FORCE AND MOTION. 27 



(2.) The direction, or the right line along which it 

 tends to move the point of application. 



(3.) The magnitude or value when compared with a 

 given standard, or the relative rate at which it is 

 able to produce motion in a body free to move. 



66. Measurement of Forces. It frequently is 

 desirable to compare the magnitudes of two or more forces. 

 That they may be compared, they must be measured ; that 

 they may be measured, a standard of measure or unit of 

 force is necessary. When this unit has been determined 

 upon, the value of any given force is designated by a nu- 

 merical reference to the unit, just as we refer quantities of 

 weight to the kilogram or pound, or quantities of distance 

 to the meter or yard. The magnitude of any force may be 

 measured by either of two units, which we shall now con- 

 sider. 



67. The Gravity Unit. The given force may be 

 measured by comparing it with the gravity of some known 

 quantity or mass of matter. This is a very simple and 

 convenient way, and often answers every purpose. TJie 

 gravity unit of force is the gravity of any unit of 

 mass. This unit of mass may be a gram, kilogram, 

 pound, or ton, or any other unit that may be more con- 

 venient under the circumstances. 



(a.) A force is said to be a force of 100 kilograms when it may be 

 replaced by the action of a weight of 100 kilograms. The pressure 

 of steam in a boiler is generally measured, at present, in pounds per 

 square inch, that is, by determining the number of pounds with 

 which it would be necessary to load down a movable horizontal 

 square inch at the top of the boiler in order to keep it in place 

 against the pressure of the steam. A cord or rope may be pulled 

 with a certain force. This force is measured by finding out how 



