What most delayed the science and art of flight, which made no progress during the whole of the eighteenth century, was an imperfect understanding of the flight of birds. The right way to achieve flight, as events were to prove, was by the study and practice of gliding. But birds were believed to support, as well as to raise, themselves in the air chiefly by what in the jargon of science is called orthogonal flight, that is, by direct downward flapping of the wings. This view received authoritative support from a famous treatise written in the seventeenth century by Giovanni Alfonso Borelli, an Italian professor of mathematical and natural philosophy. Borelli, who held professorships at the Universities of Florence and Pisa, and corresponded with many members of the Royal Society, was an older man than Wilkins, but his book on the movements of animals (_De Motu Animalium_), which included a section on the flight of birds (_De Volatu_), was not published till 1680, when both he and Wilkins were dead. It was long held in high esteem for its anatomical exposition of the action of flying, and some of its main contentions cast a damp upon the hopes of man. The bones of a bird, says Borelli, are thin tubes of exceeding hardness, much lighter, and at the same time stronger, than the bones of a man. The pectoral muscles, which move the wings, are massive and strong--more than four times stronger, in proportion to the weight they have to move, than the legs of a man. And he states his conclusion roundly--it is impossible that man should ever achieve artificial flight by his own strength. This view, dogmatically stated by one who was a good mathematician and a good anatomist, became the orthodox view, and had an enduring influence. All imitation of the birds by man, and further, all schemes of navigating the air in a machine dynamically supported, seemed, by Borelli's argument, to have been thrust back into the limbo of vanities.