62·5 lbs. 1·029 sp. gravity. ----- 5625 1250 625 ------- Answer, 64·3125 lbs. is the actual weight: but 64 lbs. is taken in practice as the weight of a cubic foot of sea water. _Example._ How many cubic feet of sea water will weigh a ton? Divide 2240 lbs. (1 ton) by 64 lbs. { _8)2240 64 { ---- { 8) 280 ---- Ans. 35 _Example._ What is the weight of a cubic foot of wrought iron? 62·5 lbs. 7·69 sp. gravity. ----- 5625 3750 4375 ------- Answer, 480·625 lbs. 480 lbs. in practice is the weight of 1 cubic foot of wrought iron. NOTE.—35 cubic feet of sea water is accounted to be a ton, as in sea water ballast for steamers, and in calculating displacement of ships. _Example._ What is the average weight of a cubic foot of Bituminous coal? 1·256 sp. gravity. 62·5 lbs. ----- 6280 2512 7536 ------- Answer, 78·5000 lbs. 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, 23-1/2 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.