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BRIDGE OVER THE BLAAUW KRANTZ RAVINE, CAPE COLONY.

In designing the structure the following points had to be considered: (1) That, on account of the great height above the ground, and on account of the high price of timber at the site, the structure could be easily erected without the use of scaffolding supporting it as a whole. (2) That, on account of the high freights to Port Alfred, the quantity of iron in the structure should be as small as possible. (3) That the single parts of the principal span should be easy to lift, and that there should be as few of them as possible. For this latter reason most of them were made in lengths of 20 ft. and more. The question of economy of material presented itself as a comparison between a few standard types, viz., the girder bridge of small independent spans; the cantilever bridge, or the continuous girder bridge in three large spans; the single girder bridge with one large span and several small spans; and the arch with small girder spans on each side. The suspension bridge was left out of question as inadmissible. A girder bridge with small independent spans on rocker piers would probably have been the most economical, even taking into account the great height of the piers near the middle of the ravine, but there would have been some difficulty in holding those piers in position until they could be secured to the girders at the top; and, moreover, such a structure would have been strikingly out of harmony with the character of the site. On the other hand, a cantilever or continuous girder bridge in three spans—although such structures have been erected in similar localities—could not enter into comparison of simple economy of material, because such a design would entirely disregard the anomaly that the greater part of the structure, viz., the side spans, being necessarily constructed to carry across a large space, would be too near the ground to justify the omission of further supports. The question was, therefore, narrowed to a comparison between the present arch and a central independent girder of the same span, including the piers on which it rests. The small side spans could obviously be left out in each case. The comparison was made with a view not only to arrive at a decision in this particular case, but also of answering the question of the economy of the arch more generally. The following table contains the weights of geometrically similar structures of three different spans, of which the second is the one here described. The so-called theoretical weight is that which the structure would have if no part required stiffening, leaving out also all connections and all wind bracing. The moving load is taken at one ton per foot lineal, and the strain on the iron at an average of four tons per square inch. The proportion of the girder is taken at 1 in 8.

It can be seen from these results that the economical advantage of the arch increases with the span. In small arches this advantage would not be large enough to counterbalance the greater cost of manufacture; but in the arch of 220 ft. span the advantage is already very marked. If the table were continued, it would show that the girder, even if the platform were artificially widened, would become impossible at a point where the arch can still be made without difficulty. The calculations leading to the above results would occupy too much space to make it desirable on this occasion to produce them. Our two views are from photographs.—The Engineer.

TORPEDO SHIPS.

Commander Gallwey lately delivered an interesting lecture on the use of torpedoes in war before the royal U.S. Institution, London, discussed H.M.S. Polyphemus, and urged as arguments in her favor: 1. That she has very high speed, combined with fair maneuvering powers. 2. That she can discharge her torpedoes with certainty either ahead or on the beam when proceeding at full speed. 3. That her crew and weapons of defense are protected by the most perfect of all armor possible, namely, 10 ft. of water. 4. That she only presents a mark of 4 ft. above the water line.

Then, he asked, with what weapon is the ironclad going to vanquish these torpedo rams? Guns cannot hit her when moving at speed; she is proof against machine guns, and, being smaller, handier, and faster than most ironclads, should have a better chance with her ram, the more especially as it is provided with a weapon which has been scores of times discharged with certainty at 300 yards. The ironclad, he answered, must use torpedoes, and then he maintained that the speed and handiness of the Polyphemus would enable her to place herself in positions where she could use her own torpedo to advantage, and be less likely to be hit herself. He then called attention to the necessity for well-protected conning towers in these ships, and prophesied that if a submarine ship, armed with torpedoes, be ever built, she will be the most formidable antagonist an ironclad ever had; and the nearer the special torpedo ship approaches this desideratum the better she will be.

A PLUMBING TEST.

A recent trial of a smoke rocket for testing drains, described by Mr. Cosmo Jones in the Journal of the Society of Arts, is deserving of interest. The one fixed upon is 10 in. long, 2½ in. in diameter, and with the composition "charged rather hard," so as to burn for ten minutes. This gives the engineer time to light the fuse, insert the rocket in the drain, insert a plug behind it, and walk through the house to see if the smoke escapes into it at any point, finishing on the roof, where he finds the smoke issuing in volumes from the ventilating pipes. The house experimented upon had three ventilating pipes, and the smoke issued in dense masses from each of them, but did not escape anywhere into the house, showing that the pipes were sound. If the engineer wishes to increase the severity of the test, he throws a wet cloth over the top of the ventilating pipe, and so gets a slight pressure of smoke inside it.

THE GAS ENGINE.^[1]

By DUGALD CLERK.

In earlier days of mechanics, before the work of the great Scottish engineer, James Watt, the crude steam engines of the time were known as "fire engines," not in the sense in which we now apply the term to machines for the extinguishing of fires, but as indicating the source from which the power was derived, motive power engines deriving their vitality and strength from fire. The modern name—steam engine—to some extent is a misleading one, distracting the mind from the source of power to the medium which conveys the power. Similarly the name "Gas Engine" masks the fact of the motors so called being really fire or heat engines.

The gas engine is more emphatically a "fire engine" than ever the steam engine has been. In it the fire is not tamed or