32 THE REALITIES OF MODERN SCIENCE 



reading scale, remembering only that we must recali- 

 brate if any doubt ever arises as to its accuracy. 



We have so far considered only standards of length 

 and of weight. Of course, units of area and of volume, 

 e.g. the square foot and the cubic foot, may be formed 

 upon the basis of a unit of length. Now it so happens 

 that all the apparently complicated quantities l which 

 enter into science, such as the heat received from the 

 sun, the intensity of a sound, the strength of an electrical 

 current, can be expressed by the aid of measurements 

 of length, weight (or more strictly mass), and time. 



The idea of time is probably one of the first abstract 

 ideas of mankind. We may think of time as flowing, 

 like an endless stream. It is when we realize that it 

 is slipping by and that changes are occurring that we 

 become most interested in how fast they are occurring. 

 In much of our study of physical science and of its 

 application to human needs we are interested in the 

 time-rate of change, that is, in "how fast." So im- 

 portant is this idea that a whole branch of mathematics 

 was invented to deal with problems involving rates. 

 This is known to-day as the " differential calculus," 

 although as originally named by the inventor it was 

 called " fluxions," that is, the mathematics which dealt 

 with things that flowed. 



An illustration not only of this idea of time as some- 

 thing which flows continuously but of the use of an 

 unusual unit for measuring it, is found in the story of 

 Galileo, a sixteenth-century scientist, and his study 

 of the pendulum. In the cathedral at Pisa, he noticed 



1 With the exception of two quantities, namely, the permeability 

 /*, and the specific inductivity K, of the ether. 



