PHYSICS
51
PHYSICS
mass of the terrestrial elciiieut. The centre of gravity
of this mass is constantly inclined to place itself
in the centre of the universe, but, within the
terrestrial mass, the position of the centre of gravity
is incessantly changing. The principal cause of this
variation is the erosion brought about by the streams
and rivers that continually wear awaj' the land sur-
face, deepening its valleys and carrying off all loose
matter to the bed of the sea, thereby producing a dis-
placement of weight which entails a ceaseless change
in the position of the centre of gravity. Now, in or-
der to replace this centre of gravity in the centre of
the universe, the Earth moves without ceasing; and
meanwhile a slow but perpetual exchange is being
effected between the continents and the oceans.
Albert of Saxony ventured so far as to think that these
small and incessant motions of the Earth could ex-
plain the phenomena of the precession of the equi-
noxes. The same author declared that one of his
masters, whose name he did not disclose, announced
himself in favour of the daily rotation of the Earth,
inasmuch as he refuted the arguments that were op-
posed to this motion. This anonymous master had a
thoroughly convinced disciple in Nicole Oresme who,
in 1377, being then Canon of Rouen and later Bishop
of Lisieux, wrote a French commentarj' on Aris-
totle's treatise "De Caelo", maintaining with quite
as much force as clearness that neither experiment nor
argument could determine whether the daily motion
belonged to the firmament of the fixed stars or to the
Earth. He also showed how to interpret the difficul-
ties encountered in "the Sacred Scriptures wherein
it is stated that the sun turns, etc. It might be sup-
posed that here Holy Writ adapts itself to the com-
mon mode of human speech, as also in several places,
for instance, where it is written that God repented
Himself, and was angry and calmed Himself and so on,
all of which is, however, not to be taken in a strictly
literal sense". Finally, Oresme offered several con-
siderations favourable to the hypothesis of the
Earth's daily motion. In order to refute one of the
objections raised by the Peripatetics against this
point, Oresme was led to explain how, in spite of this
motion, hea\-y bodies seemed to fall in a vertical line;
he admitted their real motion to be composed of a
fall in a vertical line and a diurnal rotation identical
with that which they would have if bound to the
Earth. This is precisely the principle to which
Galileo was afterwards to turn.
VIII. Plcr.^lity of Worlds. — Aristotle main- tained the simultaneous existence of several worlds to be an absurdity, his principal argument being drawn from his theory of gravity, whence he concluded that two distinct worlds could not coexist and be each sur- rounded by its elements; therefore it would be ridic- ulous to compare each of the planets to an earth similar to ours. In 1277 the theologians of Paris con- demned this doctrine as a denial of the creative omnip- otence of God; Richard of Middletown and Henrj- of Ghent (who wrote about 12S0) , Guillaume Varon (who wrote a commentary on the "Sentences" about 1300), and, towards 1320. Jean de Bassols, William of Occam (d. after 1347), and Walter Hurley (d. about 1343) did not hesitate to declare that God could create other worlds similar to ours. This doctrine, adopted by several Parisian masters, exacted that the theory of gravity and natural place developed by Aristotle be thoroughly changed; in fact, the following theorj' was substituted for it. If some part of the elements forming a world be detached from it and driven far away, its tendency will be to move towards the world to which it belongs and from which it was separated; the elements of each world are inclined so to arrange themselves that the heaviest will be in the centre and the lightest on the surface. This theory of gravity appeared in the writings of Jean Buridan of Bethune, who became rector of the University of Paris in 1327,
teaching at that institution until about 1360; and in
1377 this same theorj' was formally proposed by
Oresme. It was also destined to be adopted by
Copernicus and his first followers, and to be main-
tained by GaUleo, William Gilbert, and Otto von
Guericke.
IX. Dynamics — Theort of Impetus — Inertia — Celestial and Sublunary MECH.tNics Identical. — If the School of Paris completely transformed the Peripatetic theorj- of gravity, it was equally respon- sible for the overthrow of Aristotelean d3-namics. Convinced that, in all motion, the mover should be directly contiguous to the bodj' moved, Aristotle had proposed a strange theory of the motion of projectiles. He held that the projectile was moved by the fluid medium, whether air or water, through which it passed and this, by virtue of the vibration brought about in the fluid at the moment of throwing, and spread through it. In the sixth centurj' of our era this explanation was strenuously opposed by the Christian Stoic, Joannes Philoponus, according to whom the projectile was moved by a certain power communicated to it at the instant of throwing; how- ever, despite the objections raised by Philoponus, Aristotle's various commentators, particularly Aver- roes, continued to attribute the motion of the pro- jectile to the disturbance of the air, and Albertus Magnus, St. Thomas Aquinas, Roger Bacon. Gilles of Rome, and Walter Burley persevered in maintaining this error. By means of most spirited argumentation, William of Occam made known the complete absur- dity of the Peripatetic theorj' of the motion of projec- tiles. Going back to Philoponus's thesis, Buridan gave the name impetus to the virtue or power com- municated to the projectile by the hand or instrument throwing it; he declared that in any given body in motion, this impetus was projjortional to the velocity, and that, in different bodies in motion propelled by the same velocity, the quantities of impetus were pro- portional to the mass or quantity of matter defined as it was afterwards defined by Xewton.
In a projectile, impetus is gradually destroyed by the resistance of air or other medium and is also destroyed by the natural gravity of the body in motion, which gravity is opposed to the impetus if the projectile be thrown upward; this struggle ex- plains the different peculiarities of the motion of projectiles. In a falling body, gravity comes to the assistance of impetus which it increases at every instant, hence the velocity of the fall is increasing incessantly.
With the assistance of these principles concerning impetus, Buridan accounts for the swinging of the pendulum. He likewise analyses the mechanism of impact and rebound and, in this connexion, puts forth verj' correct views on the deformations and elastic reactions that ari.se in the contiguous parts of two bodies coming into collision. Nearly all this doctrine of impetus is transformed into a very correct mechan- ical theory if one is careful to substitute the expression vis viva for impetus. The djTiamics expounded by Buridan were adopted in their entirety by Albert of Saxony, Oresme, JMarsile of Inghem, and the entire School of Paris. .Albert of Saxony appended thereto the statement that the velocity of a falling body must be proportional either to the time elapsed from the beginning of the fall or to the distance traversed during this time. In a projectile, the impetus is grad- ually destroyed either by the resistance of the medium or by the contrary tendency of the gra\aty natural to the body. Where these causes of destruction do not exist, the impetus remains perpetually the same, as in the case of a millstone exactly centred and not rubbing on its axis; once set in motion it will turn in- definitely with the same swiftness. It was under this form that the law of inertia at first became evi- dent to Buridan and Albert of Saxony.
