Following the pneumatic came the hydraulic organ, in which water-pressure[92] took the place of wind-pressure. The invention of this _organon hydraulicon_ is ascribed to the Alexandrian mechanician Ktesibos, who flourished during the second century B. C. The description[93] left of the instrument by the inventor’s pupil Heron has been corroborated in its essentials by the discovery of a small baked clay model of an hydraulic organ, found in the ruins of Carthage in 1885 and preserved in the _Musée Lavigérie_ at Carthage. This model, 7-1/16 by 2-3/4 inches (which it is estimated would represent an actual instrument 10 feet high and 4 feet across), was made by the potter Possessoris, whose name is engraved on it, about 120 A. D., and is important as verifying the fact that a primitive keyboard was in use at the beginning of our era. It is clear that both forms of the organ, pneumatic and hydraulic, existed side by side for centuries--the hydraulic principle being best adapted to the construction of large instruments, powerful in tone, for permanent placing in amphitheatre, palace or coliseum, and the pneumatic better suited to smaller ones, easily carried about and enjoying, perhaps, a more general popularity. The stationary and moveable organs of the Roman empire thus anticipate the ‘positive’ and ‘portative’ instruments of a later day. Yet it is the hydraulic organ which is principally associated with the palmy days of Roman imperial rule. Though the poet Cornelius Severus (28 B. C.) celebrates the organ (_cortina_) which, ‘so rich in its varied strains under the master’s skill, with liquid sound makes music in the vast theatre,’ evidence tends to prove that the Romans were, musically, not a highly advanced people--their ideal was quantity and loudness of sound rather than quality, an ideal which the hydraulic organ might realize better than the pneumatic. Hence the _organon hydraulicon_, or _hydraulus_, was a luxury in vogue among the wealthy patricians of the empire. Nero, whose musical attainments history views with such grave suspicion, possessed two hydraulic organs. That they were heard in the Coliseum we know by the testimony of Petronius, the _arbiter elegantiarum_ of Nero’s Augustinian circle, who speaks of gladiators struggling to the sound of the water-organ. It is strange to note that among later Roman emperors the depraved and degenerate Heliogabalus (A. D. 219-222) and his immediate successor, the good and noble Alexander Severus, were both good performers on the water-organ. I