In the bygone era of high seas sailing, ships were supplied with small sand glasses known as "log glasses" that measured out short intervals of time (14 to 30 seconds). Their purpose was to help measure the speed of the ship by timing the release of a knotted line, the "log line" over the stern. At the end of the log glass interval, the line was retrieved and its length was resolved into the ship's speed in "knots."
Today log glasses are rare--they were easily broken, and the few surviving ones can be mistaken for egg timers. Recently, at a local antiques show, I encountered a small "hour glass" that I asked to see. On turning the glass, and measuring its emptying time with my wrist watch, it appeared to be a 14 second glass. Eureka!
A pretty glass, it is made of rosewood with a wonderful dark patina. The four posts between the ends are nicely turned, and through-mortised into the ends with wedged tenons. There is a little of the sand adhering to one of the glass chambers, but this is trivial. The good people at the New Bedford Glass Museum certified that the glass is old and hand blown.
When I looked closely at it at home, there is a faint inked mark on the top (sans serifs) saying, "This End Up," and the number "14" at the center. There can be no doubt that this is a 14 second log glass that likely dates from 1860 or thereabouts. So what were log glasses, and how were they used?
A most important factor in resolving the dead reckoning equation was that of vessel speed. This problem was solved in the 18th century, at least, with the use of a log chip, log line and a reliable clock. Captain James Cook in his fabulous three voyages of discovery understood the relation of degrees of latitude to nautical miles. Thus, if the earth is a perfect globe with a circumference of 360 degrees, each degree should should measure 1/360 of the linear circumference. With a latitudinal degree equal to 60 nautical miles the speed of a ship could be determined if there were a method available to measure the linear progress of the vessel over a discrete time interval. Cook used the idea of a "log line" -- a light line payed off the stern of the ship and "anchored" in the water to a "chip" of neutrally buoyant wood that remained essentially stationary, and was unaffected by the log line attached to it. Cook used a time interval of 30 seconds for this measurement--which was made using a small sand glass (Log Glass) constructed to run for just 30 seconds. When the log line was "heaved," the glass was turned. The log line was then stopped (nipped), when the sand had run its course. The log line was measured in equal segments that were originally marked with knots tied in it. The knots in the log line were spaced according to the purposes of calculating a ship's change in position by dead reckoning.
The actual distance of a nautical mile was not well known in Cook's day (it was overestimated), so Captain Cook's dead reckoning positions were probably a little in front of reality; but the method worked. The problem was that in the Eighteenth Century the understanding of the length of a nautical mile (1/60 of a latitudinal degree) was not exactly known (basically because the equatorial circumference of the earth was not exactly known), so that various combinations of the knotted segment lengths of the log line, and the times that the log line was deployed were used. A series of trials run by Captain Constantine John Phipps, "A Voyage Towards the North Pole," Dublin, 1773 (coincident with the Cook voyage era) determined that the knotted segment should ideally be 45 feet long with a glass time of 30 seconds. (Thanks to my brother-in-law, Ray Edinger, for calling this reference to my attention).
Subsequently the technique was refined in ways that made the calculation of vessel speed almost automatic, and very easy to accomplish. First, the log chip was improved, and was resolved into a thin (1/2") of wood (usually pine) that was shaped like a quarter of a pie. The straight edges usually had a length of about 6 inches. The rounded edge was weighted with enough lead to make the chip only slightly buoyant, and to hang vertically in the water with the apex upward. The log line was fastened to it (by a bridle) at two points (sometime three) through holes at the bottom corners. Sometimes one (or two) of these bridle lines were equipped with wooden plugs that could unsnap when the log line was nipped. This allowed the log chip to be retrieved more easily, since it would then lie flat in the water while being reeled in from only one bridle point.
This cut from Bowditch's
"Practical Navigator" 22nd Edition, 1852, shows a log reel (fig. 4), log chip
(fig. 3) and log glass (fig. 2).
It is interesting that in this edition, the 30 second log glass was still considered the standard.
This is an actual 19th century log chip with bridle attached. It is in the collection of the New Bedford Whaling Museum (00.106.20). The straight sides measure 5 inches. The lead insert on the curved side can be seen. The top corner was fastened to a lanyard with a wooden float of unknown purpose--perhaps to keep the apex of the chip upright, and be visible to from the ship. The bridle is about 3 feet long.
A second innovation made during the 19th Century was the near universal use of a 28 second interval, rather than the 30 second one used in Cook's day. This allowed a further standardization of the log line. To facilitate ease in measuring the length of the line deployed in a trial, the line was divided into equal segments. The segment was determined by making its length proportionate to a nautical mile in the same ratio as 28 seconds is to an hour (3600 seconds). When a nautical mile is considered to be 6,070 feet--the length most often used in the 19th century--the segment length becomes 47 feet, 3 inches. Through the second half of the 19th century the segments laid out on a log line were marked with "knots" tied in the line, each 47 1/4 feet apart. This means that if, during a 28 second deployment, almost 142 feet of log line (3 segments) were "pulled" overboard, the vessel's estimated speed would be about 3 nautical miles per hour. In the sailor's parlance the speed would be 3 "knots"--for the three markers for the segments deployed. So a "knot" is a nautical mile per hour. This is now the accepted manner of speaking of the progress of ships through the water, as well as the velocity of winds at sea, and the movement of ocean currents.
This is a 28 second log glass in the New Bedford Whaling Museum's collection (2001.100.3164). It is plainly made of pine and stands 4 3/4 inches high. On the top is a paper label reading, "J & A. Wallcer. 72 South Castle-Street. Liverpool." The collection also boasts one other 28 second log glass, but none of the 14 second variety.
The arithmetic shows that as sailing vessels became faster over the nineteenth century log lines had to become longer and longer. At a speed of 8 knots, say, almost 380 feet of line would be deployed, and have to be recovered following the measurement. To facilitate this, the lines were stored on wide wooden reels equipped with crank handles for the retrieval. Instead of bulky knots, the segments were marked with colored rope strands or bits of fabric spliced into the lines.
And, as ships became even faster (think of the clipper ship era, when speeds of up to 20 knots were attained), log lines of almost 1000 feet were needed. Rather than dealing with such awkward lengths of line, a final refinement was made, and this was in the timing interval--the log glass.
Log glasses apparently changed little over the almost three centuries of their use. They were hand blown in the traditional wasp-waisted "hour glass shapes", with one end initially sealed while the other was filled with fine sand until the desired emptying time was achieved. Then the second chamber was sealed, and the glass mounted in a protective wooden enclosure that had similar ends or feet. The easiest way to alter this device (to reduce the length of log line required) was simply to add less sand, so that the time interval was shortened. And to make the subsequent speed calculation simple, the time interval was cut exactly in half. So, in faster ships a 14 second watch glass was used. In practice the log lines were not altered by making the segments shorter. Rather, in using a standard log line with a 14 second log glass, the resultant number of "knots" counted after the heave was simply doubled to estimate the speed of a ship. So 7 knots deployed, let's say, resulted in a 14 knot speed estimate.
Of course, with the advances of technology, "heaving the log" is a bygone navigational practice. By the end of the nineteenth century, mechanical "taffrail logs" were introduced. These were mechanical devices that were towed aft of the moving ship and retrieved to directly read the digital record of the revolutions of an internal impellor that could be resolved into distance travelled. And of course today we enjoy the ubiquitous GPS systems that have obviated not just dead reckoning navigation, but celestial techniques as well.
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