Page:Encyclopædia Britannica, Ninth Edition, v. 9.djvu/248

 238 F I B F I B simple mechanical appliance which, simultaneously with the striking of the alarm, opens the front of the stall; the horses, trained to move at the sound of the gong, advance rapidly each to his own place at the pole. They are instantly hitched in, the men spring to their seats, and the carriage is driven at high speed to the &quot; alarm box &quot; from which the alarm was given. To make sure that there will be a working pressure of steam on reaching the fire, the water in the engines, as they stand in the houses, is kept always at boiling point by the circulation of hot water from small stationary boilers, and fire is lighted in the engine the instant it leaves the house. Every effort is made to save even a few seconds of time, so that the interval between sounding the alarm and pumping water on the fire will average three minutes, and rarely exceeds five minutes. The city is divided into 10 battalion districts. The smallest of these represent each an area of about 5000 by 2000 feet, and comprise the most exposed parts of the city; but most of the districts are from two to three times as large. The signal boxes of the electric fire-alarm telegraph are placed conspicuously in the streets about 400 feet apart in the more crowded portion of the city, and from 1000 to 1200 feet in other portions. There are 540 in all. Alarms given from these boxes are instantly tele graphed from the headquarters of the department to each company house in the city. The first alarm calls out two or more companies previously designated ; a second and third call out additional force. There are in use 57 steam fire-engines (5 of which are self-propellers), 1 steam fire- boat, 10 chemical engines, arid 18 ladder carriages, including 5 &quot;aerial ladders.&quot; The men are well-disciplined and skilful firemen. 1 (A. p. R.) FIRE-CLAY, FIRE-BRICKS. Fire-clays may be de fined as native combinations of hydrated silicates of alu mina, mechanically associated with silica and alumina in various states of subdivision, and sufficiently free from silicates of the alkalies and from iron and lime to resist vitrification at high temperatures ; the absence of the vitrifiable element is, however, merely a question of degree, as no native clays are wholly free from iron, the alkalies, lime, and the other alkaline earths. Fire-clay may be looked upon as a special term for the grey clays of the Coal -Measures, interst ratified with, and generally in close proximity to, the seams of coal, in beds varying from a few inches to many yards in thickness. They are locally known as &quot; chinches &quot; and &quot; underclays,&quot; and are supposed to represent the soil that produced the vegetation from which the coal was formed. The association of coal with the fire-clays of the carboni ferous formation has localized the manufacture of fire bricks, and by far the larger proportion are produced in the Coal-Measure districts, especially at Stourbridge, celebrated for producing a highly refractory brick, Broseley, Benthall, Madeley and Coalbrookdale in the Shropshire coal-field, and in the Midland, Yorkshire, North and South Wales, Durham, and the Scotch coal-fields : but in later years the area of fire-brick manufacture has much widened. There has been an extensive production since about 1850 from the Eocene clays in the neighbourhood of Poole and Wareham in Dorsetshire ; and a more limited supply from the Miocene between Bovey Tracey and Newton Abbot in Devonshire. Still more recently Cornwall has become the seat of the manufacture, where, as at Calstock, Tregoning Hill near Breage, St Ednor near St Columb, and Lee Moor, fire-bricks of fine quality are made from china-clay refuse and disintegrated granite. Mr Argall of the Tregoning Hill Company states that the locality was one of the ] T^he foregoing article is reprinted by permission of Messrs Little, Drown, & Co, Boston, Mass., from Great Fires and Fire Extinction, by General Alfred P. Rockwell, Boston, 1878. first seats of china-clay mining between the years 1730 and 1750, and that in 1862 the present company commenced to make fire-bricks and tiles from the refuse of the clays, taking about two-thirds of silica and one-third of mica, which are mixed together in a pug mill, moulded and burnt in round ovens holding about 16,000 bricks, and that a very superior fire-brick is made from clay direct from the &quot;stopes,&quot; containing Silica 40 00 percent. Alumina 37 00 ,, Magnesia 2 00 ,, Potash 9-00 ,, Water 12 -00 ,, which are employed by founders, smelters, gas companies, &c. The price paid at the works is from 50s. to 55s. per 1000. The source of the materials is decomposed granite, of which Tregoning Hill consists. The Kingston Down fire-clay deposit, near Calstock, sup plying the Calstock fire-brick works, the Phoenix works, and the Tamar works in the same neighbourhood, consists of a range of decomposed granite with an average width of three-quarters of a mile, running east and west for 3 or 4 miles, extending to an ascertained depth of from 300 to 400 feet, and intersected by mineral lodes. The Calstock Fire-Brick Company (limited), superintended by Mr C. B. Evate, commenced operations in the year 1870, and manufactured from the decomposed granite fire-bricks of a highly refractory character, which are delivered free on board at the port of Calstock at from 50s. to GOs. a thousand, weighing about 3| tons. Another source of fire brick material, scarcely yet developed, is the pockets or depressions occurring in the mountain limestone of North Wales, Derbyshire, and Ireland, containing white refractory clays and sands, the insoluble remnants from the local dissolution of the limestone, intermixed with the debris of the overlying millstone grit. These clays and sands when evenly mingled are sufficiently adhesive to be moulded, and their small contractility and highly refractory character render them pre-eminently suitable for fire-brick manufac ture. Fire-brick works have already been established on the estate of Captain Cooke of Colomendy Hall, near Mold, and the refractory clays and sands are largely employed for lining furnaces, 3000 tons having been sold for this purpose alone in the year 1877. The fire-clays of the Coal-Measures vary as regards their refractory character, not only in the different coal-fields, but the individual strata in close alternation often present sudden variations, refractory beds being interatratified with useless strata largely charged with disseminated car bonate of iron. The grey colour of the Coal-Measure clays is partially due to the presence of this mineral, which, whether disseminated through the mass or otherwise occurring in excess as concretionary nodules, is prejudicial to the clays as a material for fire-bricks. Carbonaceous matter is also present in variable proportions, colouring the clay from a slaty-black to a pale grey, but as this is eliminated in the earlier stages of the burning of the bricks, its presence in no way influences their refractory character. The relative proportion of silica and alumina which some manufacturers have laid undue stress upon as indicat ing heat-resisting quality is of little moment, as both these constituents, whether occurring in combination as silicates of alumina, or as free alumina and silica, are essentially the refractory elements of all good fire-bricks, being unvitrifiable per sc, excepting when associated with the alkalies, lime, or oxides of iron. The plastic character of refractory clays is also of limited influence on their suitability for fire-brick manufacture; extreme plasticity, which is generally accompanied by excessive contractility and vitrifiability, is prejudicial. As a rule few clays or