326 



FRENCH EXHIBITION. 



.Cenis. The public works recently executed by 

 the Italian Government are also shown in an 

 atlas of plans ; but they are principally military 

 barracks. 



The most important work of civil engineer- 

 ing shown by Spain is the breakwater of Tar- 

 ragona, of which there is a sectional model. Its 

 commencement dates as far back as 1790. It 

 is intended, when finished, to be about 1,500 

 yards long, with a total width at top of 100 

 yards, including a pier on the inner side of 65 

 yards wide; the base of the breakwater is 

 nearly 300, yards wide. It is formed of pierre 

 perdue. 



In the French section is by far the most 

 important exhibition of works of civil engineer- 

 ing. The admirable collection of models and 

 plans exhibited by the Public "Works depart- 

 ment cannot fail to produce the conviction that 

 the works they represent have been most sci- 

 entifically designed, and executed with great 

 care, great practical skill, and economy. There 

 are two models of the great swing bridge at 

 Brest, which spans the inlet of Penfield one 

 on a scale of ^th full size of the whole bridge, 

 another -j^tlf full size of one of the piers 

 which exhibit the construction. 



The distance between the sea-face walls of 

 the Penfield (571 feet) is spanned by two 

 wrought-iron lattice-frames, revolving upon 

 turn-tables which crown two circular towers, 

 34 feet 9 inches in diameter and 347 feet apart 

 in the clear: the latter dimension is the total 

 width of the fairway of this part of the naval 

 harbor. Each of these two frames consists of 

 two girders, 25 feet 4 inches deep over the 

 piers and 4 feet 7 inches deep at the centre ; 

 these are strongly braced together with per- 

 pendicular and diagonal braces, and support 

 the roadway, which is itself constructed so as 

 to add considerably to the rigidity of the struc- 

 ture. The shore ends of the frames form a rec- 

 tangular box, which contains the counterweights 

 of the bridge. The total weight borne upon 

 each pier is 590 tons ; the turn-table on the 

 summit of each pier is 29 feet 6 inches in 

 diameter, and has fifty rollers, each 1 foot 7 

 inches in diameter and 1 foot 11 inches in 

 length. The means of opening and closing the 

 bridge consist of a pinion fixed to the revolving 

 part of the bridge, gearing into a horizontal 

 cog-wheel on the pier. The motion is trans- 

 mitted by an intermediate to an upright shaft, 

 which comes up through the roadway of the 

 bridge, and is crowned with a capstan. Four 

 men with capstan-bars can open the bridge in 

 ten minutes. Should it be necessary to repair 

 or replace any of the trucks, or any other part 

 of the mechanism of rotation, the whole weight 

 of the bridge can be lifted off its bed by means 

 of four hydraulic presses in the centre of the 

 piers. 



A very remarkable exhibit by the Ministere 

 des Travaux Publics is a model (o^th full size) of 

 a masonry arch designed and built by M. Vau- 

 dray, engineer of the "Ponts et Ohaussees," 



as an experiment preliminary to the construc- 

 tion of a bridge over the Seine, to connect 

 the Rue du Louvre and the Rue de Rennes. 

 The description and general dimensions of the 

 arch are as follows: Its form is a segment of a 

 circle, of which the chord is 124 feet, the versed 

 sine 6 feet 11 inches. It is built entirely of cut 

 stone ; the number of the voussoirs in each 

 ring is seventy-seven, diminishing in depth 

 from 3 feet 7 inches at the springing to 2 feet 

 8 inches at the keystone; the beds and joints 

 of the voussoirs are dressed with the greatest 

 care, and are laid in Portland cement mortar, 

 the composition of which is two parts sand to 

 one part cement. The thickness allowed to 

 the mortar joints was f inch. The joints 

 next the skewback were not flushed until 

 after the completion of the ring, having been 

 meantime kept open with fir wedges. The 

 artificial abutment is 27 feet in height, 49 feet 

 in mean thictness, and 12 feet wide (this is 

 also the width of the arch) ; it was built of 

 rubble masonry, well bonded together and laid 

 in Portland cement mortar one part of cement 

 to three parts of sand ; its construction occu- 

 pied twenty days ; the laying of the voussoirs 

 seventeen days. The arrangement for striking 

 the centring was by the means of dry sand 

 contained in iron cylinders. Arrangements 

 were made to observe with the greatest exact- 

 itude the effect which should be produced on 

 the arch by the removing of the centring. 

 The arch was left to set four months ; the 

 centring was then eased by allowing the sand 

 to flow regularly from the cylinders. In an 

 hour daylight was perceptible between the 

 soffit of the keystone and the lagging ; in two 

 hours the arch and the cantring were quite 

 separate. The result upon the arch was then 

 found to be as follows : The crown had come 

 down T fi nths of an inch, the joints of the skew- 

 back had opened on the built abutment side 

 j-^ths of an inch. After the lapse of three 

 days the arch was observed to have come down 

 T ^ths of an inch more. It* was then loaded 

 with a weight of 360 tons, disposed over the 

 whole surface of the roadway; the loading 

 occupied thirteen days. When complete, the 

 crown was found to have come down f^ths of 

 an inch. Since then nothing has stirred. The 

 arch was afterward tested by a weight of five 

 tons being allowed to fall on the roadway ver- 

 tically over the keystone from a height of 1 foot 

 6 inches, but no joint has opened, nor has the 

 bridge sustained the slightest injury. 



Allusion has been made above to a method 

 of easing and striking a centring by means of 

 sand. A full-sized model in the park shows 

 this method, now frequently adopted by French 

 engineers for easing large centrings from arches 

 on the completion of works viz., by resting 

 the principals of the centring upon sand con- 

 tained in iron cylinders, from the bottom of 

 which the sand is allowed slowly to escape. 

 Each principal is supported upon round props, 

 fitting as a piston into a cylinder, containing 



