on Fleet Street contrasted with the cheaper alternatives such as octants and wooden backstaffs still widely favored among seamen who frequented shops at Wapping docks. At a price of over £15, it marked a major investment, when a captain even on a first-rate Royal Navy ship rarely earned more than £400.

One possible answer is that such instruments were not just used for discretely technical purposes, but to mediate social relations and coordinate hierarchies at sea. Eoin Philips has shown that instruments such as chronometers and sextants, which stood ‘as symbols of rationality and enlightenment,’ were vital to maintain the delicate hierarchies and complex division of labor on which astronomical fieldwork depended. He even argues that ‘the solution for discipline and visibility rested more on successful performance than it did on the functioning of the hardware itself’. 

But an instrument as expensive and fine as might well not have been purchased for use at sea. This sextant has suffered none of the wear and tear one might expect from fieldwork, with a mostly untarnished lacquered brass finish. Though the complex telescope tubes, horizon glasses, and moveable shades of sextants were acutely vulnerable to breakage and disrepair, only one of the telescopes is slightly cracked. An optical and mathematical instrument conceived for practical observation, the sextant had become a luxury item, like other ‘toys’ which made business as an eighteenth-century instrument maker profitable. As Alexi Baker has argued, such toys carried different types of meanings: ‘telescopes and microscopes were used to suggest qualities including insight and wisdom [and] globes often gestured towards worldly power and knowledge’. So too, it is worth investigating the symbolic significance of the Adams sextant as a showpiece.


For several decades, scholars in science studies have investigated broken instruments to oppose a tendency in historiography and museology to represent tools as unproblematic symbols of disembodied progress. But as Baker’s work shows, representing immaculate scientific instruments as self-contained tools of progress, far removed from the messy realities of fieldwork, was also a contemporary phenomenon. Eighteenth-century trade cards displayed instruments as ‘idealized and unproblematic tools,’ as in Adams’s engraving of his own sextant. The garbological impulses of critical theory and science studies demand us to problematize not just damage, but spotlessness.

Following the Seven Years’ War, Britain’s expanded territorial empire demanded new magnitudes of investment in administrative machinery. The heroic astronomer stood for an emerging ideology of empire, where authority was predicated on routine precision. The Adams sextant was a resonant instrument of technological imperialism, manifesting attributes such as precision, exactitude, and credibility, which were made possible by the centralized organization of calculation in the metropole. The Adams sextant belonged to a distinctive class of luxury toys and trinkets, in that it appeared to be designed for a distinctly practical purpose: to measure longitude at sea with exceptional precision, in a box designed to protect the instrument from the dangers of fieldwork. It was an emblem of imperial progress: as Adams suggested in his description of the object, ‘in every instance of the progress of science … we may trace some of the steps of that vast plan of Divine Providence … by the advancement of knowledge, the diffusion of liberty, and the removal of error, that truth and virtue may at last shine forth in all the beauty of their native colours’. Simon Schaffer describes sextants as fitting into both ‘narratives of bureaucratic administration and calculated rule as well as to those of ingenious skill and heroic belief.’ 

The administration of the maritime empire was idealized as unproblematic if rational and routinized, conducted by heroic gentlemen like Sir Joseph Banks. As Katy Barrett puts it, ‘“the longitude” articulated a problematic space between polite and impolite science, which made it a useful concept with which to negotiate wider contemporary social boundaries’ in late eighteenth-century London. Whether exhibited in the display cabinets of country houses, or displayed in the shop windows of the metropolis, unworked tools such as this sextant could insulate an imperial ideology of technical progress and genteel astronomy from its troublesome corollary, the grubby materiality of fieldwork. The mathematical and optical instruments which enabled precision, despite their provenance in the workshop and their intended application in the field, when finely crafted and kept out of their assumed context, became urbane symbols of the polite science of imperial geography.

Traditionally, the trustworthiness of precision instruments was associated with the skill and reputation of its maker. The elder Adams boasted in 1746 of his products: ‘That their Exactness may be particularly attended to, I always inspect and direct the several Pieces myself, see them all combined in my own House, and finish the most curious Parts thereof with my own Hands’. Ramsden’s dividing engine shattered assumptions about artisanal credibility. The reliability of a mechanically engraved sextant lay not in its maker’s embodied skill, so much as its maker’s access to cutting-edge machines. As Schaffer argues, the dramaturgical persona of London’s premier instrument maker Ramsden had been routinized by his dividing engine.

Jon Cooper is a PhD candidate in History at Stanford University. His interests span European intellectual history, but his academic work focuses on the history of political economy and the entangled relationships between knowledge and government in early modern Britain and its empire.

Featured Image: Adams Sextant at the David Rumsey Map Center (photo: Jon Cooper)