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E´NTASIS (ἔντασις, adjectio, Vitruv. ed. M.-Strübing, 75, 7) in a column is a swelling ( “adjectio quae adjicitur in mediis columnis,” Vitruv. l.c.) from the lower end to a certain point, after which a diminution takes place to the hypotra-chelium which forms part of the capital. Overlooked by Stuart and Revett, its discovery was due to Allason (1814), and confirmed by the measurements of Cockerell and Haller (W. Jenkins, Supplem. vol. to Stuart). In the Parthenon, the lower diameter of the column being 1.9 metre, the entasis is 1/120 (or .016 m.), the diminution 2/9 (or .425 m.) of this dimension. In some ancient columns, as at Assos, entasis is very exaggerated ; while in others, as at Corinth, it is absent. Vitruvius (3.2, 4.3) would make the width of the fillets of the flutings ( “crassitudines striarum” ) equal to the adjectio in the middle of the column; such a deviation is imperceptible. As to the profile, Penrose has shown that in the Parthenon and Propylaea it is an hyperbola, [p. 1.738]with 1 (Attic) foot for principal axis and 30 feet from centre to focus. Several theories have been advanced to account for the entasis of the column. Winckelmann's principle of Unbezeichnung, derived from the lines of the human body, has been adopted by some. Adamy (p. 159 seq.) holds that it expresses the sense of a living organic form due to the plastic tendency of Hellenism, and adduces, in confirmation, the freedom shown in small differences of form and proportion (not to be accounted for by carelessness) visible throughout the Parthenon, as, for example, the want of correspondence of the drops to the triglyphs, the inequality in height of the columns. The most generally accepted view (Gwilt, Dict. of Arch. 10.46), that entasis is a correction of an optical illusion produced by an extended line, is even accepted by Penrose, who thinks it removes the “attenuated appearance” or concavity of columns with straight sides (Investig. p. 39). The exaggeration at Assos, however, seems to indicate that the convexity was intended to be seen, and that apparent straightness was not aimed at. Originally imitated from nature, the effect was afterwards reduced and controlled by the necessities of art.
Examples of the absence and presence of entasis are represented in the woodcut,: from the great temple at Paestum (to left), from a later building in the same city (to right).
Entasis. (From Doric columns at Paestum.)
There is another kind of entasis (Vitruv., ed. M.-S., 80, 7; 123, 10; 77, 1; 75, 7) which, as affecting horizontal lines, has been called horizontal curvature or entasis. Vitruvius says that the capitals were not on a level ( “non ad libellam sed ad aequalem modulum conlocatis” ), and seems to imply that both stylobate and superior members were curved ( “ut quae adjectio in stylobatis facta fuerit, in superioribus membris respondeat” ). Further on (ib. 123, 10) he refers to the method by which this adjustment was effected ( “per scamillos impares” ); and, again (ib. 77, 1), “stylobatam ita oportet exaequari uti habeat per medium adiectionem per scamillos impares.” The reason assigned is that a level line looks hollow (ib.): “si enim ad libellam dirigetur, alveolatus oculo videbitur.” Unfortunately the figure ( “in extremo libro forma et demonstratio erit descripta,” ib. 77, 1) is not extant, and therefore the meaning of Vitruvius remains somewhat obscure. The archaeological evidence of this species of adjectio is hardly more explicit. Horizontal entasis was first noticed in the Parthenon, by Pennethorne (1837) and Hoffer (1838), and subsequently investigated with extraordinary care by Penrose (v. opp. cit. inf.). He gives (2nd Letter) drawings showing the curvature in the pavement (3 ins. in centre of East front) and entablature of the Parthenon, which, he argues, was actual, from the care bestowed on the steps, the accurate levelling instruments employed (teste Vitruv.), and the exact levelling of the sub-basement belonging to the older temple. The joints of the steps are normals to the curve, so that the stylobate forms an arch. The entablature is not quite so much or so perfectly curved as the steps, so that the angle-columns are higher than the rest--in fact, the line is more polygonal, to fit which the abaci are cut into a nail-headed figure. There is no curve in the ground-line of the Propylaea, but it exists in the upper members. The inclined lines of the pediment of the Theseium are slightly convex. As to the criticism of Penrose's view, it may be stated that Lohde and Thiersch accept it, while Adamy rejects horizontal curvature, arguing from aesthetic requirements and its absence at Aegina and Corinth. Ziller extended the theory to individual stones, and even to the vertical joints! Blümner (op. eit. p. 8) holds the question still moot--other buildings must be examined; moreover, departures from horizontality, only detected micrometrically, can scarcely have been intended to correct optical defects. Finally, the whole matter has been excellently summed up by Reber (op. cit. pp. 265-270), with the same result.
(F. C. Penrose, Two Letters from F C. P. . . . on certain Anomalies in the Constr. of the Parthenon, 1848; id. Investig., &c.; H. Blümner in Virchow and Holtzendorff's Samml.; Bötticher, Bericht üb. d. Untersuch. a. d. Akrop.; id. Tektonik; Reber, Gesch. d. Baukunst im Alterthum ; Thiersch, Optische Täuschungen; Ziller, Ueber d. urspr. Existenz d. Curvaturen d. Parth., in Erbkam's Zeitschr., 1865; G. Oehmichen, Griech. Tempelbau n. Vitruv, &c.)
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