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.

84. _Q._--Will you explain the construction and action of the single-acting
engine, used for draining mines?

_A._--Permit me then to begin with the boiler, which is common and
necessary to all engines; and I will take the example of a w