Perigusium is less than ours, being 180 one way, 120 the other. I find the amphitheatre at Silchester is of the same dimension with ours here, and built of the same materials and form, as far as I could discern, but more ruinous. These places, though of absolute necessity open at top, where usually sheltered from rain in some measure, and from the sun effectually, by great sail-cloths spread along the top from masts and ropes, which were managed by the soldiers of the marine affairs, who were more skilful in such work: a fashion first invented by Q. Catulus when he was Ædile. The places where these poles were let through the cornices of the upper order, and rested on corbels, are still visible in the great amphitheatres. This probably was done in ours by masts and poles fastened into the ground without-side, and leaning along the outside bank; which would give them a very advantageous turn in hanging over the top of the theatre; for the slope of the _agger_ externally is with an angle of forty five degrees, being half a right angle, the most natural and commodious for beauty and force to oppose against the side weight: or they might erect them in the solid work on the top of the terrace, seeing it has abundantly strength enough. But in the particularity of these modes no certainty is at this time to be expected. However, by the situation of the place, the architect has taken great care, according to Vitruvius his rules about theatres, to obviate the inconvenience of the sun-beams as well as possible; and that in three respects. 1st, As he has set it upon a plain declining northwards, and upon the higher part of the plain; upon the very tip where the declivity begins. 2dly. By taking the bearing of it exactly, I found the opening, or entrance thereto, is to the north-east precise: hence it is very plain and easy to conceive, that from nine o’clock in the morning till sun-set, in the longest day of the year, the sun will be on the backs of the spectators, upon the upper or south-west half of the building; which contrivance is worthy of notice: and that this is not done upon account of the city of Dorchester lying that way, but as a thing essential, is plain from the like in the amphitheatre of Silchester, which opens upon the same point, though directly the farthest from the city. 3dly, The breadth of the opening or entrance, level with the surface, and opposite to the falling beams of the sun, must produce a very great rebatement of the heat thereof, reflected into this vast concave, and prove a convenience the other amphitheatres are wholly destitute of: and this purpose is so much regarded, that, if we consider it with a scrupulous eye, we shall find that the western side of this upper half of the terrace and the pavilion there is somewhat broader, and nearer the upper end of the long diameter, than the eastern. In the midway of the terrace between the pavilions on both sides and the _cavea_, are still to be seen two round holes,