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Two members of the De La Hire family contributed in their own way to the history of machines and mechanical devices. Philippe De La Hire I (1640-1718) was the son of Laurent De La Hire, a founder and professor at the Académie Royale de Peinture and Sculpture. While Philippe I had no formal schooling, he was educated in an atmosphere filled with artists and technicians. Early on he was drawn to the sciences, mainly mathematics, mechanics and astronomy, but he also dabbled with zoology, physiology, and meteorology. He spent several years traveling throughout France and the rest of Europe, honing his artistic skills, developing maps of coastlines, and investigating and cataloguing nature. But, overall, Philippe I was most devoted to the sciences of mathematics and astronomy and eventually wrote several texts on these subjects. His first one, “Nouvelle methode en géometrie pour les sections des superficies coniques et cylindriques,” was published in 1673 and stemmed from his work on conic sections. After becoming a member of the Académie Royal des Sciences, and receiving a professorship, Philippe I became an active and devoted astronomer. He published his work “Traité de mécanique,” in 1695, which served as a very important work in the development of modern machines. Among the list of his many inventions was a leveling instrument for use in surveying, and the suggestion of an epicycloidal profile for gear teeth.
In mechanics, the shape of a gear tooth is vitally important to the way it transfers the driving power from tooth to tooth, from gear to gear, and ultimately to the rest of the machine. During the brief tooth contact experienced when two gears meet, the curved surfaces actually roll together like two wheels. Thus the tooth rolling action of gear teeth is essentially the same as two wheels that are in continuous contact. And just as one pair of teeth completes their contact, the next pair takes over, creating uniform pressure and motion. Known as the “involute curve,” this epicycloidal profile for gear teeth provides a rolling action rather than a rubbing action, and is Philippe I’s greatest contribution to mechanics.
Philippe I’s son, Gabriel-Philippe De La Hire II (1677-1719) was born in Paris in 1677 and raised in a similar way to his father. Although he received no formal schooling, Gabriel was raised among an academic community and lived with his family in the Paris observatory. Though his parents intended for him to pursue a career in medicine, Gabriel’s continual involvement with astronomy and mathematics from a young age led him down a path very similar to that of his father. Spending many evenings assisting his father in the observatory, Gabriel was devoted to astronomy and engineering, but also dabbled a bit in medicine, anatomy, and meteorology. He, too, became a member of the Académie des Sciences, but unlike his father, Gabriel developed an interest in architecture as well, and became a member of the Académie of Architecture. He contributed many papers to his various fields, even writing one on the organ of sight in which he argued that the aqueous humor filled the same function as the vitreous humor. Gabriel-Philippe’s main contribution to mechanics was in the form of a device he invented that detached a carriage from the horses when they got out of hand.
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