Patek Philippe watches with equation of time are extremely rare. Only 11 are known, two of which broke the world record for the most expensive watch: 24919 - 20'''; sold to Pena, Madrid on May 31, 1865, equation table printed on the back. 24920 - 20'''; exhibited in Paris in 1867, small equation sector. 27116 - 19'''; sold to Pena, Madrid; in 1867, equation sector. 47887 - 19'''; sold on Feb 20, 1879, small equation sector concentric to moon phases. 80772 - 19'''; lot 208 of this sale, equation hand concentric with minute hand traveling from 45 min. to 15 min. 111505 - 20'''; sold to Mrs. Bradford, made in 1898. 198023 - 19'''; James Ward Packard; Patek's most complicated watch until the "Graves" watch. 198385 - 25'''; Henry Graves; sold in 1933, most complicated watch until the Caliber 89. 844400 - Caliber 89, sold in 1989. The most complicated watch ever. 866569 - 19''', this lot. 866714; - 17'''; sold to Beyer, Zurich; in 1982; sold by Antiquorum on April 1989, lot 253.Before the first application of the pendulum to clocks by Christian Huygens in 1657, clocks were regulated by a foliot, or balance wheel, and seldom kept time to within a quarter of an hour per day. Therefore, the difference between mean time, the time of the clock, and solar time, the time indicated by a sundial, was of little consequence.After the invention of the pendulum, clocks' accuracy having improved to roughly two minutes a day, the question of the variation between mean and solar time could be addressed. Books were written on the subject, and both Flamsteed and Huygens produced tables showing the equation of time. The sundial was then consulted using an equation table, a number of which, including one by Tompion, were compiled. An alternative - albeit expensive - to finding the difference between mean and solar time by euation table was a clock which showed the equation of time on its dial. In the late 17th century, the equation clock was regarded as a very ingenious piece of mechanism, a master clock by which all the other clocks in the household were set. The earliest equation clock recorded was one designed by Nicholas Mercator (1640-1687), mathematician and Member of the Royal Society: "Next day, to the Royal Society, where one Mercator, an excellent Mathematician, produced his rare clock and new motions toperform the equation..." wrote Evelyn on the 28th August 1666.Naturally Hooke was interested in equation of time and instructed Tompion on the subject. It has been suggested that the equation kidney was invented by Huygens and that Tompion and other English clockmakers copied it. But we do not know when Huygens first designed an equation clock, with or without the kidney mechanism. However, this could not have been before 1669, for at that date he maintained that equation tables should be used rather than "....overload the clocks with a great many wheels sthat they may show unequal time". It is not known whether the mechanism of Hooke and Tompion's equation clock followed that of Nicholas Mercator and Fromenteel's, but it is almost certain that Hooke and Tompion invented the revolving kidney-shaped cam upon which the working of the equation mechanism depends, and thus perfected the equation clock. Tompion indeed, looked upon himself as the inventor and not just the maker of this equation clock design, for the two earliest extant Tompion clocks wth the equation kidney are bear on their dials the signature : Tho. Tompion Invenit.Watches with equation of time are extremely rare. Very few were produced in the 18th century and the first part of the 19th century, and always by very eminent makers such as Ferdinand and Louis Berthoud, Lepaute, Lepine and Le Roy on the continent, and Mudge and Ellicott in England.Even fewer have been made in modern times, and most of them were produced in Switzerland and carry the most prestigious of signatures, such as Louis Audemars, Audemars Piguet and Patek Philippe.