Inside the book

The work opens with the author's candid address to the reader, in which Sguario states that his theory is presented as probable and likely, rather than demonstrated, and that he was only at the beginning of his fifth lustrum (so about twenty-five years of age), and the treatise was completed in approximately thirty days. The book is illustrated with a particularly striking large folding engraved plate depicting the aurora over Venice, showing rays and luminous columns rising above the horizon; curiously, the references to the "second" and "third" hours of the night follow the Venetian system of timekeeping, counting the hours from nightfall rather than from midnight. It is complete with 2 tables of geometrical demonstrations supporting the author's optical theory. The dissertation begins with a survey of the history and geography of the aurora, collecting reports from across Europe, including the observations at Brévillepont in 1726 and 1731 and the Venetian display of 1737. Sguario reviews the regions in which the phenomenon might be observed and reproduces, with corrections, the engraved "Tavola delle Aurore Boreali del Sig. de Mairan". Rejecting both the ancient and contemporary theory that the aurora resulted from combustible vapors or subterranean fires, Sguario proposes a distinctly Newtonian explanation, attributing the phenomenon to the reflection and refraction of sunlight in a lofty atmospheric cloud identified with the zodiacal light, or solar atmosphere. His argument draws upon planetary attraction, cometary atmospheres, the mechanics of rarefied matter, and the geometry of parabolic and hyperbolic motion, illustrated by detailed mathematical diagrams.