Before the first application of the pendulum to the clocks by Christian Huygens in 1657, when clocks were regulated by a foliot, or balance wheel, they seldom kept time within a quarter of an hour per clay, and the difference between mean-time, the ti me of the clock, and the solar time, or apparent time, the time indicated by the sundial or measured by the passage of the sun at the meridian, was of little consequence. After the invention of the pendulum, the rate of clocks being of roughly two minutes a clay, the variation between mean and solar time, had to be adjusted. Books were written on the subject, and both Flamsteed and Huygens produced tables which showed the equation of ti me. The sundial was then consulted with an equation table, a number of which, including one by Tompion, were compiled. An expensive alternative to find the difference between mean and solar time by equation table, was a clock which showed the equation of time on its dial. In the late 17th century, an 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 to perform the equation..." wrote Evelyn on the 28th August 1666. Naturally Hook 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, it was not before 1669 for then he maintained that equation tables should be used rather than "....overload the clocks with a great many wheels so that they may show unequal time", and Hook was Tompion's adviser in these matters, not the Dutchman. Whether the mechanism of Ilook and Tompion's equation clock followed that of Nicholas Mercator and Frornenteel's, we do not know. But it is almost certain that Hook 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 design of equation clock, for the two earliest extant Tompion clocks with the equation kidney are describes on their dials: 77w. Tompion Invent. Fore more information on the subject, please refer to the book on Tompion by R.W. Symonds: Thomas Toncpion, Second edition, Spring Books, 1969. As opposed to most watches with equation of time, on which the difference between the mean time and the solar time is only shown on a sector, giving the number of minutes that must be withdrawn or added to the solar time to find the mean time, in this watch, the solar time is permanently shown by the gold "sun" minute hand which revolves, following or preceding the regular minute hand of the mean time. Watches with equation of time are extremely rare. Very few were produced in the 18th century and the first part of the 19th century, by very eminent makers such as Ferdinand and Louis Berthoud, Lepaute, Lepine and Le Roy, on the continent, Mudge and Ellicott in England; even less were made in modern ti mes, most of them produced in Switzerland, carrying the most prestigious signatures such as Louis Audemars, Audemars Piguet and Patek Philippe. Only few watches by Patek Philippe, with equation of time are known to exist, most of them are described and illustrated by Martin Huber and Alan Ranbery in their book Patek Philippe Pocket matches : - No. 24920 (Fig.176) was exhibited in Paris in 1867; - No. 27116 (Fig 181) was made the same year; - No. 47887 (Fig. 215) was sold in 1879; - No. 111505 (Fig. 173) started in 1898 was sold in 1916, - No. 198023 (Fig. 206) was sold to Packard in 1927; - No. 198385 (Fig. 250) was sold to Grave in 1933; - No. 844000 (Fig. 247) known as Calibre 89 is to date the most complicated watch ever made. It was also produced in rose gold, white gold and platinum. - No. 866714, made in 1982 for Beyer in Zurich, was sole] by Antiquorum in Geneva, 7 'he Art of Patch Philippe, on 9 April 1989 (lot 253). - No. 80772, the watch now offered for sale.