A.—­Precisely so, it will be the diameter multiplied by the length of the bearing.

62. Q.—­What is the amount of friction in the case of surfaces sliding upon one another in sandy or muddy water—­such surfaces, for example, as are to be found in the sluices of valves for water?

A.—­Various experiments have been made by Mr. Summers of Southampton to ascertain the friction of brass surfaces sliding upon each other in salt water, with the view of finding the power required for moving sluice doors for lock gates and for other similar purposes.  The surfaces were planed as true and smooth as the planing machine would make them, but were not filed or scraped, and the result was as follows: 

Area of Slide    Weight or Pressure on      Power required to move the
rubbing          rubbing Surface.            Slide slowly in muddy
Surface.                                     Salt Water, kept stirred up.

Sq. in.                  Lb.                             Lb.
8                       56                             21.5
"                      112                             44.
"                      168                             65.5
"                      224                             88.5
"                      336                            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?