senting the architecture of different countries or workshops, or houses for the industrial classes, there was little deserving of especial record or imitation.

A distinguishing feature of the Exhibition building was its system of ventilation, which consists in the use of jets of compressed air for carrying in by induction the requisite supply of fresh air to the various departments of the building. Around the whole of the exterior of the Exhibition-building is carried a large subterranean gallery, divided by rows of pillars into three, each about 9 ft. 10 in. in width. A wall completely divides the two inner galleries from the outer one; the inner serve as cellars for the various restaurants, the outer for ventilating purposes. This outer annular gallery communicates with the external air by means of 16 shafts, each 9 ft. 10 in. in diameter, disposed symmetrically around the building, and having their openings distant about 66 ft. from the external covered way.

In order to conduct the air from the annular gallery to the interior of the palace, 16 radial subterranean shafts were constructed, each extending under the palace for a length of 394 ft., or from the annular subterranean gallery nearly to the centre.

Under each of the three main annular avenues of communication between the machinery gallery and the central court, ventilating conduits were constructed, communicating with the 16 radial shafts, the shafts under the annular avenues being formed in sections extending from one radial shaft to the next, each section being in communication with one radial shaft only. Each section of the building can thus have its supply of air regulated independently of that of the others. The air is admitted into the building from the circular branch shafts. This induction of the external air is effected by placing in each radial subterranean gallery, almost under the external wall of the building, a jet or nozzle supplied with compressed air, this air as it escapes acting like the steam issuing from a blast-pipe, and carrying in along the radial gallery a quantity of air with it by induction. The 16 jets are formed by their connecting pipes into four groups, and these groups are supplied with compressed air by the four sets of air-compressing machinery. The diameters of the pipes leading from the air-compressing machinery to the jets vary from 1 ft. to 2 ft. Each jetpipe has a flat end, having formed in it four sector-shaped openings disposed symmetrically around the centre. These openings when completely uncovered have a united area of 2,015 sq. in.; but this area can be reduced by means of a valve.

The engines for furnishing the necessary supply of compressed air are four in number, and they have a total power of 105 horses nominal. The air is supplied to the jets at a pressure of from 29 to 314 in. of water. The escape of the vitiated air is allowed to

take place through venetians in the roof. The cost of ventilating the building has been stated by M. Piarron de Mondesir, in a paper read by him before the Société des Ingénieurs Civils of France, to be about 0.1 franc per 353,165 cubic feet of air supplied.

CIVIL ENGINEERING AND PUBLIC WORKS.-In the French section only is there a satisfactory exhibition in civil engineering proper; the other European nations, as well as the United States of America, are but slightly represented.

The French collection is unrivalled. It contains models, admirably got up, of bridges, viaducts, reservoirs, docks, tunnels, etc., with complete plans illustrative of all recent public works; a brief but clear report of each work is to be found in a volume published under the auspices of the Ministère d'Agriculture, du Commerce, et des Travaux Publics.

The most important exhibit in the English section is the application to light-houses of the dioptric system of light of Augustin Fresnel. The dioptric system has been recently admirably described by Mr. Chance in a paper read at the Institution of Civil Engineers.

The machine for the production of the light consists essentially of six brass wheels, with sixteen bobbins of insulated copper attached at equal distances to the circumferences of each wheel; inside each bobbin is a hollow core of soft iron; the wheels are all fixed upon a shaft, which is driven by a steam-engine. In turning, every core of each wheel is brought at the same instant between the opposite poles of two magnets, which pair of poles it also quits at the same instant. The core of every bobbin has its magnetism thus reversed by the revolution of the wheels 107 times per second. This reversing of the magnetism induces a current of electricity in the bobbins; the combination of the currents produces one of sufficient intensity to give a powerful light.

Prussia exhibits a system of centring lately used in tunnelling, the essential feature of which is the substitution of a portable iron framework for the ordinary timber centring. This system has been adopted in the construction of the tunnels of Narusen and Ippensen.

In the Southern States of Germany a remarkable example of cheap railway-bridge construction is exhibited in the Bavarian section, by a model, on a scale of th full size of a bridge of boats over the Rhine, between Maximiliansau and Maxau. The bridge consists of twelve rafts, six of which are easily removable to allow the passage of boats; these rafts are carried by thirty-four pontoons, or boats, substantially built of oak, of which material are also the principal beams and the upper planking of the roadway. These pontoons are 65 feet 6 inches long, 12 feet 2 inches broad, and 4 feet 7 inches deep: except the two pair next the shore, which are somewhat longer.

In the Italian section is an atlas containing plans and sections of the great tunnel of Mont

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 engineering shown by Spain is the breakwater of Tarragona, 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 engineering. The admirable collection of models and plans exhibited by the Public Works departinent cannot fail to produce the conviction that the works they represent have been most scientifically 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 th full size of one of the pierswhich 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 perpendicular and diagonal braces, and support the roadway, which is itself constructed so as to add considerably to the rigidity of the structure. The shore ends of the frames form a rectangular 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 transmitted 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 Ministère des Travaux Publics is a model (th full size) of a masonry arch designed and built by M. Vaudray, engineer of the "Ponts et Chaussées,"

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as an experiment preliminary to the construction 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 inch. The joints next the skewback were not flushed until after the completion of the ring, having been meantime kept open with fir wedges. artificial abutment is 27 feet in height, 49 feet in mean thickness, 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 occupied 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 exactitude 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 centring were quite separate. The result upon the arch was then found to be as follows: The crown had come downths of an inch, the joints of the skewback had opened on the built abutment side

ths of an inch. After the lapse of three days the arch was observed to have come down 7ths 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 downths 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 vertically 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 contained 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