118 



WELLS'S NATURAL PHILOSOPHY. 



What Is 

 tonguin" ? 



■^^'^lat is dove- 

 tailing? 



which the 



edges of boards are wholly, or partially received 

 by channels in each other. 



266. Dovetailing is a ^lo- loo. 



method of insertion in 

 parts are connected hy ^^^g^r 

 ivedge-sliaped indentations which per- 

 mit them to be separated only in one 

 direction. (See Fig. 100.) 

 What is mor- 267. Mortising is a method of insertion in 

 usjng? which the projecting extremity of one timber is 

 received into a perforation in another. (See Fig, 101.) The 

 Fig. 101. opening or hole cut in 



one piece of wood to re- 

 ceive or admit the pro- 

 jecting extremity of an- 

 other piece, is called a 

 mortise ; and the end of 

 the timber which is re- 

 duced in dimensions so 

 as to be titted into a mor- 

 tise, for fastening two timbers together, is called a tenon. 

 268. The form in which a given quantity of 

 matter can be arranged in order to oppose the 

 greatest resistance to a bending force, is that 

 of a hollow tube, or cylinder ; and the strength 

 of a tube is always greater than the strength 

 of the same quantity of matter made into a solid rod. 



The most beautiful and sti'iking illustrations of this princi- 

 ple occur in nature. The bones of men and animals are hol- 

 low, and nearly cylindrical, because they can in this form, 

 with the least weight of material, sustain the greatest force. The stalks of 

 numerous species of vegetables, especially the grain-bearing plants, as wheat, 

 rice, oats, etc., which are required to bear the weight of the ripened ear of 

 grain, or seed, are hollow tubes, and their strength, compared with their 

 lightness, is most remarkable. In this form they not only sustain the crush- 

 ing weight of the ear which they bear at the summit, but also the force of the 

 wind. Iq the construction of columns for architectural purposes, especially 

 those made of metal, this principle is taken advantage of* 

 • In that most gigantic work of modem engineering, the Britannia Tubular Bridge. 



'wm 



In what form 

 can a given 

 quantity of 

 matter be ar- 

 ranged to op- 

 pose the jjreat- 

 est resistance ? 



What are il- 

 lustrntions of 

 this principle ? 



