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The metric standards of length and mass are kept at the International Bureau of Weights and Measures in the Pavilion de Breteuil, Sevres, near Paris. The Bureau is jointly maintained by the principal civilized governments as members of the Metric Convention. The use of metric weights and measures was legalized in the United Kingdom in 1897. LENGTH Unitthe centimetre, 1/100 of the international metre, which is the distance, at the melting-point of ice, between the centres of two lines engraved upon the polished " neutral web " surface of a platinum-iridium bar of a nearly X-shaped section, called the International Prototype Metre. DERIVED C.G.S. UNITS Velocity :-Unit-\ cm. per second. Angular Velocity :-Units-I radian (57°-2C;6) per sec. ; I revolution per sec. Acceleration :-Time rate of alteration of velocity. Unit-(i cm. per sec.) per sec. Angular Acceleration :-Units-I radian per sec. 2 ; i revolution per sec. 2 Momentum :-Mass multiplied by velocity. Uniti gm. cm. sec.-1. Moment of Momentum :-Momentum multiplied by distance from axis of reference. Unit-I cm. 2 gm. sec.-1. Moment of Inertia :-~s ,md?,where m is the mass of any particle of a body, and d its distance from the axis of reference. Unit-i cm. 2 gm. (see p. 16). Angular Momentum :-Moment of inertia multiplied by angular velocity round axis of reference. Unit-I cm. 2 gm. sec.-1. Porce :-Measured by-the acceleration it produces in unit mass. Unitthe dyne = cm. gm./sec. 2 Gravitational unitthe weight of I gram = g dynes. Couple, Torque, Turning Moment :-Force multiplied by distance from point of reference. Uniti dyne cm. Work :-Force multiplied by distance through which point of application of force moves in direction of force. Unitthe erg = I dyne cm.; I joule = lo 7 ergs. [i calorie = 4-18 joules]. Gravitational unitweight of I gm. x i cm. =^dyne cms. = g ergs. Energy :-Measured by the work a body can do by reason of either (i) its motion-Kinetic Energy (= mv 2 /2) or (2) its position-Potential Energy. Unit-the erg. (See "Work.") 1 Board of Trade Unit = 1 kilowatt hour = 3'6 x io 8 watt-sees. Power :-Work per unit time. Unit-1 erg per sec. 1 watt = io T ergs per sec. = 1 joule per sec. = 1 volt-ampere. 1 kilowatt = 1*34 horsepower. Pressure, Stress :-Force per unit area. Unit-1 dyne per cm. 2 1 megabar = io 6 dynes per cm. 2 =; 750 * mm. mercury at 0°C, lat. 45 , and sea-level (g = 980-6). 1 atmosphere = 760 mm. mercury at 0°C, lat. 45°, and sea-level = 759/4 mm. mercury at o°C. in London = J/0132 x io 6 dynes per cm. 2 = 147 lbs. per inch 2 = o-94 ton per foot 2. »^,. t. ... 7T tr • • Correct to I part in 5000. Elasticity :-Ratio of stress to resulting strain. Unit-1 dyne per cm. 2 , since the dimensions of a strain are zero. HEAT UNITS Temperature :-The melting-point of pure ice under 1 atmosphere is defined as o°C, and the boiling-point of water under 1 atmosphere as ioo°C. This fundamental interval is divided into 100 parts by use of the constant-volume hydrogen thermometer (see p. 44) ; each part is a degree Centigrade. Dimensions of temperature ...

[Plate 6]The determination of e by a new oil-drop method in which the electric field is horizontal has been described. The expression for e in terms of quantities measured is similar to that which applies to H. A. Wilson's method. The correction for the departure from Stokes's law is obtained from Millikan's relation is derived. This contributes an error of 1 in 500 in e and is its major un certainty by the oil-drop method. Recent determinations of the electronic charge by the X-ray method have been analysed. The mean value of e by this method is deduced to be (4-8044 + 0-0007) x 10-10 e.s.u., and this result differs from the mean of the determinations of Millikan and the authors (4-8007 ± 0-002) x 10-10 e.s.u., by 0-0037 x 10-10 e.s.u., which is less than the error due to the viscosity of air. Assuming the X-ray value of e, the viscosity of air can be deduced and is found to be (1830-9 ± 0-5) x 10~7 c.g.s.unit at 23° C.T h e e le c tro n ic c h a r g e I ntroductionThe values of three of the'general constants of physics, e, h and e/m, have been the subject of much discussion and experiment. Discrepancies exist such as the disagreement in the value of e as found by the drop and the X-ray methods, and the disagreement between the value of Rydberg's constant calculated from e, h and e/m and its observed value.

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Natural Philosophy P r in c iples of th e methodIf a lightning discharge may be idealized as equivalent to a vertical hertzian (dipole) oscillator, then its electric field at any instant at a point P, distant r from the axis of the oscillator, may be described as being the vector sum of:(1) the " electrostatic" field of the dipole, which is proportional to 1 (2) the induction field, proportional to 1/r2; (3) the radiation field, proportional to 1 Jr. The phase relations of these fields change with r.The design of an instrument to record without distortion the wave form of this field for an atmospheric is very exacting. It must record changes in the electric field which occur in a few microseconds, but which may last for a