Two Roman Amphitheatres Revolving to Form a Theatre
A revolving structure on pivot r, its two halves joined by the beam-chain a d
Recalling the great Roman works, Leonardo describes two amphitheatres set back to back on a single pivot that, with the crowds aboard, swing round and lock together into a single theatre. The upper drawing shows the completed drum-shaped structure; the lower diagrams show the two ring-halves and the hinged clasps that fasten them. The notes explain the pivot r, the meeting of points S and t, and a chain of beams (c b, a d) that is unhooked from one amphitheatre and made fast to the back of the other once it reaches position q. He judges the invention not very subtle and difficult chiefly on account of its weight.
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Two amphitheatres swinging together into one theatre
Among the great Roman works he finds two amphitheatres built so that they touched at their backs and then, with all the crowd, turned and closed together in the form of a theatre. The upper drawing shows the resulting drum-like enclosure; the lower ring-diagrams show the two halves and their clasps.
Pivot r and the beam-chain c b / a d joining the halves
The amphitheatre g moves as far as q, whereupon the chain of beams c b is detached entirely from amphitheatre p and fastened to the back of g. When the theatre is made, the chain a d clothes part p, fixed by one end to p and by the other to q, so the two stand joined and bound. r is the pivot and S and t come to touch.
The revolving theatre made hard by its own weight
Leonardo remarks that this invention was not very subtle, and that it was more difficult on account of the gravity (weight) involved, standing as figured above.