On Engines, dated 1433, is an early and previously unknown treatise on engineering which was prepared for King Sigismund's arrival in Siena as he traveled to Rome for rites of coronation. Mariano Taccola has been described as a designer of military devices, but the drawings and texts of De ingeneis show his interest in the field of technology to encompass far more than the machines of attack and defense. He deals actively, at times creatively, with bridges and their foundations, with harbors, harbor equipment for the loading of freight, aqueducts both above and below ground, equipment for operating the wells at the end of an aqueduct, mill houses, and the machines and power plants associated with them. The numerous figures and landscapes that accompany his texts also reveal Taccola to be an interesting and original Sienese artist. He portrays human figures in situations that seldom appear in the major arts, and the nature studies among the vignettes beside his texts are distinctive to his art. As in a previous study of Brunelleschi's technology and inventions (MIT Press 1970), the authors represent different viewpoints in their analysis—Frank Prager, the history of technology, and Gustina Scaglia, the history of art. Together they have transcribed and translated the Latin texts and indicated the original form of De ingeneis from manuscripts in two libraries. They have summarized as much as is known about Taccola, whose work earned him the name of "the Sienese Archimedes," and have evaluated his achievements as writer, graphic artist, and engineer. The drawings and descriptions of the four books of De ingeneis are presented clearly and concisely, to inform and to teach. For it was about Taccola's time, and with his help, that the long stagnation of many technical practices of the Middle Ages came to an end, and De ingeneis became the starting point for a long line of copybooks. Taccola, unlike his predecessors, was particularly interested in describing or suggesting classes of machines, their common parts and basic functions or general rules. He was moved to experiment with different forms of cross-reference between chapters and to develop various means of illustration, using at times a distinct and original method of balancing drawings, texts, and the marginal vignettes used to identify each illustration. Although Mariano Taccola's work was soon surpassed, his manuscripts were copied throughout the fifteenth century as textbooks, and their rediscovery provides invaluable source material for studying the early, transitional history of mechanical technology and adds a new dimension to the history of Quattrocento art.