510 



CARPENTRY. 



Theory of again, hooping the part ef must be useful ; and the 



Carpentry. carpe nter of the bridge of Schaffhausen has very 



T-~ p/ properly introduced it. And upon similar principles, 



we should doubt the propriety of such erections as 



the Coalbrookdale bridge and others, where several 



parallel and curved bars are exposed to strains of this 



kind, without proper pains being taken to secure the 



connection between them. 



Joints. 



luilding 

 up beams. 



PLATE 

 CXIII. 

 Fig 3. 



Dowelling. 



rig. 4, 5. 



Coaking 

 and ta- 

 bliiig. 

 Fig- 6, 7. 



OF JOINTS. 



Having now considered in general the various kinds 

 of strains which occur in the works of the carpenter, 

 and having endeavoured, as far as was in our power, 

 to shew how the strength of the timber is made to 

 resist them, and how this resistance can be brought 

 under calculation, we proceed to examine upon si- 

 milar principles, the modes of connection by which 

 these strains are propagated through the different 

 parts of a framing. Upon this part of our subject 

 we shall be the more brief, as it is handled in more 

 detail in the practical part of this article, by a gen- 

 tleman of well known ability. Another opportunity 

 will occur for treating of the more complicated fra- 

 mings, as trusses, roofs, centres, bridges, ship frames, 

 &c. where we can consider this subject upon prin- 

 ciples that are common both to carpentry and iron 

 work. In the latter we can form frames composed of 

 one single piece, and we can, in like manner, accommo- 

 date the thickness and breadth of the parts, or their 

 length to the strength we require, or the uses they 

 are applied to, without any weaker connecting parts. 

 But in the former, we must accommodate our framings 

 to the size of our materials ; and when these are too 

 small, we must endeavour to join or piece them, till 

 the requisite magnitude or strength be attained. The 

 best method of constructing these joinings, is one of 

 the most delicate parts of the art of carpentry. 



The different forms are very numerous. Each of 

 them has its advantages in particular cases. The 

 leading principle with the carpenter and joiner should 

 be, to make every joint, even those intended only to 

 please the eye, as strong as the case will admit. He 

 will attend to the changes that may take place in his 

 work, by swelling, or shrinking ; for as this is a 

 force altogether insuperable, unless the change in 

 the whole assemblage be made to act in the same di- 

 rection, he cannot prevent the smaller parts from 

 crushing or tearing away. 



When the thickness or depth of a beam is to be 

 increased, other pieces are applied over or under it. 

 If the strength of the beam is to be combined with 

 pliability, these pieces are not fixed to the beam, but 

 bend and yield gradually like the plates of a coach- 

 spring, Fig. 3. Where the assemblage is intended 

 to act like an entire log, the parts must be attached 

 together. Iron or wooden bolts may be driven 

 through them, or they may be dowelled within, by 

 letting pieces of tough wood into the solid on each 

 side of the joint, Fig. 4, 5. The pieces may be 

 fitted into the face of the joint like tenons, or driven 

 across it like keys or wedges. They may be also 

 attached by cooking, or tabling, Fig. 6, 7, &c. 

 which is uniting two or more pieces together in the 

 middle by small tabular parts formed from the solid of 



one, and sunk into corresponding cavities in the other : Theory m 

 the butts or ends of these prevent the pieces drawing Carpentry, 

 asunder lengthwise. """Y"*" 



When the butting surfaces brought into action are 

 equal, dotvelling (Fig. 5.) is certainly preferable to P LAT " 

 coaking or tabling, for this obvious reason, that the _,. * 

 depths of the two beams are preserved entire ; and *' 

 we know that the thicker beam will be stronger, in 

 proportion to the greater distance of the compressed 

 or dilated fibres from the neutral line. Observe, how- 

 ever, that the beam is not stronger in proportion to 

 the square of this greater depth, for the quantity of 

 fibres left entire is supposed, in both cases, to be the 

 same. If the fibres which are cut across be contain- 

 ed within that part of the beam that is compressed, 

 and if close work be made, the assemblage will, in- 

 deed, be nearly as powerful as a solid beam of equal 

 depth, which is a still stronger reason for preferring 

 the method of dowelling ; nevertheless, perhaps, be- 

 cause it appears more ingenious, many carpenters ap- 

 pear to prefer the other. 



The intention of both these methods of joining, is 

 to prevent the two pieces from slipping on each other. 

 Where there is no great risk of that, the indenting 

 need not be very deep ; at all events, when the but- 

 ting surfaces are so great as to be able, without crip- 

 pling, to shove off the coak along the plane, there 

 can be nothing gained by making them deeper. 

 When the one piece is tabled into the other, we have 

 an increase of hold and consequent strength, by deep- 

 ening the indenting so far ; but we have also a dimi- 

 nution of strength by lessening the scantling of the 

 beam. There is therefore a maximum here, or cer- 

 tain depth, to which we should cut, so as to make 

 the strongest compound beam of two given balks. 

 Duhamel has made a number of experiments, with a 

 view to determine this point ; and it appears from 

 them, that the depth should be about one eighth of 

 the depth of the two balks, or one seventh of the 

 compound beam. 



Many artists use an oblique scarfing, instead of Scarfing 

 square tabling, as Fig. 8. This practice does not ap- and dove 

 pear advisable ; the sharp corner is apt to be crip- tailm g- 

 pled. Dovetailing them, as in Fig. 9. seems also Fig. 8, 9* 

 injudicious ; the pieces are then apt to tear each other 

 up. The experiments ^of Duhamel appear clearly 

 to favour the method of square tabling, and it has 

 the advantage of acting both ways. The dovetail- 

 ing appears intended to take a hold of the two pieces, 

 and prevent them from parting. There is a very in- Foxing, 

 genious method of fixing dowel pins and tree nails 

 in that way, which may be applied to other joints in 

 carpentry. A little split is made in the end of the 

 tree nail, and a small wedge struck in it ; this wedge 

 is driven home when the bolt reaches the bottom, 

 expands the end of it, and squeezes it firmly against 

 the side of the hole. 



Two beams are often connected side by side, with Building 

 the intention of resisting strains in the direction of the niasts and 

 plane of the joining, or rather with the view of resist- ax es ' 

 ing a strain in every direction, as is the case with the 

 parts of a mast, or an axle. The feather and groove, 

 used in flooring, is of the first kind, and it if some- 

 times employed in joining beams. The feather loosens 

 by the unequal shrinking of the beams, and then it 



