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18 TRE MaRINE. REVIEW in broken stone from 34 to 54 per cent. The reduction in voltiime of cement and sand when mixed and made into mortar varies with different ratios of cement to sand, with different ratios of voids in the sand, and with the amount of moisture in the sand; damp sand (such as is often used for concrete) bulking much more largely than dry sand. The author found that the reduction in volume of cement and ordinary dry sea-sand by being made into mortar varied from 12 per cent for I to 4 mortar to 22 per cent for I to I mortar, and that the reduction in volume for I to I mortar when the sand was damp increased to about 30 per cent. Taking the maximum reduction in volume of cement and sand when made into mortar at 30 per cent, the only other reduction between the volume of the dry materials used and of the. concrete produced is that due to the amount of mortar which goes to fill the voids in the gravel or stone, These voids should be taken at the maximum--viz., 40 per cent for gravel, and 54 per cent for broken stone. These reductions combined make a total reduction between the volume of the dry materials, and that of the concrete, "varying from 35 per cent for concrete in the proportion of 1 cement, 2 sand, 3 gravel, to 44 per cent for concrete in the proportion of 1 cement, 4 sand, 7 broken stone. DISPLACERS. The proportion of ordinary-sized rubble displacers which can be satisfactorily deposited by hand in concrete was as- - certained by the author not to exceed one-fourth of the volume of the set concrete, : MODE OF ASCERTAINING WEIGHTS OF MATERIALS AND RELATIVE COSTS OF CONCRETES. Taking, as an example, impervious concrete in the propor- tion of I cement, 2 sand, and 3 gravel :with displacers, the weights of the materials and 'the cost of a cubic yard of con- crete will be ascertained as follows:--1 cement, 2 sand = 3 vols. allow 30 per cent, or 0.9 vol. for reduction by being made into mortar. Gravel 3 vols., allow 40 per cent, or 1.2 vols., for voids occupied by mortar. Total, 6 vols., reduced by 2.1 vols, = 35 per cent reduction, or the ratio of dry ma- terials = 1.538 to 1-of concrete. 1:538 cubic yards = 41.5 cubic feet, which, divided in the ratio of the materials--viz, I--2--3,---will give:-- Assumed rates per ton. ase d. 6.9 cu. ft. cement X. 857% | Tb. = 5.26 cwt...... at Bs 7 «4 1g.8 eu. ft. sand'X 85:lb. == 10.5 cwt.....2..... at: ap T 9 20.8 cu. ft. gravel X 112 lb. = 20.8 cwt........ a ae a 41.5 . Iz 0 Average cost of mixing and conveying materials. in- cluding use of plant (So ce 8 0 Timber boarding and water...... (AAS as os be ce 13 m3 Centracters' swofit (say) 15. percent... 6c. .ecces ces. 2 5 C ost of one cu. yd. of concrete without displacers. , 1 8 Deduet mertounth for dieplacers.. 2... 3665 es 4 8 14 0 Add % cu. yd. == 634 cu. ft. of stone as displacers X 140 lb. per cubic foot = = 8.44 cwt. at 4s per ton...... tr 8 Cost of 1.cu. yd. of concrete with displacers........... i 8 The foregoing estimated cost would be about 5 per cent above the average cost of concrete estimated in the same way, on account of maximum reduction of cement and sand, and maximum voids having been taken. The following tables show proposed standard proportions for coneretes for sea-works and concretes for general pur- poses, and their relative costs estimated in the foregoing manner; also the volumes of mortar produced from cement and sand, and the ratios of mortar to gravel or broken stone. It will be seen that more mortar is allowed for concrete in which broken stone is used, as it does not yield to treading or ramming like concrete made with gravel, and does not al- low displacers to sink readily into it. Concrete made with broken stone, although it can be made impervious when dis- placers are used, is therefore not so well adapted to receive them. : Concretes Nos, 7 to 10 and Nos. 12 and 13 have a ratio of mortar which would not allow of displacers being used, and for these the gravel may vary in size up to what would be gauged by meshes 2% in. square, and broken stone up to what would be gauged by meshes 1% in. square. The tables show the reduction in cost, without diminution in strength of concrete, which is obtained by the use of dis- placers; No. 6 concrete of I cement, 2 sand, 3 gravel with displacers, costing 19 per cent less than the same concrete without displacers. The cost of No. 6 is also the same as that of the much weaker concrete No. 12, consisting of I cement, 4 sand, and 7 broken stone without displacers. HOWDEN'S FORCED DRAFT. A noteworthy fact in connection with what may now almost be termed the "triumphal march" of the steam turbine, as applied to ship propulsion, is that practically all the turbine steamers so far produced have been fitted with boilers in which the combustion is controlled by Howden's forced draft. - In these steamers the enhanced economy attained over the few which have been fitted with closed stokehold, or other type of forced draft, has been so pronounced as to make it certain that the application of this system of hot-air forced draft, in itself places turbine engines in a most favorable position as respects economy compared with well-designed triple or quadruple reciprocating engines. Both the latest Cunard vessels--the Caronia and Carmania--on which at present exhaustive experiments are being made as to the comparative steam consumption of the two types of engines, the ordinary reciprocating engines in the first, and the turbine engines in the last named, are fitted with duplicate boilers wotked with the Howden forced draft. Both vessels, there- fore, have boiler plants of the highest efficiency, whereby means are afforded of making a thoroughly accurate com- parison. The two new 25-knot express steamers, now build- ing for the Cunard Line, are also being fitted with this system, as well as the new Canadian Pacific express steamer Empress of Britain, recently launched by the Fairfield Co., and her siste® still on the stocks, the Empress of Ireland. It is now also being adopted largely in express channel steamers--the Ostend-Dover turbine steamer Princesse Elizabeth, for exam- ple, which is understood to be the fastest passenger steamer afloat, having her boilers fitted with the Howden system. About 2,000 steamers in all have now been fitted with an indicated horsepower of considerably over 6,000,000. War- ships are also now very largely fitted with the Hiowden sys- tem of controlled combustion; in short, practically every ship of any importance now launched is fitted with this system, which gives a greatly increased efficiency of the boiler in- stallation, thereby reducing correspondingly the weight and space occupied. The reduction in coal consumption effected admits, in the case of merchant ships, of increased cargo space being secured through the reduction of bunker space necessary. The system also renders the propulsive power of the ship entirely independent of the state of the atmosphere. The combustion is effectively regulated quite independently of what may be the direction of the wind or the state of the temperature. This latter advantage is, obviously,, a most valuable one for ships trading in the tropics. By this system also, the stokeholds in vessels so employed are rendered quite cool, and' easy to work in. A considerable portion of the forced draft apparatus for vessels so equipped, including the air-heating plant, is constructed by the contracting en- gineers, from complete working drawings supplied,' but the