great success, and the distribution problems are reduced to a minimum.  In the United States a notable installation has been on the New York, New Haven & Hartford Railroad, where the section between Stamford and New York has been worked by electricity exclusively since July 1, 1908.  Here the single phase motors use direct current while running over the tracks of the New York Central from Woodlawn to the Grand Central Terminal.  On both the New York, New Haven & Hartford and the New York Central locomotives the armature is formed directly on the axle of the driving wheels, so consequently much interest attaches to the new design adopted for the Pennsylvania tunnels, where the armatures of the direct current motors are connected with the driving wheels by connecting rods somewhat after the fashion of the steam locomotive, and following in this respect some successful European practice.

APPENDIX.

Units of measurement.

(From Munro and Jamieson’s Pocket-book of Electrical Rules and Tables).

I. FUNDAMENTAL UNITS.—­The electrical units are derived from the following mechanical units:—­

    The Centimetre as a unit of length;
    The Gramme as a unit of mass;
    The Second as a unit of time.

The Centimetre is equal to 0.3937 inch in length, and nominally represents one thousand-millionth part, or 1/1,000,000,000 of a quadrant of the earth.

The Gramme is equal to 15.432 grains, and represents the mass of a cubic centimetre of water at 4 degrees C. Mass is the quantity of matter in a body.

The Second is the time of one swing of a pendulum making 86,164.09 swings in a sidereal day, or 1/86,400 part of a mean solar day.

II.  DERIVED MECHANICAL UNITS.-

Area.-The unit of area is the square centimetre.

Volume.—­The unit of volume is the cubic centimetre.

Velocity is rate of change of position.  It involves the idea of direction as well as that of magnitude.  Velocity is uniform when equal spaces are traversed in equal intervals of time The unit of velocity is the velocity of a body which moves through unit distance in unit time, or the velocity of one centimetre per second.

Momentum is the quantity of motion in a body, and is measured by mass x velocity.

Acceleration is the rate of change of velocity, whether that change take place in the direction of motion or not.  The unit of acceleration is the acceleration of a body which undergoes unit change of velocity in unit time, or an acceleration of one centimetre-per-second per second The acceleration due to gravity is considerably greater than this, for the velocity imparted by gravity to falling bodies in one second is about 981 centimetres per second (or about 32.2 feet per second).  The value differs slightly in different latitudes.  At Greenwich the value of the acceleration due to gravity is g=981.17; at the Equator g=978.1; at the North Pole g=983.1.