is reduced; the yield of the higher gauges of macadam from the stone breakers is less, with an increase of dust; and the waste in dust lime in the lime kilns is sensibly increased. Gunpowder, with its comparatively small impact, is therefore used for the principal work, though in special circumstances, gelignite proves a valuable adjunct. The excavation of the quarry face is done where possible by large blasts, and some computation is essential to determine the charge of explosive. This subject has been investigated by officers of the Royal Engineers for many years, in their work of demolishing fortifications. By means of exact drawings of the erections and their elevations, the lines of least resistance were worked out for determining the loci of the charges, which were estimated upon the cube of the resistances. The resistances of simple and compound masonry structures were calculated from known formulæ, and the ratio of the charge worked out on this basis. For simple masonry revetments, the charge of gunpowder was placed at of the cube of least resistance in feet, and proved adequate to reduce the fort to ruins. The ratio differs with the strength of the object to be removed, and the nature of the strata to be excavated. In blasting the chalk cliff at Seaford, the ratio adopted was of the cubed resistance; and the charges, amounting to 28 tons of gunpowder, were effective in displacing the mass; which, measured after the event, proved to be nearly 292,000 tons, or over 10 tons of chalk per lb. of gunpowder.

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Another method used by the Royal Engineers was to base the charge upon the weight of rock to be displaced, and this was adopted in blasting a cliff in connection with the harbour works at Holyhead. The rock was extremely hard quartzose schist; and in the conditions there, the charges were fixed at 1 lb. of gunpowder per 3 tons of rock. The result of the blast was computed at 3 tons per lb. of explosive, but this low yield was evidently due to ineffective adjustment of the charges, as a few years later a similar blast yielded 7 tons of rock to the lb. factor. This method has no claim to exactness nor certainty; the blasting of huge masses of rock is a problem in which all the factors should

be gauged as accurately as possible; concealed elements estimated, and the resistances to displacement calculated; in this way only can the operation be accomplished with any certainty, or without a large waste of explosive. An illustration is afforded by a blast at Yr Eif Quarry, Carnarvonshire, some years ago, which yielded something between 14 and 20 tons of granite per lb. of gunpowder, showing that if the factor of three tons adopted at Holyhead, or even the later result of 7 tons, had been used here, there would have been an enormous waste of gunpowder.

In the limestone districts of Derbyshire, immense quantities of stone are obtained in large blasts by the usual method of tunnelling into the cliff, and forming chambers on opposite sides of the tunnel. The chambers are charged with gunpowder. On one occasion a nitroglycerine explosive was used, but developed its characteristic local action by forming funnelshaped craters over the chambers, and leaving the mass otherwise undisturbed. The results have varied, and although the mass of rock was always more or less displaced, there has been room for considerable improvement, which could have been gained by applying exact methods for determining the charges. This precaution is the more necessary because the limestone is intersected with numerous divisional planes and fissure joints, sometimes concealed, which easily vitiate an empirical estimate made without regard for such contingencies. The excavation by tunnel and chambers also left room for economy in the condition of the rock as delivered by the blast, which involved considerable labour in handling. Another method is now adopted of terracing the rock with faces 25 feet deep, boring vertical holes on the top, chambering them by firing unstemmed charges of gunpowder, and then introducing the blasting charge. By this method a smaller extent of rock is blasted, but it is delivered to the floor in convenient-sized pieces that admit of more economical handling. The method has been developed to great advantage by the new explosive "cheddite"; and recently one such hole prepared for a charge of 400 lb. of gunpowder was charged with 120 lb. of cheddite, which rent the floor of the terrace for a length exceed

ing 250 feet, and delivered over 1,600 tons of rock in condition for rapid handling and despatch; giving a useful effect of over 13 tons per lb. of explosive and economical delivery.

The above principles and methods of blasting are also applied in the other branches of engineering, for cuttings in railway, canal, and every enterprise involving excavation; and with such agents as gunpowder, cheddite, and gelignite, the scope of the steam navvy has been so largely extended, that even the intercalated beds of Clay and Lias Limestone can be removed by this machine, as was done recently in the construction of the London and South Wales Direct Railway.

Blast Main. The main pipe which conveys blast to a furnace.

Blast Pipe.-Specifically the pipe which discharges the exhaust steam from the cylinders of locomotives into the chimney. See Locomotive Engine.

Blast Stove.-A Hot Blast Stove. Blazed Pig.-An inferior grade of pig iron. It is highly silicious.

Blazing off.—A method of tempering used for springs, and many small articles in which the temperature is judged by the flashing or burning of a grease or fat.

