Sherlock Holmes comments, in The Hound of the Baskervilles , that ‘the world is full of obvious things that no one by any chance ever observes.' The remark is apropos to the wristwatch, and to where the attention of the modern collector is directed. Distracted by elaborately shaped cases, novel time displays, and exotic complications, it is easy to forget the almost offensively simple fact that at its most basic, a watch is a machine that allows a mainspring to unwind at a particular rate. The hands simply exist, in a very real sense, to let the owner know how fast the mainspring is unwinding.

An almost equally offensively simple fact is that for a mainspring to continue to power a watch, it has to be wound up periodically. While it seems obvious that a watch that is wound once a day should only need the ability to run for 24 hours, in fact a longer power reserve is needed. In the first place, a watch with only 24 hours of power reserve would be apt to stop should the owner be tardy in remembering to wind his or her timepiece by so much as an hour or two. Less obvious, and less well known to many owners is the fact that not every part of the mainspring's power delivery is uniform.

But consider: as a spring unwinds, the amount of power it delivers is variable; the more tightly the spring is wound, the more energy it delivers through the power train to the escapement, and as it unwinds, power delivery begins to drop.

At the end of a watch's power reserve, when the spring is almost completely unwound, balance amplitude- the number of degrees through which the balance wheel swings as it ticks and tocks- begins to drop off, and stability of the watch's rate begins to suffer.

A longer power reserve ensures that during ordinary use, a watch wound daily will use only that part of the mainspring's output which is most uniform- thus ensuring a better chance of fulfilling the owner's expectations of accuracy.

In the dimmest, darkest, earliest days of watchmaking just keeping a watch running for a day was a challenge.

The first mechanical clocks were almost certainly driven, as modern pendulum clocks still are, by a falling weight (leaving aside earlier and largely lost exotica such as the Greek and Chinese clepsydras, or water clocks) and spring driven horologia of the earliest type had only three wheels in their power train (modern watches have four) and typically ran for a mere fourteen or so hours.

Such watches and clocks were remarkable not because they ran accurately but rather because they ran at all. Good timekeeping in clocks had to wait until the development of pendulum escapements, whereas in watches, it was not until the development of the spiral balance spring that watches could reasonably be expected to keep time to within a few minutes a day.

Under such circumstances, the fact that a watch would often run for less than 24 hours was not a matter of great practical concern. Early mainsprings reflected all the relative deficiencies of the early days of watchmaking; they were made of steels that had a tendency to rapidly lose their elasticity and delivery of power to the escapement was variable to say the least.

Long power reserves, of days or even months duration, were until relatively recently the exclusive domain of clocks (which, unlike their portable brethren, could often be expected to keep reasonably accurate time). The length of a power reserve is to some extent a function simply of how large a mainspring a watch or clock can contain (or in the case of a weight driven clock, how long a fall is available for the weight) and the metallurgy necessary to provide a powerful, long mainspring which would not break or set in an inelastic and inefficient shape did not develop until fairly recently.

Late 1500's, anonymous weight driven striking and alarm clock with 3 wheel gear train. Image courtesy Antiquorum

The late 19th century saw the development of eight day pocket watch movements by a variety of manufacturers. Of the many makers who produced eight day pocket watches, perhaps the best known is Hebdomas.

The name itself reflects the nature of the watch; Hebdomas is from the Latin word for ‘week,' and was originally taken from the Greek hebdomos , seventh, or a group of seven things.

The eight day watch's claim to fame is not, of course, that it will run for eight days but rather that it will run for a week; the additional day's power reserve helps to ensure that over the very long running time of the watch, the last turnings of the mainspring in the barrel are not used, as these would deliver low energy and negatively affect the rate.

God may have rested on the seventh day; the owner of an eight day watch however is advised to rouse himself and wind his timepiece.

Unusual, astronomical eight-day carriage clock with lever escapement by Breguet, 1816. Image courtesy Antiquorum

Hebdomas watches are fairly easily found and form an interesting area of specialization for the collector. They achieved their extremely long power reserve by the expedient of having a mainspring so large that the barrel was the diameter of the entire case, and as a result, the balance cock and balance were shifted to the dial side of the plate.

Hebdomas eight day watches are conspicuous in that they usually have a cutout in the dial to expose the balance which is held in a bridge screwed to the dial side of the watch; they were among the first watches in which the balance could actually be seen through an opening in the dial.

Hebdomas was perhaps the most popular manufacturer of eight day watches and their movements were of good quality, jeweled to the center wheel, and with a Breguet overcoil hairspring.

8-day Hebdomas pocket watch, circa 1900. Image courtesy Antiquorum

Other manufacturers made eight day watches as well, and some of these movements, originally intended for use in pocketwatches, came to be used as wristwatches; there are numerous examples which were designed to be used as military wristwatches.

Eight day watches were generally rather big as the mainspring was considerably larger as a rule, and such watches, when adapted for military use were like most modified pocket watches, worn on the wrist in a cuplike receptacle of leather, and held in place with a hefty leather strap that would have done well as a toddler's belt.

Parmigiani Fleurier Kalpa XL Hebdomadaire Squelett, with 8-day power reserve.

Although Hebdomas was perhaps the best known and most popular maker of eight day pocket and wristwatches they certainly did not hold a monopoly and eight day movements were made by a variety of other manufacturers, including Jaeger LeCoultre, Longines, Cortebert, and Arogno (the latter often in the form of eight day alarm pocket watches).

Eight day movements in addition to finding their way into wrist and pocket watches were also useful as car dashboard clocks, particularly as a motorist was unlikely to remember to wind the clock on an automobile dashboard as frequently as a watch carried in the pocket or on the wrist

Extremely rare, valuable complicated 8-day striking watch made for the Sixth Nizam of Hyderabad , Mahbub Ali Khan (reigned 1869–1911). Image courtesy Antiquorum

Though popular as a niche product, from a practical standpoint, eight day watches were doomed by two factors:

One: the increasing fashion, as the decades passed, for flat, small wristwatches (inherently incompatible with long power reserves given the metallurgy of the time, as the necessary large mainsprings made for bulky watches)

Two: the invention of practical, reliable automatic watches which in most cases made the ability of a watch to hold a power reserve longer than the usual 36 to 40 hours simply unnecessary.

Jaeger LeCoultre Master 8 Days Perpetual Squelette and Panerai Radiomir 10 Day GMT in Pink Gold

Practically speaking, modern watches with an extended power reserve serve an impractical purpose, as a general rule, if they can be said to serve a purpose at all.

They simply allow the avaricious modern connoisseur who is laboring under the pleasant burden of more watches than can possibly be worn, to be reasonably sure that when picked up after being ignored for several days a watch will still be found to be running.

Nonetheless, they're technical accomplishments; a very long running time requires a mainspring with very particular metallurgical characteristics.

Panerai Radiomir 10 Day GMT in Pink Gold.

Modern alloyed mainsprings can be made many times longer than the blued steel mainsprings of days gone by; they can be wound and unwound comparatively indefinitely without taking a set and experiencing a reduction in power (or breaking under the strain of repeated cycles of winding), and they can be shaped in such a way as to produce more consistently even torque at the beginning and end of running time.In addition, multiple mainspring barrels are frequently used, which can be coupled to run either in series or in tandem.

These barrel configerations, combined with better tolerances in the power train and escapement which maximize the efficiency of delivery of energy to the balance, allow running times of many days to be achieved in sometimes remarkably small spaces - though extremely long power reserves in general still tend to make for watches somewhat bigger than average.

IWC ‘Big Ingenieur' with 7 Day automatic caliber 51112.