238 SCIENTIFIC AGRICULTURE. 



its highest point in a few seconds: two pieces of wood attain 

 their utmost friction in one or two hours : when iron runs upon 

 oak the friction will increase for four or five days. 



Friction is less after motion becomes well established and 

 rapid, than when it first commences. The whole efficacy of 

 the screw depends upon the friction between the threads of 

 the external and internal screw: the screw being an inclined 

 plane, if there was no friction, it would unscrew, or the inter- 

 nal screw would descend by its own gravity when placed ver- 

 tically. Query ? What relation has the development of fric- 

 tion to. electricity ? 



The resistance of the atmosphere, which in some machines 

 must be considerable, is another obstruction to the action of 

 machinery. The weight or gravity of a machine itself, or of 

 some of its parts, is sufficient in some cases to require a consi- 

 derable part of its power to overcome it. 



STRENGTH OF MATERIALS. 



It is important, in the construction of all pieces of archi- 

 tecture and machinery, that the mechanic should know the 

 strength of the materials which he is to employ in the work. 

 By strength, we understand the power which a body has, by 

 the cohesive force of its particles to resist fracture : stress is 

 the power or tendency in a body to produce fracture by its 

 own weight. 



A joist eight inches wide and two inches thick, is four times 

 as strong when laid on its edge as when laid on its side. 



" A triangular beam is twice as strong when resting on its 

 broad base as when resting on its edge." 



" The strength of a column in the direction of its length, is 

 directly proportional to the area of its transverse section." 



" Half the length of a beam supported at both ends, will 

 bear four times as great a pressure as the whole beam ; and a 

 prop placed under the centre of a beam increases its strength 

 in the same ratio." 



