512 



CARPENTRY. 



Theory of into action as a bolt, and being so much shaken, is 

 Carpentry. not eX posed to any leverage, and consequently can- 

 """Y~""' not cripple the fibre in that event. This key need 

 be no thicker than just to take hold of the wood, 

 without tearing it up ; the greater distance between 

 these mortices the deeper they must be cut, since the 

 greater will be the lateral adhesion. As to the 

 breadth of the key itself, this must depend on its 

 hardness ; and, in order to have it as small as possi- 

 ble, it is common to make it of some much harder 

 substance than the beam, as of oak, or of iron. The 

 latter is said to be apt to cripple the fibre ; but per- 

 haps this is more to be ascribed to injudicious driving, 

 for these keys are usually made with a small taper, 

 and, in order to obtain firmness and closeness, the 

 workmen is apt to proceed too far. Every species 

 of connection in carpentry is liable to this defect, and 

 the necessity of guarding against it must be carefully 

 and constantly impressed upon the workman. The 

 joints should be close, and all motion prevented, and 

 the pieces made firmly to adhere, that no corner or 

 single fibre may be unequally loaded ; but any thing 

 beyond that is useless and dangerous. 



Since, therefore, the apparent firmness of adhe- 

 sion, produced by bolts and screws, does not com- 

 municate any real additional strength, and that an 

 equal adhesion may be produced even by a wooden 

 connection, the ingenuity of the carpenter is left 

 more at liberty ; and it may not be improper to in- 

 quire, whether stronger modes of adhesion are not to 

 PLAT* be found than those either of Fig. 17, or 18. It 

 \ XIII. is unquestionable, that if the two surfaces of No. 2. 

 Fig. 17, 18. Fig, 17. were tabled together, the joint would be 



greatly stronger, and this can be done with very lit- Theory of 

 tie, if any loss of wood ; nay, the adhering parts near Carpentry. 

 the ends of the joint being so near the edge,, and of ^ """V"" 1 

 course so much farther removed from the neutral fi- 

 bre, would render the assemblage stronger against a 

 transverse strain than even No. 1. The tabling will PLATE 

 be more perfect by lengthening the joint, that is, by CXIII. 

 making it more oblique. But there is a limit to that Fl S- 17 - 

 independent of the waste of timber ; for when the * 

 distance between the alternate projections is so great, 

 as to tear the fibre ere the connecting matter part, 

 there can nothing be gained by increasing the surface 

 of attachment. Fig 20. exhibits another method of Fig. 20. 

 piecing a beam, which will enable it to resist a con- 

 siderable transverse strain ; and, as a tie, it is at least 

 as strong as any such form as Fig. 17. The lower Fig- 17, 

 piece is supposed to be oak, and by tightening the 

 key above, its transverse strength is greatly increa- 

 sed. Duhamel has made many experiments on an 

 assemblage somewhat similar to this, and has shown 

 us, that this built beam of three pieces may, in some 

 cases, be much stronger than an entire one. The in- 

 genious carpenter will see, that the principle is capa- 

 ble of being applied in many other situations. As a 

 tie, its chief value consists in taking hold of several 

 plates of fibres. We may clap another such fish on 

 the top, and so double the connection. But we have 

 already said enough on this subject, which would re- 

 quire much more room than we can spare to treat of 

 it completely. Hereafter we shall see, that much 

 more powerful assemblages may be formed by fra- 

 ming. hSee the article FRAifli^e, (A. N.) 



PART IT. 



