The Mariner's Chronometer

As usual, figures may be enlarged by clicking on them.

Like the Hamilton M22, this is not strictly a chronometer, in that it does not have a chronometer escapement and other features associated with mechanical chronometers. Rather, it is a high quality lever escapement watch with some special features, adapted mainly to the needs of the aviator and the requirement to manufacture many tens of thousands to meet war-time demands. The Hamilton Watch Company increased its production of precision watches following the entry of the United States into the Second World War in 1941. Its 16-size 992b railroad quality pocket watch formed the basis for their Watch, Navigational, Master, most of which, 73,285, found their way into Air Force use, while 21,1512 were assigned to the US Navy, with 312 going to the Royal Canadian Navy. The movement was designated 4992b. Between 1941 and 1969, when production ceased, about 140,00 had been made. An advertisement in Flying Magazine in February 1948 showed the watch selling new for US$67, while reconditioned ones were available for US$28 or $660 and $266 respectively in 2014 dollars, allowing for a cumulative rate of inflation of 885%. While it retained the fine quality of the 992b, it differed from it in having a sweep second hand , a 24 hour dial reading “GCT” (Greenwich Civil Time) and a hacking function, allowing the movement to be stopped easily by pulling out the crown so that the time shown could be set precisely to GMT. It was also up-jewelled to 22 jewels. Figure 1 shows the layout of the face. The case was generally of chromed nickel silver, though some early ones, made for the Navy, were of silver. Both the back and the bezel screw on.

Figure 1: Face (bezel and crystal removed)

Figure 2 shows the movement. Except for the hacking mechanism, regulator and centre seconds wheel, the layout is that of a conventional lever escapement pocket watch.

Figure 2: General arrangement of the movement.

However, a complication  is introduced by having the sweep centre hand. The third wheel pinion is driven by the centre wheel as usual, but the third wheel arbor has an extension through the (jewelled) barrel bridge and the centre seconds wheel is a tight, friction fit on the end of the arbor. This wheel drives the centre seconds pinion whose arbor passes through the centre wheel and its pinion on its way to the dial. A delicately forked stabiliser spring bears on the underside of the pinion to keep its pivot engaged with its jewel. The regulator acts on the Breguet over-coil of the Elinvar-type balance spring in the usual way, but is micro-adjustable by means of the regulator screw, with counter-pressure applied by a goose-necked spring, as shown in Figure 3. Elinvar is a nickel-iron alloy whose co-efficient of elasticity varies hardly at all at normal room temperatures, so that the compensation required at the balance wheel is small and taken care of by the ovalising balance, for details of which see my post number 11  . This combination gives exceptionally good temperature compensation. The regulator screw has two slots at right angles  and is rather difficult to access with a screwdriver, as the centre seconds bridge gets in the way of aligning the ‘driver square with the screw-head.

Figure 3: Regulator.

When the crown is pulled out, the watch stops. The origin of the term “hack” in this context  seems to be obscure. “Hack” can mean “for common or ordinary use” and was used in this way to describe a lesser watch, a “hack watch” used to carry time from the chronometer to the deck when taking sights of celestial bodies. Such a watch was also called a comparing or deck watch. The term can also mean a stammer or intermittent cough and it was perhaps by analogy with this that the term arose. Figure 4 shows how the mechanism works.

Figure 4: Hacking mechanism.

When the crown is pulled out, several things happen: the winding pinion is disconnected from the winding stem; the clutch is engaged so that the hour and minute hands can be set; and pressure on the end of the setting lever is released. The setting lever spring then causes the setting lever to rotate about its pivot and the delicate spring finger is brought into contact with the rim of the balance wheel, bringing it and the watch to a halt. The spring finger is labelled in Figure 2 and is also clearly visible above the regulator screw in Figure 3. The watch was used mainly by aircraft navigators and was then provided with a round metal container in which it was suspended on springs. The face was viewed through a window in the lid and the ferrous material of the container shielded the watch from stray magnetic fields.. This case, beautifully photographed by the Smithsoniam Museum, is shown in Figure 5.

Figure 5: Outer case for aircraft use (Courtesy of Smithsoniam Museum)

Naval users preferred their watches to be in a more traditional case and one such is shown in Figure 6. In the original outer cases, the watch lay in a pocket suspended by foam rubber pads and when the lid was closed a further pocket suspended on foam rubber lay over the watch so that it was prevented from moving between the pockets while being isolated from shocks. The case illustrated is made from African mahogany and the pockets are suspended on springs, which have the merit of not perishing like rubber, while being susceptible to corrosion. Happily, this old watch, dating from 1942, will not be going to sea again.

Figure 6: Naval-type outer case.

At sea, a navigational watch has as its main requirement a predictable daily rate or in other words an essentially constant daily rate of gain or loss. It matters little that this gain is great or small as long as it is constant, so that the exact time can be predicted from its rate. For example, if it loses 4 seconds a day every day and is showing the correct time at the beginning of the first day, it should be showing 12 seconds slow at the end of the third day and this twelve seconds should be added to give the correct time. Air voyages in the 1940s were generally much shorter than sea voyages, seldom lasting  longer than 10 hours, and the navigator could not rely on his celestial observations to give him position lines much better than 8 or 10 miles from the correct position, unimportant when making landfall from the air, but important at sea. A 4 second error in time leads at the equator to an error in position east or west of one nautical mile and less as the latitude increases. The aviator’s time requirements are thus much less severe than that of the sailor. The makers specification required that the average daily rate should not exceed 5 seconds and that the average of the daily deviations from the average daily rate should not exceed two seconds, rather poorer than for a marine chronometer in good condition. The average daily rate of the 72 year-old watch shown in Figure 6, taken over ten days, was 3.78 seconds losing, with an average deviation from the mean rate of 1.90 seconds, a very creditable performance after so many years.