It is built of brick and is one of the finest specimens of Renaissance fortification, and exemplifies especially the transition from the old girdle walls to the system of bastions; it still has round corner towers, not polygonal bastions (Burckhardt).
This may be rectangular in shape (" straight " shed), containing a series of parallel tracks on which the engines stand and which are reached by means of points and crossings diverging from a main track outside; or it may take a polygonal or circular form (round house or rotunda), the lines for the engines radiating from a turn-table which occupies the centre and can be rotated so as to serve any of the radiating lines.
They contain, however, a fragment of a separate tract on Polygonal Numbers.
The difference in form and content suggests that the Polygonal Numbers was not part of the larger work.
An air-tube consists of an epithelium of large polygonal cells with a thin basement-membrane externally and y a chitinous layer internally, the lastnamed being continuous with the outer cuticle.
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