3When the forum is placed, a spot as healthy as possible is to be chosen for the theatre, for the exhibition of games on the festival days of the immortal gods, according to the instructions given in the first book respecting the healthy disposition of the walls of a city. For the spectators, with their wives and children, delighted with the entertainment, sit out the whole of the games, and the pores of their bodies being opened by the pleasure they enjoy, are easily affected by the air, which, if it blows from marshy or other noisome places, infuses its bad qualities into the system. These evils are avoided by the careful choice of a situation for the theatre,
2taking especial precaution that it be not exposed to the south; for when the sun fills the cavity of the theatre, the air confined in that compass being incapable of circulating, by its stoppage therein, is heated, and burns up, extracts, and diminishes the moisture of the body. On these accounts, those places where bad air abounds are to be avoided, and wholesome spots to be chosen.
3The construction of the foundations will be more easily managed, if the work be on a hill; but if we are compelled to lay them on a plain, or in a marshy spot, the piling and foundations must be conducted as described for the foundations of temples in the third book. On the foundations, steps (gradationes) are raised, of stone and marble.
4The number of passages (præcinctiones) must be regulated by the height of the theatre, and are not to be higher than their width, because if made higher, they will reflect and obstruct the voice in its passage upwards, so that it will not reach the upper seats above the passages (præcinctiones), and the last syllables of words will escape. In short, the building should be so contrived, that a line drawn from the first to the last step should touch the front angle of the tops of all the seats; in which case the voice meets with no impediment.
5The entrances (aditus) should be numerous and spacious; those above ought to be unconnected with those below, in a continued line wherever they are, and without turnings; so that when the people are dismissed from the shows, they may not press on one another, but have separate outlets free from obstruction in all parts. A place which deadens the sound must be carefully avoided; but, on the contrary, one should be selected in which it traverses freely. This will be effected, if a place is chosen wherein there is no impediment to sound.
6The voice arises from flowing breath, sensible to the hearing through its percussion on the air. It is propelled by an infinite number of circles similar to those generated in standing water when a stone is cast therein, which, increasing as they recede from the centre, extend to a great distance, if the narrowness of the place or some obstruction do not prevent their spreading to the extremity; for when impeded by obstructions, the first recoil affects all that follow.
7In the same manner the voice spreads in a circular direction. But, whereas the circles in water only spread horizontally, the voice, on the contrary, extends vertically as well as horizontally. Wherefore, as is the case with the motion of water, so with the voice, if no obstacle disturb the first undulation, not only the second and following one, but all of them will, without reverberation, reach the ears of those at bottom and those at top.
8On this account the antient architects, following nature as their guide, and reflecting on the properties of the voice, regulated the true ascent of steps in a theatre, and contrived, by musical proportions and mathematical rules, whatever its effect might be on the stage (scena), to make it fall on the ears of the audience in a clear and agreeable manner. Since in brazen or horn wind instruments, by a regulation of the genus, their tones are rendered as clear as those of stringed instruments, so by the application of the laws of harmony, the antients discovered a method of increasing the power of the voice in a theatre.