* University of California, San Diego. In the preparation of this paper the National Edition of Galileo's work, Le Opere di Galileo Galilei, ed. A. Favaro, 20 vols. (Florence: G. Barbera, 1890-1909 and 1929-1934), has been the main source. However, for convenience the quotations reported are from the translation of the Dialogo sopra i Due Massimi Sistemi del Mondo by Stillman Drake and from the translation of the Discorsi e Dimostrazioni Matematiche intorno a Due Nuove Scienze by Henry Crew and Alfonso de Salvio, as will be indicated below, together with the corresponding loci of the National Edition Opere. I would like to acknowledge the helpful suggestions and criticism of the anonymous Isis referee who read this paper. 1 role of the pendulum in the emergence of the concept of absolute time in 17th-century science is discussed in my Ph.D. dissertation, The of the Concept of Time in Science in the 16th and 17th Centuries (University of Melbourne, Australia, 1966). 2 Neither in the Dialogo nor in the Discorsi is the pendulum made the object of an independent study. Its role is that of a very convenient demonstrative device. pendulum is used by Galileo in connection with the doctrine of impetus, with the refutation of the Aristotelian doctrine of contrary motions, with Galileo's theory of tides and nonuniform planetary revolutions, and with his doctrine of freely falling bodies. Yet, Galileo's work on the pendulum is sufficiently extensive that it can be regarded as a piece of research in its own right. 3 isochrony of the pendulum may have been recognized in the 10th century by the Arabic astronomer Ibn Yunus, according to some writers, e.g., R. Wolf in Geschichte der Astronomie (Munich, 1877), p. 368, and P. Miiller, Elementi di Astronomia (Rome, 1904), Vol. I, p. 106, as reported by Adriano Carugo and Ludovico Geymonat, eds., Galileo Galilei. Discorsi e Dimonstrazioni Matimatiche intorno a Due Nuove Sciense (Turin: Boringhieri, 1958), pp. 694-695. There are also numerous references to the properties of the pendular process in the manuscripts of Leonardo. Among other things Leonardo recognizes the conservation of motion which makes the bob of the pendulum ascend as much, or nearly as much, as it has descended. Leonardo states that the fall of the body along the circular arc takes less time than the fall along the corresponding chord. This is an anticipation of the brachistochrone curve dealt with by Galileo. Yet Leonardo gives no hint that he recognizes either the isochronism of the oscillations or the independence of the period from the weight of the pendulum. In other words, Leonardo does not seem to have even come near to grasping the fundamental properties of the pendulum (Carugo and Geymonat, Discorsi, loc. cit.). Galileo has other precursors in Jean Buridan, Nicole Oresme (Ariotti, Development of the Concept of Time), and especially in Girolamo Cardano. As it is shown in his work De subtilitate, published in Nuremberg in 1550, Cardano states that the longer the thread suspending the bob, the smaller the force required to move this bob from the perpendicular; for, he states, the bob ascends less (Carugo and Geymonat, Discorsi, loc. cit.).