Galileo's compass - History of an invention

Dinamic version

Throughout the Renaissance (fig.1) many attempts were made to develop a universal instrument (fig.2) that could be used to perform arithmetical calculation and geometric operations easily. (fig.3) This need was felt especially in the military field, where the technology of firearms called for increasingly precise mathematical knowledge. To satisfy these requisites, the first proportional compasses (fig.4) were developed in the second half of the sixteenth century, among them some singular instruments known as the “radio latino(fig.5) and the “proteo militare(fig.6). The geometric and military compass of Galileo belonged to this class of instruments. Invented in Padua in 1597, the instrument is also linked to Galileo’s activity (fig.7) in the Accademia Delia (fig.8), founded in Padua to provide mathematical instruction for young noblemen training for a military career. (fig.9) With the seven proportional lines traced on the legs of the compass and the four scales marked on the quadrant, it was possible to perform with the greatest of ease all sorts of arithmetical and geometric calculations, ranging from calculating interest to extracting square and cube roots, from drawing polygons to calculating areas and volumes, from measuring gauges to surveying a territory. Between 1598 and 1604, Galileo instructed several European sovereigns on the use of his compass, (fig.10) among them Prince John Frederick of Alsace, Archduke Ferdinand of Austria, the Landgrave Philippe of Hesse and the Duke of Mantua.

The success of the instrument encouraged Galileo to divulge his invention still further. In 1606 he published 60 copies of Le operazioni del compasso geometrico e militare (fig.11), each of which he sold privately along with one of the instruments. (fig.12) The production of compasses, from which Galileo earned a substantial profit, was entrusted to an instrument-maker whom the scientist housed for some years in his own home. The publication of the treatise immediately aroused great interest, so intense as to provoke bitter arguments in the academic world over the authorship of the invention. Already in 1607 Baldassarre Capra, one of Galileo’s pupils, tried to claim credit for the invention of the instrument among erudite circles by publishing a treatise in Latin on its operations (fig.13). Other adversaries of Galileo (fig.14) claimed that the instrument had been invented first by the Dutch mathematician Michel Coignet. Many variations in the instrument were made (fig.15) and, with the addition of new proportional lines, its fields of application were later extended. Specific treatises (fig.16) were written by Michel Coignet, who called it “compasso pantometro”, by Muzio Oddi who called it “compasso polimetro(fig.17), by Ottavio Revesi Bruti who, adding proportional lines for architectural drawing, called it “archisesto(fig.18), by Girard Desargues and other French mathematicians who, adding proportional lines for perspective drawing, called it the “optical or perspective compass”. Numerous variations (fig.19) were developed throughout the seventeenth and eighteenth centuries, while during the course of the nineteenth, the proportional compass was gradually replaced by the dissemination of highly refined slide rules (fig.20) which survived in the technical studios of engineers, architects and geometers up until the very recent advent of the computer.

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