hard. For convenience it is best to give the standard a value of one hundred, and express the hardness of other substances in hundredths of the standard. The following table shows the relative hardness of various substances, Ormuz diamond being taken as a standard: SHRINKAGE OF CASTINGS. In steam cylinders, ' inch in a foot. 100 71 70 64 22 15 65 40 38 37 47 32 30 34 32 26 24 24 20 Girders, beams, etc., % inch in 15 inches. Engine beams, connecting rods, etc., % inch in 16 inches. Large cylinders, say 70 inches diameter, 10 feet stroke, the contraction of diameter, 3% inch at top. Ditto, 1⁄2 inch at bottom. Ditto in length, % inch in 16 inches. Thin brass, % inch in 8 inches. Thick brass, % inch in 10 inches. CHAPTER XXV. STEAM YACHTS AND LAUNCHES. HITHERTO the design and construction of a steam yacht has been carried on in a hap-hazard sort of way, that is only too apparent from a casual inspection of the vessel when completed. One firm is called upon to whittle out a model of a certain length, another is asked to build the engine, while a third supplies a boiler of certain dimensions. The interior fittings are supplied by individuals, perhaps totally unacquainted with marine work. Each individual concerned may be competent to carry out his own particular part of the work, but he pays but little attention to any other portion, and the result, while perhaps correct in detail, is decidedly lacking in harmony. The first requisite is speed, and accommodation comes next in the scale. The former can only be obtained by an easy model and great power, while the latter demands close attention to the utilization of space. In order to produce an average of these essentials, the engine and boiler must be as small as possible, while at the same time capable of exerting great power, and so designed as to be free from vexatious break-downs under continuous work. The consumption of fuel should be reduced to a minimum in order to economize space, and to avoid the inconvenience of frequent coaling. In addition to these requisites a certain amount of beauty is necessary in order to harmonize with the purpose for which the vessel is to be used, The most important point in naval design is harmony of details, and this can only be attained by the influence of one competent mind. Errors in steam launches resulting from incorrect design are of comparatively less importance, but in steam yachts, where the amount of money involved is much greater, it is important that the design should be correct. |