1848.] 



THE CIVIL ENGINEER AND ARCHITECTS JOURNAL. 



153 



column (here drawn from the posticum;, -srhich is really perpendicular, is 

 made to appear to lean slightly in the same direction as the outer columns. 



I have ohserved that a perpendicular pilaster, when brought very near to 

 a column in a portico or doorway, has the effect of making the column appear 

 to lean forward. Tlie inclination of the anta; is not itself visible, except to a 

 spectator in the narrow space between the inner row of columns and the wall ; 

 ■ and when the eye is brought very near to a line, so that it cannot take it in 

 all at once, it is scarcely possible to judge of perpendicularity. 



The last peculiarities I shall mention are the differences of the abacus. 

 Those on the east front are of the largest class, the north-east and south-east 

 having the largest in the whole temple; they are 6-858 ; the others in the 

 east front are (v/Sf) ; on the south side they are 6-580, and so are they on 

 the west front. The north-west angle is the same as the ordinary size of 

 the east front, G-755 ; the south-west angle is somewhat less; on the north 

 side they are 6-750 in the middle, and regularly decrease towards the 

 angles. 



The entasis of the columns is the most wonderful and beautiful of all the 

 curves. It is so delicate, that its existence was for some time doubted ; and 

 yet I found by careful measurements, in a manner which was suggested to 

 me by Professor Willis, which I will here describe. A fine harp wire was 

 strained from the top to the bottom, as tight as it would bear, close to the 

 edge of such flutes as preserved a sufiicient number of points, with the 

 original surface, and by means of a rule similar to the one 1 here produce, 

 which is supplied with a vernier, I was enabled lo measure from the flute to 

 the wire with the greatest accuracy to about half -^jL foot. I took such 

 measures in several columns of the Parthenon, which I found to be won- 

 derfully true and identical. I measured also the entasis of columns from 

 the Erectheum, Propylaea (both orders), temple of Theseus, and Jupiter 

 Olympus. In those of the Parthenon, Erectheum, and Propylaea, I find the 

 correspondence with hyperbolic arcs, which I have calculated so exact, that 

 the mean of from 14 to '20 measurements in each column differs from the 

 calculated curve less than Jjj;^ foot, and none of them, where the surface 

 was to be depended upon, differs by so much as jj,,. The entasis of Jupiter 

 Olympus gives also a very true hyperbolic arc. The columns of Theseus 

 are so much worn in their outer edges that I was obliged to content myself 

 -with measures within the flutes, which never give such regular curves as the 

 fillets, although the flutes are worked with a nicety far exceeding that found 

 in any other style of architecture. Still, a mean from four different sections 

 within the flutes gives a very accurate hyperbolic arc, although no one is a 

 perfectly regular curve. These hyperbolas are all chosen with their axes, 

 multiples, or aliquot parts of the attic foot. The Greek architects acted 

 with great judgment and knowledge of the nature of tlie curve they were 

 employing, as it is the only one of the conic sections which can produce 

 variety in such delicate curvature as they have chosen for their entasis. 



I must now advert to some of their mouldings, which are woiked with 

 the same perfection, and, as far as I have examined, are all different forms of 

 the conic sections. 



The echinus of the capitals of all the Doric columns agrees with various 

 forms of the hyperbola. The soffit of pediment, Parthenon, and Propylaea, 

 and I think of Theseus also, shows a hyperbolic arc. This is a magnificent 

 moulding, and worked with the utmost perfection. That of the Erectheum 

 is an equally true parabola. The cymatium of the Parthenon is the only 

 certain circular form which I know, except the torus of base, Erectheum, 

 and Ionic order Propyla;a. The flutes, also, are all parts of circles, whose 

 centres are proportioned to the width of the flutes. In this they showed 

 their judgment, as it would have been almost impossible to have worked 

 pure ellipses ; and in these retiring surfaces the value of the perfect variety 

 of curvature of the ellipse would scarcely have been appreciated. Talking 

 of flutes, there is a peculiarity in the flutes of the Parthenon, which does 

 not occur in any other of the temples of .Athens. The flute at the neck is 

 deeper in proportion to its width than in the rest of the shaft : during the 

 ■whole rise, utrtil about 3 feet below the neck, the sagitta or depth of the 

 flute, from its chord, is jsths of chord. At the neck it is 'jijths, which is 

 about ith part greater. This has a good deal of effect on the column, and 

 gives a richness of effect to the upper part, at the same time that it 

 diminishes the light in that part where it can best afford it, viz., where it is 

 contrasted to the deep hack-ground of shade of the upper part of the cella 

 wall. 



I have not yet much to say with regard to the colouring of the temple, 

 nor have I much time to say that litlle, for I fear I must have exhausted your 

 patience. The drawings which are at present made represent the architrave, 

 band, &c., the triglyph, and the string which carries the marble beams which 

 supported the ceiling. There is not a great deal of positive colour remain- 

 ing in the Parthenon. The underside of the mutules show some vestiges 

 of blue and red colour, and the upper part of the nook of the triglyphs, 

 here and there on the east front, preserves some blue. One of the antae 

 of the posticum has a tolerable supply among its eggs of blue and green, 

 and some red. The flowers which decorate the cymatium and other mould- 

 ings have no trace of positive colour, but the drawing of the ornaments 

 upon them is in many places clearly to be made out. 



CAST AND WROUGHT IRON BRIDGES.— (Part II.) 



