A.--It will be most affected by percussion. Long-continued impact on the side of a tube, producing a deflection of only one fifth of that which would be required to injure it by pressure, is found to be destructive of the riveting; but in large riveted structures, such as a ship or a railway bridge, the inertia of the mass will, by resisting the effect of impact, prevent any injurious action from this cause from taking place.
81. Q.--Will the power of iron to resist shocks be in all cases proportional to its power to resist strains?
A.--By no means. Some cast iron is very hard and brittle; and although it will in this state resist compression very strongly, it, will be easily broken by a blow. Iron which has been remelted many times generally falls into this category, as it will also do if run into very small castings. It has been found, by experiment, that iron of which the crushing weight per square inch is about 42 tons, will, if remelted twelve times, bear a crushing weight of 70 tons, and if remelted eighteen times it will bear a crushing weight of 83 tons; but taking its power to resist impact in its first state at 706, this power will be raised at the twelfth remelting to 1153, and will be sunk at the eighteenth remelting to 149.
82. Q.--From all this it appears that a combination of cast iron and malleable iron is the best for the beams of engines?
A.--Yes, and for all beams. Engine beams should be made deeper at the middle than they are now made; the web should be lightened by holes pierced in it, and round the edge of the beam there should be a malleable iron hoop or strap securely attached to the flanges by riveting or otherwise. The flanges at the edges of engine beams are invariably made too small. It is in them that the strength of the beam chiefly resides.
CHAPTER I.
GENERAL DESCRIPTION OF THE STEAM ENGINE.
THE BOILER.
83. Q.--What are the chief varieties of the steam engine in actual practical use?
A.--There is first the single-acting engine, which is used for pumping water; the rotative land engine, which is employed to drive mills and manufactories; the rotative marine engine, which is used to propel steam vessels; and the locomotive engine, which is employed on railways. The last is always a high-pressure engine; the others are, for the most part, condensing engines.