Energy is that property which, possessed by a body, gives it the capability of doing work. Kinetic energy is the work a body can do in virtue of its motion. Potential energy is the work a body can do in virtue of its position. The unit of energy is the Erg. Power or Activity is the rate of work; the practical unit is called the Watt--10^7 ergs per second. A Horse-power = 33,000 ft--Ibs per minute = 550 ft--Ibs per second, but as seen above under Work, 1 ft--Ib = 1 356 X 10^7 ergs, and under Power, 1 Watt = 10^7 ergs per sec a Horsepower = 550 X 1 356 X 10^7 ergs = 746 Watts; or, =EC/746=C^2R/746=E^2/(746 R) =HP where E = volts, C = amperes, and R = ohms. The French "force de cheval" = 75 kilogramme metres per sec = 736 Watts = 542 48 ft--lbs. per sec. = .9863 H.P.; or one H.P. = 1.01385 "force de cheval." DERIVED ELECTRICAL UNITS.--There are two systems of electrical units derived from the fundamental "C.G.S." units, one set being based upon the force exerted between two quantities of electricity, and the other upon the force exerted between two magnetic poles. The former set are termed electro-static units, the latter electro-magnetic units. III. ELECTROSTATIC UNITS.-- UNIT QUANTITY of electricity is that which repels an equal and similar quantity at unit distance with unit force in air. UNIT CURRENT is that which conveys unit quantity of electricity along a conductor in a second. UNIT ELECTROMOTIVE FORCE, or unit DIFFERENCE OF POTENTIAL exists between two points when the unit quantity of electricity in passing from one to the other will do the unit amount of work. UNIT RESISTANCE is that of a conductor through which unit electromotive force between its ends can send a unit current. UNIT CAPACITY is that of a condenser which contains unit quantity when charged to unit difference of potential. IV. MAGNETIC UNITS.-- UNIT MAGNETIC POLE is that which repels an equal and similar pole at unit distance with unit force in air. STRENGTH OF MAGNETIC FIELD at any point is measured by the force which would act on a unit magnetic pole placed at that point. UNIT INTENSITY OF FIELD is that intensity of field which acts on a unit pole with unit force. MOMENT OF A MAGNET is the strength of either pole multiplied by the distance between the poles. INTENSITY OF MAGNETISATION is the magnetic moment of a magnet divided by its volume. MAGNETIC POTENTIAL.--The potential at a point due to a magnet is the work that must be done in removing a unit pole from that point to an infinite distance against the magnetic attraction, or in bringing up a unit pole from an infinite distance to that point against the magnetic repulsion. UNIT DIFFERENCE OF MAGNETIC POTENTIAL.--Unit difference of magnetic potential exists between two points when it requires the expenditure of one erg of work to bring an (N. or S.) unit magnetic pole from one point to the other against the magnetic forces. V. ELECTRO-MAGNETIC UNITS.--