Question

This passage is adapted from an essay on eighteenth-century horology and maritime navigation.

For centuries, seafaring navigators could easily determine their latitude—their north-south position on the globe—by measuring the angle of the sun or the North Star above the horizon. Determining longitude—their east-west position—proved far more elusive. Because the Earth rotates 360 degrees every twenty-four hours, a difference in time of one hour corresponds to a fifteen-degree difference in longitude. To calculate longitude at sea, a navigator needed a clock that could maintain the precise time of a reference meridian, such as Greenwich, England, while the ship traveled across unpredictable ocean currents and varying climates.

In 1714, the British Parliament passed the Longitude Act, offering a reward of £20,000—a massive sum at the time—to anyone who could develop a practical method for determining longitude within half a degree after a voyage to the West Indies. Most astronomers believed the solution lay in celestial navigation, specifically tracking the movement of the moon against background stars. However, an unschooled Yorkshire carpenter and self-taught clockmaker named John Harrison envisioned a purely mechanical solution: a sea-going clock of unprecedented accuracy.

Harrison began his quest in the 1720s. His first three marine timekeepers, designated H1, H2, and H3, were massive, intricate machines. Weighing upwards of seventy pounds, these mechanisms were suspended inside gimbaled brass frames designed to counteract the rolling motion of sailing vessels. To eliminate the need for liquid lubricants, which thickened or evaporated in extreme temperatures, Harrison crafted many internal parts, including gear teeth, out of lignum vitae, a naturally oily tropical hardwood. While H1 through H3 demonstrated remarkable stability during localized sea trials, Harrison gradually realized that their large, swinging pendulums and heavy balance wheels remained vulnerable to severe oceanic turbulence.

In 1755, Harrison took a radical departure from his previous designs. Abandoning the large clock format entirely, he began constructing a portable timepiece roughly five inches in diameter, resembling an oversized pocket watch. Completed in 1759 and designated H4, this fourth timekeeper represented a revolution in horological engineering. Unlike his earlier sea clocks, which utilized wooden components made of lignum vitae, the H4 watch relied primarily on polished steel and brass gear wheels to withstand thermal expansion and mechanical stress. Furthermore, H4 incorporated a fast-beating balance wheel driven by diamond pallets, allowing it to maintain precise oscillation even when subjected to sudden, violent ocean swells.

In late 1761, Harrison’s son, William, embarked on a trial voyage aboard the HMS Deptford bound for Jamaica, carrying H4 inside a locked box. Over the nine-week transatlantic journey, the pocket watch lost just five seconds, corresponding to a longitude error of less than two nautical miles—far exceeding the accuracy mandated by the Longitude Act. Despite this resounding triumph, the Board of Longitude delayed awarding Harrison the full prize money for years, demanding further tests and complete disclosure of his manufacturing techniques. Nevertheless, Harrison’s H4 had irrevocably demonstrated that mechanical precision could conquer the perils of oceanic navigation.

Based on the passage, how did Harrison's H4 timekeeper differ from his earlier marine clocks (H1 through H3) regarding its internal components?