This 78·5 lbs. is the weight of a cubic foot in a solid block, but loose, as used for fuel, a cubic foot weighs about 49.7 lbs. which is the average of 13 kinds.
Example. What is the weight of a solid cast cylinder of copper, 4 inches diameter and 6 inches high?
8·607 sp. gravity.
62·5 lbs.
——-
43035
17214
51642
————
537·9375 lbs. per cub. ft.
————
Say 538 lbs.
·7854
16 diam. squared.
——–
47124
7854
——–
12·5664 area of base.
6 high
———–
75·3984 cu. in. in volume
———–
Say 75·4 cubic inches.
cub. in.cub. in.lbs.
Then, as 1728 : 75·4 :: 538 : Answer.
538
——
6032
2262
3770
————
{ 12)40565·2
{ ————–
1728 { 12)3380·433
{————–
{ 12) 281·702
———–
23·475 lbs.
Answer, 231⁄2 lbs. nearly.
FRICTION AND VISCOSITY OF FLUIDS.
Frictional Resistance.—The resistance with which bodies oppose the movement of one surface on another is termed friction. It depends on the nature, and the roughness of the surfaces in contact; at the commencement of the sliding, it is greater than when the motion is continued.
Friction is in effect an equivalent force exerted in a direction opposite to that in which the sliding occurs. Its whole amount is the product of two factors: the first of these, which sums up the effect of the nature and condition of the surfaces, is called the coefficient of friction; the second, which is the sum of all pressures, as weight strain, and the adhesion due to magnetism (when employed), which act to urge the two bodies together, i.e., perpendicularly to the surface of contact, is called the normal pressure. But this law holds only where, with dry surfaces, the pressure is not enough to indent or abrade either; or, with wet surfaces, not enough to force out the unguent. In either of these cases, the friction increases more rapidly than the ratio of normal pressure.
No surfaces can be made absolutely hard or smooth; when one surface is made to slide over another, the slight roughness of the one interlock with those of the other, so that the surfaces must be separated or the points abraded to allow of the motion; but if one surface roll upon another, the prominent points are successively raised, without the need of complete lifting of the body or wearing off those points. Hence, there are two kinds of friction, the sliding and the rolling. The former of these in amount greatly exceeds the latter; it is a leading element in the stability of structures and fabrics of all lands, and the most important resistance and source of waste in all machinery, and is therefore a chief object of regard in the arts of construction and the science of engineering.