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contemporary, Albert de Saxe (see Saxe, Albert de). In opposition to the Aristotelean theory of weight, according to which the natural location of heavy bodies is the centre of the world, and that of light bodies the concavity of the moon's orb, he proposes the following : The elements tend to dispose themselves in such manner that, from the centre to the periph- ery their specific weight diminishes by degrees. He thinks that a similar rule may exist in worlds other than this. This is the doctrine later substituted for the Aristotelean by Copernicus and his followers, such as Giordano Bruno. The latter argued in a manner so similar to Oresme's that it would seem he had read the "Traits du ciel et du monde". But Oresme had a much stronger claim to be regarded as the precursor of Copernicus when one considers what he says of the diurnal motion of the earth, to which he devotes the gloss following chapters xxiv and xxv of the "Traits du ciel et du monde". He begins by establishing that no experiment can decide whether the heavens move from east to west or the earth from west to east; for sensible experience can never establish more than relative motion. He then shows that the reasons proposed by the physics of Aristotle against the move- ment of the earth are not valid; he points out, in par- ticular, the principle of the solution of the difficulty drawn from the movement of projectiles. Next he solves the objections based on texts of Holy Scripture; in interpreting these passages he lays down rules uni- versally followed by Catholic exegetists of the present day. Finally, he adduces the argument of simplicity for the theory that the earth moves, and not the heav- ens, and the" whole of his argument in favour of the earth's motion is both more explicit and much clearer than that given by Copernicus.

Meunier, Essai sur la vie et les ouvrages de Nicole Oresme (Paris, 1857): WoLowsKi ed., Traictie de la premihe invention des mon- noies de Nicole Oresme, teites fTani^ais et latin d'apr^s les manu- scrits de la Bibliotheque Imperiale, et Traits de la monnoie de Coper- nic, texte latin et traduction fran^aise (Paris, 1864); Jocrdain, Memoire sur les cmnmeiicements de VEconomie politique dans les (coles du Moyen-Afje in Memoires de I'Academie des Inscriptions el Belles-Lettres. XXVIII, pt. II (l-:t>: ( ri,i/E, Der Algorismus proportionum des Nicolaus Ore^tn. i <>^r Mathcmatik u.

P/ij/siit, XIII, Supplcmentarj- (I. It i '.",-79; Idem. Dcr

Tractatus de Latiludinibus Formnrun: ,/. A - ../,i;n Oresme (Ibid., 1868), 92-97; Idkm. Die malhemalischen Schriften des Nicole Oresme (Berlin, 1870) ; Suter, Eine bis jetzt unbekannte Schrift des Nic. Oresme in Zeitschr. fiir Mathematik und Physik, XXVII, Hist.-litter. AUheiUnw (LfipziK, 1SS2). 121-25; Cantor. For- lesungen Uber dir t',, /, ,/. y MiO., ut.rr. , TI (2nd ed., Leipzig, 1900), 128-36; III m - /, - :, Copernic: Nicole

Oresme 0377) in // - ( I'aria, 15 Nov.,

1909); Idem, i)..,,.. ',,.,.,- -,.,;., ,/ /,, ,n, ,,/,, ,■,., ,, ,„i,-isieime in BW- letin hispanique {H'nili-3.ux, l'JU^-li). PlERRB DUHEM.

Organ (Greek bpyavov, "an instrument"), a musi- cal instrument which consists of one or several sets of pipes, each pipe giving only one tone, and which is blown and played by mechanical means. I. Origin AND De\elop.ment. — \s far as the sounding material is concerned, the organ has its prototype in the syrinx, or Pan's pipe, a little instrument consisting of several pipes of differing length tied together in a row. The application of the mechanism is credited to Ctesibius, a mechanician who lived in Alexandria about 300 B. c. According to descriptions by Vitruvius (who is now generally believed to have written about A. D. 60) and Heron (somewhat later than Vitruvius), the or- gan of Ctesibius was an instrument of such perfection as was not attained again until the eighteenth century. The blowing apparatus designed by Ctesibius con- sisted of two parts, just as in the modern organ; the first serving to compress the air (the "feeders"); the second, to store the compressed air, the "wind", and keep it at a uniform pressure (the "reservoir"). For the first purpose Ctesibius used air-pumps fitted with handles for convenient working. The second, the most interesting part of his invention, was constructed as follows: a bell-shaped vessel was placed in a bronze basin, mouth downwards, supported a couple of inches above the bottom of the basin by a few blocks. Into

the basin water was then poured until it rose some distance above the mouth of the bell. Tubes connecting with the air-pumps, as well as others connecting with the pipes of the organ, were fitted into the top of the bell. When, therefore, the air-pumps were worked, the air inside the bell was compressed and pushed out some of the water below. The level of the water consequently rose and kept the air inside com- pressed. Any wind taken from the bell to supply the pipes would naturally have a tendency to raise the level of the water in the bell and to lower that outside. But if the supply from the air-pumps was kept slightly in excess of the demand by the pipes, so that some of the air would always escape through the water in bubbles, a very even pressure would be maintained. This is what was actually done, and the bubbling of the water, sometimes described as "boiling", was al- ways prominent in the accounts given of the instru- ment.

Over the basin there was placed a flat box containing a number of channels corresponding to the num- ber of rows of pipes. Vitruvius speaks of organs having four, six, or eight rows of pipes, with as many channels. Each channel was supplied with wind from the bell by a connecting tube, a cock being in- serted in each tube to cut off the wind at will. Over the box containing the channels an upper-board was placed, on the lower side of which email grooves were cut trans- versely to the channels. In the grooves close-fitting "sliders" were inserted, which could be moved in and out. At the intersections of channels and grooves, holes were cut vertically through the upper board and, correspondingly, through the top covering of the chan- nels. The pipes, then, stood over the holes of the upper-board, each row, representing a scale-like pro- gression, standing over its own channel, and all the pipes belonging to the same key, standing over the same groove. The sliders also were perforated, their holes corresponding to those in the upper board and the roof of the channels. When, therefore, the slider was so placed that its holes were in line with the lower and upper holes, the wind could pass through the three holes into the pipe above; but if the slider was drawn out a little, its solid portions would <'ut olT the connexion between the holes in the roof of t lir cliaiiiiels and those in the upper-board, and no wind ('(mid jkiss. There was thus a double control of the pipes. By means of the cocks, wind could be admitted to any one of the channels, and thus sujjply all the pipes standing over that channel, but only those pipes would get the wind whose slide was in the proper position. Again, by means of the slide, wind could be admitted to all the pipes standing in a transverse row, but only those pipes would be blown to whose channels wind had been admitted by the cocks. This double control is still a leading principle in modern organ- building, and a row of pipes, differing in pitch, but having the same quality of tone, is called a sto]), because its wind supply can be stopped by one action. It is not quite certain what the stops in the ancient organ meant. It is very unlikely that different stops produced different qualities of tone, as in the mod- em organ. Most probably they represented different "modes". For the convenient management of the slides each was provided with an angular lever, so that on pressing down one arm of the lever, the slide was pushed in; the lever being released, the slide was pulled out again by a spring.

This organ, called hydraulua, or orgrniinii h)iilnnili- cum, from the water used in the blowing ii|ip;ir:i I us, en- joyed great popularity. Writers like ( 'ice id nn- hmd in its praise. Even emperors took pride in playing it. It was used to heighten the pleasures of banquets and was associated particularly with the theatre and the circus. Numerous representations, particularly on coins called contorniates, also testify to its general repute. At an early period we meet organs in which