Strain. — The deformation produced by a stress divided by the original di- 

 mension. 



Stress. — The force per unit area of a body that tends to produce a deforma- 

 tion. 



Tenth-meter. — 10" 10 meter = 1 angstrom. 



Torque, moment of a couple, about an axis is the product of a force and the 

 distance of its line of action from the axis. 



Volume. — Extent of space. Unit, a cube whose edge is the unit of length. 

 The volume of a body is expressed as V = CL a . The constant C depends on 

 the shape of the bounding surfaces. 



Velocity ( v= — J is distance traversed per unit time. 



Viscosity. — The property of a liquid by virtue of which it offers resistance 

 to flow. The coefficient of viscosity is the tangential force that must be applied 

 to the upper surface of a 1-cm cube of the liquid on an edge to produce a 

 velocity of 1 cm/sec in the face when the lower face is at rest. 



Work (W). — The work done by an unbalanced force is the product of the 

 force by the component of the resulting displacement produced in the direction 

 of the force. 



Young's modulus. — Ratio of longitudinal stress within the proportional 

 limit to the corresponding longitudinal strain. 



Part 2.— Heat Unltao 



Blackbody. — A body that absorbs all the radiation that falls upon it. From 

 this definition and certain assumptions it can be shown that its total radiation = 

 <rT* ( Stefan-Boltzmann Law) and that the spectral distribution of the radia- 

 tion is given by the Planck Law : 5a 



Jx= ~nr~ 



Brightness temperature (S). — The temperature of a non-blackbody de- 

 termined from its brightness (with an optical pyrometer, see Table 77) as if 

 it were a blackbody. Such temperatures are always less than the true tempera- 

 tures. 



British thermal unit (Btu).— The amount of heat required to raise 1 

 pound of water at 60°F, 1°F. This unit is defined for various temperatures, 

 but the general usage seems to be to take the Btu as equal to 252 calories. (See 

 calorie. See Table 7.) 



Calorie. — The amount of heat necessary to raise 1 gram of water at 15°C, 

 1°C. 



5 For dimensional formulas see Table 30, part 2. 

 58 An easier way to write this exponential term is : 



A=^/i(W(#))-n 



This form will be used hereafter. 

 SMITHSONIAN PHYSICAL TABLES 



