The character and function of a pendentive may be readily grasped by a practical experiment. Cut an orange into two hemispheres. Lay the flat of one on four reels, placed at the four angles of a square, inscribed within the circle. These reels represent the piers on which the pendentives are to be constructed. Now by four perpendicular incisions of the knife cut off the segments of the hemisphere that project beyond the square. The lateral spaces between the piers will now be spanned by four arches. Finally, a trifle above the top of the arches, make a horizontal cut, removing the upper part of the hemisphere. The rind which remains represents the four pendentives. The flesh inside of it may be likened to the timber centering used in the construction of the pendentives and, now that the latter are completed, may be removed. Remove also the flesh from inside the upper part of the hemisphere. It will then be a hollow cap, which you can replace on the top of the pendentives. You now have an instance of a dome and pendentives included in a single hemisphere. More usually, however, the architect makes the curve of the dome different from that of the pendentives. Frequently, too, to give the dome superior distinction, he constructs a cylindrical wall on the circle of the pendentives, and on this _drum_, as it is called, elevates his dome. Scientifically stated: “If a hemisphere be cut by five planes, four perpendicular to its base and bounding a square inscribed therein, and the fifth parallel to the base and tangent to the semi-circular intersection made by the first four, there will remain of the original surface only four triangular spaces bounded by arcs of circles. These are called pendentives.” (Professor Hamlin.) * * * * * The first church built by Justinian was =SS. Sergius and Bacchus= in =Constantinople=. The part dedicated to the latter saint--a small basilica--was destroyed by the Turks. The remainder presents the plan of a rectangle enclosing an octagon on which rests a dome of a curious, fluted, melon shape. A few years later was erected the church of =S. Vitale= in =Ravenna=, probably as the Court Church. Its plan is an octagon within an octagon; the inner one being surmounted by a dome.