mensions in space. In passing from one part of the spectrum to another X is the only 

 quantity which varies, and we have the important law : 



When light is scattered by particles which are very small compared with any of the 

 wavelengths, the ratio of the amplitudes of the vibrations of the scattered and incident light 

 varies inversely as the square of the wavelength, and the intensity of the lights themselves 

 as the inverse fourth power. 



The dimensional and conversion- factor formulae for the more commonly 

 occurring derived units are given in Table 30. 



TABLE 2.— SOME FUNDAMENTAL DEFINITIONS 



Part 1. — Geometrical and mechanical units * 



Activity (power). — Time rate of doing work; unit, the watt. 



Angle (<f>). — The ratio of the length of its circular arc to its radius; unit, 

 the radian. 



Angstrom.— Unit of wavelength = 10" 10 meter. (See Table 522.) 



Angular acceleration I a = -^ V — The rate of change of angular velocity. 



Angular momentum (/w). — The product of its moment of inertia about 

 an axis through its center of mass perpendicular to its plane of rotation and its 

 angular velocity. 



Angular velocity. — The time rate of change of angle. 



Area. — Extent of surface. Unit, a square whose side is the unit of length. 

 The area of a surface is expressed as S = CL 2 , where the constant C depends 

 on the contour of the surface and L is a linear dimension. If the surface is a 

 square and L the length of a side, C is unity ; if a circle and L its diameter, C 

 isTr/4. (See Table 31.) 



Atmosphere. — Unit of pressure. (See Table 260.) 



English normal= 14.7 lb/in. 2 = 29-929 in.Hg = 760.18 mmHg (32°F) 

 U. S. = 760 mmHg (0°C) =29.921 in.Hg= 14.70 lb/in. 2 



Avogadro number. — Number of molecules per mole, 6.0228 X 10 23 mole- 

 cules/mole. 



Bar. 4a — International unit of pressure 10 6 dyne/cm 2 . 

 Barye. — cgs pressure unit, one dyne/cm 2 . 



Carat. — The diamond carat standard in U. S. = 200 mg. Old standard = 

 205.3 mg = 3.168 grains. The gold carat: pure gold is 24 carats; a carat is 

 1/24 part. 



Circular area. — The square of the diameter = 1.2733 X true area. True 

 area = 0.785398 X circular area. 



Circular inch. — Area of circle 1 inch in diameter. 

 Cubit = 18 inches 



* For dimensional formula see Table 30, part 2. 



4a Some writers have used this term for 1 dyne/cm 2 . 



SMITHSONIAN PHYSICAL TABLES 



