Le Comte ("The Count") d'Esterno's 1864 pamphlet "Du Vol des Oiseaux" ("On The Flight Of Birds") presented his "... seven laws of flapping flight and the eight laws of soaring flight" the results of years of observing birds. Of particular note are Count d'Esterno's three requirements for flight, 1) "equilibrium", 2) "guidance" and 3) "impulsion." In that pamphlet, d'Esterno made the statement, "Gliding seems to be a characteristic of heavy birds; therefore, the odds are not stacked against humans doing the same in a fair wind."
D'Esterno designed a flying machine to reproduce the various motions he thought were necessary for flight. Not surprisingly, his machine was quite bird-like; the outer and rear portions of the wings were to be flapped, while the front inner parts of the wings were fixed. The prominent horizontal tail surface was mounted on a universal joint and the operator was able to move the machine's seat to change the center of gravity. While it seems clear that d'Esterno never attempted to build his machine, he did devote considerable thought to its construction. He estimated that the total weight, including an operator, would have been about 330 pounds, which would have required a total wing surface area of some 215 square feet. The wings would each have been 15 feet 6 inches wide and would have had a maximum chord of 7 feet. D'Esterno hoped to derive the power to soar from moving air and so no powerplant, save for the motions of the operator, was provided. The broad outlines of his design can be seen in numerous later attempts to construct a flying machine, most notably Gustave Whitehead's #21.