10 



APPLIED SCIENCE 



ribbon tapes on which the graduations are accurately etched 

 are to be preferred for extremely careful measurements. 

 They are very convenient in measuring curvilinear or irregu- 

 lar surfaces, as is done in measuring the circum- 

 ference of a gas tank, the length of a belt to run 

 over pulleys, or the length of band iron around 

 a packing case. When using a tape measure for 

 any considerable distance, care should be taken 

 to see that the tape is supported at frequent in- 

 tervals or rests on the floor. Otherwise an error 

 will occur, due to the sagging and stretching of 

 the unsupported tape. 



Compass-like devices with curved legs, called 

 calipers, are used to measure the diameters of 

 round bodies (Figs. 5 and 6). 



FIG. 5. 



Inside 

 Calipers. 

 For measur- 

 ing inside 

 bore dia- 

 meters. 



12. Mass and Weight. Mass is the quantity 

 of matter contained in a body. When we speak 

 of a pound of lead, the word pound expresses a definite quan- 

 tity of matter. Commercially, weight 

 always stands for mass. A merchant 

 estimates his stock in pounds and usually 

 understands by those weights nothing 

 more -than the quantity of matter pos- 

 sessed. The unit of mass is the quantity 

 of matter in a standard pound. 



The weight of a body is the measure of 

 the force attracting it towards the center of 

 the earth. Figure 7 illustrates the principle 

 of ordinary weighing scales. outside diameters. 



13. Density. The simplest way to determine the weight 

 of a large body is to measure its volume and then multiply 



