| Area of slide rubbing surface. | Weight or pressure on rubbing surface. | Power required to move the slide slowly in muddy 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 |
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 strength; or, in other words, that it will take twice the strain to break a bar of malleable iron by drawing it asunder endways, than will cripple it by forcing it together endways like a pillar; whereas a bar of cast iron will be drawn asunder with one sixth of the force that will be required to break or cripple it when forced together endways like a pillar.
65. Q.--What is the cohesive strength of steel?