
COLUMN ENTASIS
It will be noticed that the shaft of the classic column is smaller at the top than at the bottom. The diameter of the shaft does not diminish in direct proportion to the height, but in such a way as to cause an effect of swelling just above the center of the shaft. This curvature is called entasis. It begins one third of the way up the shaft in most Orders, the lower third being cylindrical except in the Greek Doric shaft where the entasis begins at the bottom.
If the shaft were left straight from bottom to top, it would seem to be slightly curved in near the center, thereby giving to the shaft an appearance of weakness.
The entasis prevents this optical effect and, at the same time, gives to the shaft a certain life which it would otherwise lack. The entasis may be drawn on all small work, by the proper manipulation of the pencil against the straightedge and without any construction. The method of doing this is described by Figs. 73 to 76 on Plate 73.
First the cap and base of the column are drawn and then the vertical part of the shaft is drawn to point B, Fig. 73, onethird of the distance up. The straightedge is placed as shown and the pencil placed against the straightedge and sloping so that the point rests on the line at B. Now draw the line along, at the same time throwing the point of the pencil gradually farther away from the straightedge until location C, Fig. 74, is reached. From here on, the pencil is straightened up as the line proceeds. This is indicated at D, Fig. 75, and the pencil point is finally brought up against the straightedge just as the line reaches point E. After a little practice, the student can draw a nicely curving entasis by this simple method.
For all work where greater accuracy is required and where the change in rate of curvature must be constant, the method of Fig. 77 may be employed.
Here the shaft from B to E is divided into any number of equal parts (in this case six) and the part plan of the shaft is drawn at B. Point E is now projected down to the plan at f and the circle arc fB is divided into the same number of parts as the shaft above. From each of the points thus determined on the circle arc, project up to the corresponding line above. This will give the points through which the curving shaft line is to be drawn.
After the entasis has been thus drawn for accurate work, the shaft is dimensioned by giving the diameters at each of the horizontal lines. From such a drawing the work can be gotten out exactly as drawn.
