PHYSICS
53
PHYSICS
seventeenth century, was likewise to baffle Baliani and
Gassendi.
Vinci was much engrossed in the analysis of the de- formations and elastic reactions which cause a body to rebound after it has struck another, and this doc- trine, formulated by Buridan, Albert of Saxony, and Marsile of Inghein he applied in such a waj' as to draw from it the explanation of the flight of birds. This flight is an alternation of falls during which the bird compresses the air beneath it, and of rebounds due to the elastic force of this air. Until the great painter discovered this explanation, the question of the flight of birds was always looked upon as a problem in statics, and was likened to the swimming of a fish in water. Vinci attached great importance to the views developed by Albert of Saxony in regard to the Earth's equilibrium. Like the Parisian master, he held that the centre of gra\-ity within the ter- restrial mass is constantly changing under the in- fluence of erosion and that the Earth is continually moving so as to bring this centre of gravity to the centre of the World. These small, incessant motions eventually bring to the surface of the continents those portions of earth that once occupied the bed of the ocean and, to place this assertion of Albert of Saxony beyond the range of doubt, Vinci devoted himself to the stud)' of fossils and to extremely cautious observa- tions which made him the creator of Stratigraphy. In many passages in his notes Vinci asserts, like Nicholas of Cusa, that the moon and the other wandering stars are worlds analogous to ours, that they carrj- seas upon their surfaces, and are surrounded by air; and the development of this opinion led him to talk of the gravity binding to each of these stars the elements that belonged to it. On the subject of this gra^'ity he professed a theory similar to Oresme's. Hence it would seem that, in almost everj- particular, Vinci was a faithful disciple of the great Parisian masters of the fourteenth century, of Buridan, Albert of Saxony, and Oresme.
XI. It.\li.\n A^•ERROIS^I and Its Tendencies to Routine — Attempts .\t Restoring the Astronomy OF Homocentric Spheres. — ^^■hilst, through the anti- Peripatetic influence of the School of Paris, Vinci reaped a rich harvest of discoveries, innumerable Ital- ians devoted themselves to the sterile worship of de- funct ideas with a servility that was truly astonishing. The Averroists did not wish to acknowledge as true anything out of conformity with the ideas of Aristotle as interpreted by Averroes; with Pompanazzi (1462- 152G), the Alexandrists, seeking their inspiration fur- ther back in the past, refused to understand Aristotle otherwise than he had been understood by Alexander of Aphrodisias; and the Humanists, sohcitous only for purity of form, would not consent to use any tech- nical language whatever and rejected all ideas that were not sufficiently vague to be attractive to orators and poets; thus Averroists, Alexandrists, and Human- ists proclaimed a truce to their vehement discussions so as to combine against the "language of Paris", the "logic of Paris", and the "physics of Paris". It is difficult to conceive the absurdities to which these minds were led by their slavish surrender to routine. A great number of physicists, rejecting the Parisian theory of impetus, returned to the untenable dj'namics of Aristotle, and maintained that the projectile was moved by the ambient air. In 1499 Xicolo Vernias of Chieti, an Averroist professor at Padua, taught that if a heavy body fell it was in consequence of the mo- tion of the air surrounding it.
A servile adoration of Peripateticism prompted many so-called philosophers to reject the Ptolemaic system, the only one w'hich, at that time, could satisfy the legitimate exigencies of astronomers, and to re- adopt the h3'pothesis of homocentric spheres. They held as null and void the innumerable observations that showed changes in the distance of each planet
from the Earth. Alessandro Achillini of Bologna
(1463-1512), an uncompromising Averroist and a
strong opponent of the theory of impetus and of all
Parisian doctrines, inaugurated, in his treatise "De
orbibus" (1498), a strange reaction against Ptolemaic
astronomy; Agostino Nifo (1473-1538) laboured for
the same end in a work that has not come down to us;
Girolamo Fracastorio (1483-1553) gave us, in 1535,
his book "De homocentricis", and Gianbattista Amico
(1.536), and Giovanni Antonio Delfino (1.559) pub-
lished small works in an endeavour to restore the
system of homocentric spheres.
XII. The Copernican Revolution. — Although directed by tendencies diametrically opposed to the true scientific spirit, the efforts made by Averroists to restore the astronomy of homocentric spheres were perhaps a stimulus to the progress of science, inas- much as they accustomed ph3-sicists to the thought that the Ptolemaic system was not the only astro- nomical doctrine possible, or even the best that could be desired. Thus, in their own way, the Averroists paved the waj' for the Copernican revolution. The movements forecasting this revolution were noticeable in the middle of the fourteenth century in the writings of Nicholas of Cusa, and in the beginning of the fif- teenth centiu-y in the notes of Vinci, both of these eminent scientists being well versed in Parisian phys- ics.
Celio Calcagnini proposed, in his turn, to explain the daily motion of the stars by attributing to the Earth a rotation from West to East, complete in one sidereal day. His dissertation, "Quod ccclum stet, terra vero moveatur", although seeming to have been wTitten about 1530, was not published until 1.544, when it appeared in a posthumous edition of the author's works. Calcagnini declared that the Earth, originally in equilibrium in the centre of the universe, received a first impulse which imparted to it a rotary motion, and this motion, to which nothing was op- posed, was indefinitely preserved by virtue of the principle set forth by Buridan and acceptetl by Albert of Saxony and Nicholas of Cusa. According to Cal- cagnini the daily rotation of the Earth was accom- panied by an oscillation which explained the move- ment of the precession of the equinoxes. Another oscillation set the waters of the sea in motion and determined the ebb and flow of the tides. This last hypothesis was to be maintained by Andrea Cesalpino (1519-1603) in his "Qusstiones peripateticae" (1569), and to inspire Galileo, who, unfortunately, was to seek in the phenomena of the tides his favourite proof of the Earth's rotation.
The "De revolutionibus orbium coelestium hbri sex" were printed in 1543, a few months after the death of Copernicus (1473-1543), but the principles of the astronomic sj'stem proposed by this man of genius had been published as earlj- as 1539 in the "Narratio prima" of his disciple, Joachim Rhaeticus (1514-76). Copernicus adhered to the ancient astro- nomical hj-potheses which claimed that the World was spherical and limited, and that all celestial motions were decomposable into circular and uniform motions; but he held that the firmament of fixed stars was immovable, as also the sun, which was placed in the centre of this firmament. To the Earth he attrib- uted three motions: a circular motion by which the centre of the Earth described with uniform velocity a circle situated in the plane of the ecliptic and eccentric to the sun; a daily rotation on an axis in- clined towards the ecliptic, and finally, a rotation of this axis around an axis normal to the ecliptic and passing through the centre of the Earth. The time occupied by this last rotation was a little longer than that required for the circular motion of the centre of the Earth which produced the phenomenon of the precession of the equinoxes. To the fi\'e planets Copernicus ascribed motions analogous to
