MECHANICAL UNITS. Various units or standards used in different countries and under different conditions for the expression of me eh:init.:1 1 quantities. One system, the C. G. S. system (q.v.), is based upon the eentimeter, the gran. and the mean solar second. Another uses the foot, the pound, and the mean solar second. The yard and the pound are legally defined as follows: "The straight line or distanee be tween the centres of the transverse lines in the two gold plugs in the bronze bar deposited in the office of the Exchequer [London] shall be the genuine standard yard at 62° F.: the pound is time 111?1,i3 of a certain piece of platinum marked 'P. S. 1S44, 1 pound,' and deposited at the office of the Exchequer." Both the standard yard and pound are now preserved at the Standards Office of the Board of Trade, London. Still another system is based upon the foot (one-third of the yard), the mean solar second. and the weight of one pound at sea-lcrel and latitude 45° as the unit of force, (A unit mass is given an aceelera Hen unity by a unit force; hence, since the unit force gives per second an acceleration 32.172 feet per second to a mass of one pound. it will give an acceleration 1 to 32.172 pounds; therefore, on this system, the unit of mass is 32.179 pounds.) The relations between these units is as follows: The units for the various important mechanical quantities are derived from them. These de
rived units and a few others are given in the following sections: clocity.—One centimeter per second; one yard (or foot) per second; one nautical mile, knot ( 6080 feet), per hour.
Acceleration.—Cnit velocity per second.
Forec.—One grain with unit C. G. S. accelera tion = dyne; one pound with unit (ft.-lb.-see.) acceleration = poundal.= 13,825 dynes. Weight of one pound = 44,520 dynes.
ll'ork.—One dyne acting through one centi meter = cry; ergs =jade. One pound raised one foot = foot-pound = 1.326 joules. One kilo gram raised one meter = kilogram-meter = 9.81 joules. (The last two relations are approxi mate.) Power.—One joule per second = watt; 33,000 foot - pounds per minute = horse - power = 746 watts; 'force de chevaP= 75 kilogram-meters per second = 736 watts.
Pressurr.—One dyne per square centimeter = `kale.' One megadyne (10i dynes) per square centimeter = `megabarie."Weight of one pound per square foot' = 47.9 dynes per square centi meter; one poundal per square foot = 14.SS dynes per square centimeter: 'one centimeter of mercury' = 13.5950 X 980.692 dynes per square centimeter = 13.332.5 dynes per square centi meter; hence 75 centimeters of mercury = 1 megabarie (very closely) 76 centimeters of mer cury, 'one atmosphere' := 1.0133 nnegab:n•ies.