Blechynden Boiler.-A water tube boiler of the accelerated circulation type. It resembles the Yarrow boiler in its general build. The tubes are only very slightly curved, are inclined towards the vertical, and are expanded into thickened plates in two lower chambers, and one upper chamber or drum. There are no external downtakes, the water descending by two outside rows of tubes removed from the main sets of tubes. The tubes are withdrawn through the upper drum, by removing its upper portion.

Bleeding. Red rust from underneath boiler scale, which denotes hidden corrosion. Blind Holes.-See Blank Holes. Blister Copper. See Copper. Blister Steel. See Cementation. Block. A term which, used either alone or as an affix or prefix, denotes a large number of objects, the principal of which will be found under suitable heads, as Pulley Blocks, &c. Pieces by which the weight of a swing bridge

is taken off the central point are blocks. Cubical masses of concrete are concrete blocks, and their moulds are block moulds, and the work is block work, to distinguish it from monolithic work. The block system relates to railway signalling; teeth used in moulding wheels by machine are tooth blocks. Blocking up, and blocking down denote various workshop processes. The snatch block of a crane is the lower lifting sheave with its entire fittings, a tail block denotes a fitting at the rear of a crane, and some other objects; a tuyere block is the back of a smith's hearth, &c.

Blocking Girders.-A device employed in portable balance cranes to afford artificial stability to them when lifting medium and maximum loads across the gauge. Though cranes are stable when lifting in line with the wheel base, comparatively few are built to be stable in all other positions, hence the reason for "blocking them up" between the angle of upset, and the position at right angles with the wheel base. The blocking girders provide a base about equal to that afforded by the centres of the running wheels.

In their simplest form these girders comprise two rolled I joists supported in straps suspended from the ends of the truck. When in use, timber blocking is wedged between the girders and the ground. At other times the girders are carried with the crane, standing out awkwardly. To obviate this, two joists are fitted side by side on some cranes, so that they can be slid in and out as required. When not in use their ends are flush with the truck sides. Another method which has been adopted, though rather expensive, is to hinge the girders on vertical pivots at the corners of the truck, so that they can lie close to the truck when not in service.

Blocking Screws.-Screws which either support blocking girders instead of wood chocks, or which are substituted for them. They are of coarse pitch, properly square threaded, carrying round-faced circular blocks to rest on the ground, and the screws are carried in bearings coming out from the sides of the crane truck.

Block Mould.-A large framed wooden box, the nearest resemblance to which is a massive foundry core box, used for making concrete

blocks in, Fig. 228. Its sides, ends, and bottom enclose a rectangle corresponding with the dimensions of the block to be moulded. It is framed of 3-in. or 4-in. deals, battened together to make up the depth. Cleats at the ends of the side members prevent the ends of the box from being pushed outwards by the mass of concrete, and a transverse bolt about midway in the length prevents the sides from being bulged outwardly. The sides are clamped against the ends by means of bolts. two at each end, which are long enough to allow the sides

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at the angles suitable, and are secured at the top of the mould by a notched board (just as similar pieces are supported in a core box by means of a steady bar). These, which correspond with core prints, are drawn out before the concrete has set hard.

Besides this, provision has to be made for bonding or keying the concrete blocks after they are set in place. If above water, grout is poured in recesses formed in the sides of the blocks. Or recesses may be formed by fitting wood blocks of nearly semicircular section

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to be opened sufficiently to permit of the removal of the concrete block. The concrete is deposited in the box from a mixer or a skip above, to permit of which a line of rails is run along flanking the top of the moulds. Men in the moulds shovel the concrete into the corners, and distribute it properly over the area.

The interior of the box is seldom quite plain. As the blocks have to be lifted with lewis bars, holes are formed to receive these. Two tapered timbers (not shown), each of a section equal to that of the holes required, are fixed in the box

on the inside of each side piece, arranged to form a complete approximately circular recess with the adjoining block; this is filled with a bag of concrete, to key the whole together when set in breakwaters.

The ends of concrete blocks which form the facings of sea walls are frequently protected with blocks of granite or other stone. When these are used, the stone facings are first built in position within the block mould, and the concrete is shovelled against them. The outer faces are squared, but the inner ones are left rough and broken, and of different lengths, to afford a better hold for the concrete. After the concrete has set sufficiently, the box sides are unbolted, the sides and ends tilted outwards, and the block can be lifted with a Goliath or other crane and deposited in the block yard to harden thoroughly. See also Block Work.

Block Setting. The setting of concrete blocks is effected either by a Block-setting Titan crane, or by a Goliath, or a Derrick. The blocks are suspended therefrom, and set by men making minute adjustments, and signal