MATERIALS FOR ARCHITECTURAL PURPOSES. 119 



269. A beam, supported at its two ends, when bent bv its 

 "Why IS a beam . , . , . , ,, , . ,.,.,• i i , ' • 



beut in the weight m the middle, has its habihtj' to break greatly in- 



r^'f '^'"k ,^'**''^ creased, because the destroying force acts with the advantage 

 of a long lever, reaching from the end of the beam to the cen- 

 ter ; and the resisting force or strength acts only with the force of a shart lever 

 from the side to the center ; at the same time, a httle only of the beam on the 

 under side is allowed to resist at alL 



Tliis last circumstance is so remarkable, that the scratch of a pin on the 

 under side of a beam, resting as here supposed, will sometimes suffice to begin 

 the fracture. 



SECTION II. 



APPLICATION OF MATERIALS FOR ARCniTECTCRAL OR STRUCTURAL PURPOSES. 



whatisArchi- 270. Architecture, in its general sense, is the 

 " "^^ art of erecting huilJings. In modern use, the 



name is often restricted to the external forms, or styles of 

 buildings. 



ltd '^^^® diflferent varieties of architecture undoubtedly owe theur 



the different origin to the rude structures which the cUmate or materiiils of 

 architecture "' ^^^ countrj' obliged its early inhabitants to adopt for tempo- 

 ppjbablj' owe rary shelter. These structures, with all their prominent fea- 

 eir origin lures, have been afterward kept up by their refined and 



opulent posterity. Thus the Egyptian slj'le of architecture had its origin in 

 the cavern, or mound. The Chinese architecture is modeled from a tent ; the 

 Grecian is modeled from the wooden cabin ; and the Gothic, it has been sug- 

 gested, from the bower of trees. 



On what does 271. Tho Strength of a building will princi- 

 abniidinfprhf- P^^lj dopcud ou the walls being laid on a good 

 cipaiiydepend? j^^j ^^^^ foundation, of Sufficient thickness at 

 the bottom, and standing perfectly perpendicular. Its 

 usefulness will depend upon a proper arrangement of it3 

 parts. 



crossing the Menai Straits, which separate the island of Anfrlesea from the mainland of 

 Great Britain, advantage has been taken of the strength of matter arranged in the form 

 of a tube or hollow cylinder. The entire bridge is formed of immense rectangular tube* 

 •f iron, 56 feet hicjh in the center, 14 feet wide, and having an entire length of 1513 feet, 

 with an elevation above the water of more than 100 feet. The sides of the tubes are also 

 composed of smaller tubes, united together in a peculiar maimer, so as to obtain the 

 maximum of strength from the form of structure; and so great is this strength, that a 

 train of loaded cars, weighing 2S0 tons, and impelled with great velocity, deflects the 

 tubes in their centers less th.in three fourths of an inch. The entire weight of the tubes 

 composing this bridge is upward of lO.,50O tons, the length of two of the spans, or dist.ances 

 between the points of support, being -tGO feet each. The same amount of iron in the fbna 

 of a solid rod or beam, would not probably have sustained its own weight. 



