The proportional compass was used for geometry calculations related to Euclid's postulates. With complex scales on its two rulers, Galileo adds from 1597 new practical applications : calculation of the optimal elevation of the gun, calculation of the powder load with respect to the size and metal of the bullet. He includes with each delivered compass a handwritten manual prepared by copyists.
In 1606 the commercial success of his compass remains important. Galileo has his manual printed in Italian as a 28 x 19 cm folio book.
Jealousies between scientists are not a recent phenomenon. The young Baldassare Capra had not accepted that Galileo did not recognize his skill in the discovery of a new star that could question the principle of Aristotle on the inalterability of the sky. Reciprocal insults lead to hatred.
In 1607 Capra adapted in Latin the instructions for use of the proportional compass to claim as his own the recent developments, in a 19 x 14 cm quarto book. Galileo is angry. His copy of this book in which he annotated all the errors of interpretation made by Capra is preserved in the Biblioteca di Firenze.
The trial required by Galileo against Capra at the University of Padua is easily won by the real inventor. A witness tells that he had a compass from Galileo as early as 1597, when Capra was only 17 years old. Capra refuses a request from the court to demonstrate his competence in the use of the instrument. The copies of Capra's book are destroyed.
Galileo is upset once again : 30 copies that were already distributed could not be retrieved. He publishes in the same year as a counter-attack a new book in Italian, the Difesa, 24 x 17 cm quarto, detailing his arguments against Capra's slanders.
A copy of each of these three books is included in the sale of the Tomash Library by Sotheby's in London on September 18. Lot 197 estimated £ 60K is the 1606 user's manual. Lot 101 estimated £ 30K is Capra's book. Lot 198 estimated £ 300K is the Difesa in a presentation copy inscribed by Galileo to one of the three administrating judges of the University of Padua. Please watch the video shared by the auction house.
RESULTS including premium :
Operation manual : £ 162K
Capra : £ 50K
Difesa : £ 470K