Probably the first documented mention of an equation clock is in the diary of John Evelyn who recorded that in 1666 he visited the Royal Society where he witnessed a curious clock, which showed the equation of time, being presented by a certain Mercator. More data on the subject appeared in the first two decades of the 18th century, when Henry Sully, Joseph Williamson, Daniel Quare, and Thomas Tompion--who were among the foremost English clockmakers of all time--produced elaborate examples of these timepieces. Another significant maker was Dowe Williamson, who became Court Clockmaker to Emperor Charles VI of Austria. In London, Joseph Williamson produced some of the finest astronomical timepieces of this type that have been known. The interest in the subject next shifted to France where many fine examples were produced during the first half of the 18th century. Just after the middle of the 18th century, the subject of astronomical clocks suddenly became a major horological preoccupation in another region, namely, Austria, where the work in this field was apparently done exclusively by members of the clergy. The earliest was Father Philipp Matthaeus Hahn (1739-1790) of Wuerttemberg.[4] Father Hahn considered the equation of time as only one part of a plan to represent astronomical occurrences by means of clockwork. In addition to planetaria and similar mechanisms, Father Hahn produced two extraordinary astronomical, tall-case clocks, both of which survive in public museums. * * * * * ACKNOWLEDGMENTS The author wishes to acknowledge the valuable assistance received from the following: DR. AMOS AVERY, Amherst, Massachusetts; MR. EDWIN A. BATTISON, curator of light machinery and horology, U.S. National Museum; DOTT. RICHARD BLAAS, Oesterreiches Staatsarchiv, Vienna; DOTT. ADOLFO CETTO, librarian, Biblioteca Comunale di Trento, who made copies of Borghesi's two volumes available; SIGNOR MARIO DI MARIO, editor of _La Clessidra_, Rome, who permitted several of the illustrations in Sig. Luigi Pippa's article to be used herein; MR. WALTER A. GILBERT, Norwich, Connecticut; DR. HEINRICH LINARDI, Uhrenmuseum der Stadt Wien, Vienna; SIGNOR LUIGI PIPPA, Milan, Italy; CAV. ING. GUIDO UCELLI DI NEMI, Presidente, and DOTT. FEDERICO MORELLI and CAV. ORAZIO CURTI of the Museo Nazionale della Scienza e della Tecnica, Milan, for their cooperation on the descriptions and illustrations of the restored clockshop of Bartolomeo Antonio Bertolla; and DR. EDWARD WATERS, Division of Music, Library of Congress, Washington, D.C. The translation from Francesco Borghesi's Latin texts, which made this study possible, were made by: REV. NEIL HERLIHY, S.J., REV. FRANCIS J. HEYDEN, S.J., and REV. STEPHEN X. WINTERS, S.J., Georgetown University, Washington, D.C.; and REV. DANIEL HUNTER, O.P., and REV. ROBERT STENGER, O.P., Dominican House of Studies, Washington, D.C. * * * * *