36 SCIENCE OF COMMON THINGS. 



Strength of materials. Hollow tubes. 



the Chinese many centuries, before the Christian era. 

 In Europe, its composition and properties were dis- 

 covered by Berthold Schwartz, a Prussian monk, ins 

 the twelfth century. It was first used in battle inl 

 1346. 



CHAPTEE II. 



STRENGTH OF MATERIALS. 



SOQ When materials are employed for mechanical purposes, upon what 

 does their power or strength, apart from the nature of ti*e material, depend 

 for resisting external force? 



Upon the shape of the material, its hearing, and the 

 nature of the force applied to it. 



SO7 In what position witt a bar or learn sustain the greatest application 

 of force f 



When it is strained in the direction of its length. 



SOS What do we mean by stiffness of a material f 



It is the resistance to the application of force tending 

 to bend it. 



509 How much stiffer is a beam supported at both ends, than one of 

 hall the length firmly faced at only one end? 



Twice as stiff. 



510 In what form can a given quantity of matter be arranged so as to 

 oppose the greatest resistance to a bending force f 



In the form of a hollow tube or cylinder. 



511 Why are the bones of man and animals hollow and cylindrical f 



Because in this form they can with the least weight 

 of material sustain the greatest force. In man and 

 animals, the hollow part of the bones is filled with an 

 oily substance called marrow. 



SIS Why are the quills of birds hottow and empty of marrow t 



