PHYSICS
60
PHYSICS
V. The Science of Observation and Its Prog-
ress — Astronomers — The Statics of Jordanus —
Thierry of Freiberg — Pierre of Maricourt. —
Averroism had rendered scientific progress impossible,
but fortunately in Latin Christendom it was to meet
with two powerful enemies: the unhampered curi-
osity of human reason, and the authority of the
Church. Encouraged by the certainty resulting from
experiments, astronomers rudely shook off the yoke
which Peripatetic physics had imposed upon them.
The School of Paris in particular was remarkable for
its critical views and its freedom of attitude towards
the argument of authority. In 1290 William of Saint-
Cloud determined with wonderful accuracy the ob-
liquity of the ecliptic and the time of the vernal
equinox, and his observations led him to recognize the
inaccuracies that marred the "Tables of Toledo",
drawn up by Al-Zarkali. The theory of the pre-
cession of the equinoxes, conceived by the astron-
omers of Alfonso X of Castile, and the" "Alphonsine
Tables" set up in accordance with this theory, gave
rise in the first half of the fourteenth century to the
observations, calculations, and critical discussions of
Parisian astronomers, especially of Jean dcs Linieres
and his pupil John of Saxonia or Connaught.
At the end of the thirteenth century and the begin- ning of the fourteenth, sublunary physics owed great advancement to the simultaneous efforts of geome- tricians and experimenters — their method and dis- coveries being duly boasted of by Roger Bacon who, however, took no important part in their labours. Jordanus de Nemore, a talented mathematician who, not later than about the beginning of the thir- teenth century, wrote treatises on arithmetic and geometry, left a very short treatise on statics in which, side by side with erroneous propositions, we find the law of the equilibrium of the straight lever very cor- rectly established with the aid of the principle of virtual displacements. The treatise, De ponder- ibus",by Jordanus provoked research on the part of various commentators, and one of these, whose name is unknown and who must have written before the end of the thirteenth century, drew, from the same prin- ciple of virtual displacements, demonstrations, ad- mirable in exactness and elegance, of the law of the equilibrium of the bent lever, and of the apparent weight {gravitas secundum situm) of a body on an inclined plane.
Alhazen's "Treatise on Perspective" was read thor- oughly by Roger Bacon and his contemporaries, John Peckham (1228-91), the English Franciscan, giving a summary of it. About 1270 Witelo (or Witek; the Thuringopolonus), composed an exhaustive ten-vol- ume treatise on optics, which remained a classic until the time of Kepler, who wrote a commentary on it.
Albertus Magnus, Roger Bacon, John Peckham, and Witelo were deeply interested in the theory of the rainbow, and, like the ancient meteorologists, they all took the rainbow to be the image of the sun reflected in a sort of a concave mirror formed by a cloud resolved into rain. In 1300 Thierry of Freiberg proved by means of carefully-conducted experiments in which he used glass balls filled with water, that the rays which render the bow visible have been reflected on the inside of the spherical drops of water, and he traced with great accuracy the course of the rays which produce the rainbows respectively.
The system of Thierry of Freiberg, at least that part relating to the primary rainbow, was reproduced about 1360 by Themon, "Son of the Jew" (Theinoju dcei), and, from his commentary on "Meteors", it passed on down to the days of the Renaissance when, having been somewhat distorted, it reappeared in the writings of Ale.ssandro Piccolomini, Simon Porta, and Marco and Antonio de Dominis, being thus propa- gated until the time of Descartes.
The study of the magnet had also made great
progress in the course of the thirteenth century; the
permanent magnetization of iron, the properties of
the magnetic poles, the direction of the Earth's ac-
tion exerted on these poles or of their action on one
another, are all found very accurately described in
a treatise written in 1269 by Pierre of Maricourt
(Petrus Peregrinus). Like the work of Thierry of
Freiberg on the rainbow, the "Epistola de magnete"
by Maricourt was a model of the art of logical se-
quence between experimen' and deduction.
VI. The Articles of Paris (1277) — Possibility OF Vacuum. — The L'niversity of Paris was very un- easy because of the antagonism existing between Christian dogmas and certain Peripatetic doctrines, and on several occasions it combatted Aristotelean influence. In 1277 Etienne Tempier, Bishop of Paris, acting on the advice of the theologians of the Sor- bonne, condemned a great number of errors, some of which emanated from the astrology, and others from the philosophy of the Peripatetics. Among these errors considered dangerous to faith were several which might have impeded the progress of physical science, and hence it was that the theologians of Paris declared erroneous the opinion maintaining that God Himself could not give the entire universe a recti- linear motion, as the universe would then leave a vacuum behind it, and also declared false the notion that God could not create several worlds. These con- demnations destroyed certain essential foundations of Peripatetic physics; because, although, in Aris- totle's system, such propositions were ridiculously un- tenable, belief in Divine Omnipotence .sanctioned them as possible, whilst waiting for science to confirm them as true. For instance, Aristotle's physics treated the existence of an empty space as a pure absurdity; in virtue of the ".\rticles of Paris" Richard of Mid- dletown (about 1280) and, after him, many masters at Paris and Oxford admitted that the laws of nature are certainly opposed to the production of empty space, but that the realization of such a space is not, in itself, contrary to reason; thus, without any ab- surditj-, one could argue on vacuum and on motion in a vacuum. Next, in order that such arguments might be legitimatized, it was necessary to create that branch of mechanical science known as dynamics.
VII. The Earth's Motion — Oresme. — The "Ar- ticles of Paris" were of about the same value in sup- porting the question of the Earth's motion as in furthering the progress of dynamics by regarding vacuum as something conceivable.
Aristotle maintained that the first heaven (the firmament) moved with a uniform rotarj- motion, and that the Earth w;us absolutely stationary, and as these two propositions necessarily resulted from the first principles relative to time and place, it would have been absurd to deny them. However, by declaring that God could endow the World with a rectilinear motion, the theologians of the Sorbonne acknowledged that these two Aristotelean propositions could not be imposed as a logical necessity and thenceforth, whilst continuing to admit that, as a fact, the Earth was im- movable and that the heavens moved with a rotary diurnal motion, Richard of Middletown and Duns Scotus (about 127.5-1308) began to formula*e hy- potheses to the effect that these bodies were animated by other motions, and the entire school of Paris adopted the same opinion. Soon, however, the Earth's motion was taught in the School of Paris, not as a possibility, but as a reality. In fact, in the specific setting forth of certain information given by Aristotle and Simplicius, a principle was formulated which for three centuries was to play a great role in statics, viz. that every hca\v body tends to unite its centre of gravity with the centre of the Earth.
When writing his "Questions" on .Aristotle's "De Cfflo" in 136S, Albert of Helmstadt (or of Saxony) admitted this principle, which he apphed to the entire