(Continued from page 126 J 

 At the request df the council of the Royal Scottish Society of Arts, the 

 second part of a paper " On the Strength of Materials, as ajiplicable to the 

 construction of Cast or IVrong/it Iron Bridges, including an account of tlic 

 Tubular Bridges over tlie Cunway and Menai Straits ifc," was read, April 

 10th. By George Buchanan, Esq., President. 



In the first part of this paper, Mr. Buchanan described, on a former even- 

 ing, the principle and construction of the High Level Bridge at Newcastle, 

 which is intended to complete the communication by railway between Lon- 

 don and Berwick-upon-Tweed. Some inquiry having been then made from 

 the chair regarding the bridge over the Tweed, the only remaining link un- 

 completed between London and Edinburgh, he had received the following 

 particulars from Mr. Harrison, the resident engineer under Mr. Stephenson ; 

 — This bridge is to be of stone, and is to consist of 28 semicircular arches, 

 each 6I2 feet span, resting on lofty piers, carrying the level of the railway 

 103 feet above high-water mark, 126 feet above low-water mark, and 135 

 feet above the deepest part of the bed of the river. The whole length of 

 the bridge, with abutments and wing-walls, is 2140 feet. The 28 arches are 

 divided into two series by a broad pier, 28 feet in thickness in the middle. 

 The piers of the arches are 8.1 feet in thickness at the springing, increasing 

 by steps towards the bottom. The bridge will not be completed for 16 or 

 18 months, but it is intended to have a temporary bridge ready for traftic in 

 the month of July next. This viaduct is a work of great rnagnitude, and 

 will form, when finished, a striking and imposing structure, and one of the 

 many to which the extension of railways has given rise. While on this sub- 

 ject, he would mention two other remarkable works, recently designed and 

 executed by Mr. Miller on the North British and Ayrshire Kailways. The 

 one is the viaduct overthe Valley of Dunglass, between Dunbar and Berwick, 

 not far from the once-celehrated Pease Bridge. This viaduct crosses the 

 valley and banks by six semicircular arches, each 60 feet span, and then the 

 deep ravine by a single arch, 135 feet span, and rising 105 feet above the 

 bed of the stream. A large and beautiful model of this structure was ex- 

 hibited, which Mr. Miller, at the President's request, had allowed to be 

 shown to the Society. The other viaduct is that of Ballochmyle, across the 

 Water of Ayr, on the Cumnock Extension of the Ayrshire Railway, and is 

 similar to that of Dunglass in crossing the valley on three semicircular arches, 

 each 50 feet span on each side, hut is still more remarkable in crossing the 

 deep ravine in the middle by a single semicircular stone arch no less thaa 

 180 feet span, and rising 150 feet above the bed of the stream — a bold and 

 noble design, and which has been executed with complete success, the adja- 

 cent rocks furnishing such vast blocks of stone as greatly to facilitate the 

 construction, and to render, indeed, the plan itself practicable. The arch 

 stones are 5 ft. 3 in. deep at the springing, and 4 ft. 9 in. at the crown, and 

 the appearance from below of the stupendous arch rising to such a height is 

 singularly grand and striking. The whole arrangements connected with the 

 quarrying and raising and depositing the stones on the building, by the im- 

 proved machinery of modern times, have been most efficiently conducted by 

 the contractors, Messrs. Ross and Mitchell, and the simple mode of centering 

 adopted and shown in the Dunglass model is recommended by the advantage 

 of preserving the timbers entire. 



The subject of stone bridges opens a wide and interesting field, but ex- 

 tending beyond the limits of this paper. He would, therefore, resume the 

 one more immediately prescribed, namely, the strength of materials, particu- 

 larly iron for bridges. Some interesting experiments, which the time on the 

 previous evening did not permit to be shown, were then made on the tensile 

 strength of stone from ilailes and Craigleith quarries. The Hailes stone 

 bore on the square inch 360 lb., the Craigleith considerably more ; and a re- 

 markable effect was observed here after the load had hung for a little: it. 

 was suggested by a member to give it a slight tap with a hammer, and, on 

 this beirrg done, it immediately snapped asunder, showing the effect of vibra- 

 tion or concussion when the materials are greatly strained in aiding and 

 completing the fracture, a circumstance which appears to throw light on 

 what may sometimes occur by the i-apid and violent actions of the trains on 

 railways. The compressive strength of the Ilailes and Craijleith stones was 

 then shown, by experiment, to be much greater than the tensile strength; 

 and as it required, indeed, more weight and a more powerful apparatus than 

 could be commanded, these experiments on different stones were deferred to 

 another evening. 



The compressive strength on posts or pillars was then considered, and the 

 remarkable effects of the length of the pillar in diminishing its strength. On 

 this subject much light has been thrown by the experiments of Messrs. 

 Hodgkinson and Fairbairn. Pillars or rods were tried of different lengths, 

 from 3 inches to 5 feet, and of different diameters ; rods half an inch diame- 

 ter, with 3| inches length, bore 11 tons; but when the length was 7i inches 

 it onlycarrred 5 tons, when 15 inches long, 3 tons; and at 30 inches only 

 13 cwt. From these experiments, a general rule may be drawn for different 

 lengths. Taking the strength of cast-iron as formerly given at 50 tons per 

 square inch, this will hold good in pillars till the length reaches five times 

 the diameter, and then it begins to di-ninish. When the length is ten times 

 the diameter, the strength is reduced in the proportion of 1| to 1 ; with the 

 length at 15 times the diameter, it is reduced as 2 to 1 ; 20 times as 3 to 1 ; 

 and 40 times as 6 to 1. 



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