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For this reason the two tubes should not be of too near the same size, or if this cannot be avoided, a small bulb should be blown on the end where the joint is to be made. If this bulb be made with the same wall-thickness as the rest of the tube, and somewhat pear-shaped, it may be drawn out to the same size as the rest of the tube, if necessary, after the joint has been made.
The beginner should examine such a joint on regular factory-made apparatus, and note the uniformity of wall-thickness and the "clean-cut" appearance of the joint, as a model for his imitation.
Care must be taken not to blow the bulbs d and f too thin as they then become very difficult to handle, and the joint is usually spoiled. The wall-thickness of these bulbs must never be much less than that of the original tube. If the joint as completed has thinner walls than the rest of the tube, it will be more easily broken.
At a point about two inches from the open end of the tube, it is slowly warmed and finally heated to the softening point. Grasping the open end with a pair of crucible tongs, it is cautiously pulled out, a little at a time, usually during rotation in the flame, to make a constriction of moderate wall-thickness, but of sufficient internal diameter to admit the tube containing the substance.
If the tube is not rotated during the blowing, the bulbs will be lop-sided and it will be impossible to get a joint of uniform wall-thickness; if rotation is omitted during the drawing, the tube will almost invariably be quite crooked.
The wall-thickness is similarly to be preserved above ground likewise, and the intervals between these walls should be vaulted over, or filled with earth rammed down hard, to keep the walls well apart. The entasis as given by Fra Giocondo in the edition of 1511.
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