140.5
" 448 170.75

[Illustration: Fig. 2. Sketch of Slide. The facing on which the slide moved
was similar, but three or four times as long.]

These results were the average of eight fair trials; in each case, the
sliding surfaces were totally immersed in muddy salt water, and although
the apparatus used for drawing the slide along was not very delicately
fitted up, the power required may be considered as a sufficient
approximation for practical purposes.

It appears from these experiments, that rough surfaces follow the same law
as regards friction that is followed by smooth, for in each case the
friction increases directly as the pressure.

STRENGTH OF MATERIALS AND STRAINS SUBSISTING IN MACHINES.

63. _Q._--In what way are the strengths of the different parts of a steam
engine determined?

_A._--By reference to the amount of the strain or pressure to which they
are subjected, and to the cohesive strength of the iron or other material
of which they are composed. The strains subsisting in engines are usually
characterized as tensile, crushing, twisting, breaking, and shearing
strains; but they may be all resolved into strains of extension and strains
of compression; and by the power of the materials to resist these two
strains, will their practical strength be measurable.

64. _Q._--What are the ultimate strengths of the malleable and cast iron,
brass, and other materials employed in the construction of engines?

_A._--The tensile and crushing strengths of any given material are by no
means the same. The tensile strength, or strength when extended, of good
bar iron is about 60,000 lbs., or nearly 27 tons per square inch of
section; and the tensile strength of cast iron is about 15,000 lbs., or say
6 3/4 to 7 tons per square inch of section. These are the weights which are
required to break them. The crushing strain of cast iron, however, is about
100,000 lbs., or 44 1/2 tons; whereas the crushing strength of malleable
iron is not more than 27,000 lbs., or 12 tons, per square inch of section,
and indeed it is generally less than this. The ultimate tensile strength,
therefore, of malleable iron is four times greater than that of cast iron,
but the crushing strength of cast iron is between three and four times
greater than that of wrought iron. It may be stated, in round numbers, that
the tensile strength of malleable iron is twice greater than its crushing