C A R P E N T R Y. 



490 



ry of the internal parts of the holes should be no smaller 

 than tin- edges. Great irregularities occurred in the 

 * first experiments from this cause, because the pins 

 \vcic somewhat tighter within than at the edges; but 

 when this was corrected, they were extremely regu- 

 lar. He employ I'd three sets of holes, viz. a circle, a 

 square, (which was occasionally made a/ rectangle 

 whose length was twice its breadth,) and an equila- 

 teral iri:mgle. In all these experiments, the strength 

 appeared exactly proportional to the area of the sec- 

 tion, and quite independent of its figure or position : 

 hut it was considerably abov* the direct cohesion ; 

 that is, it took considerably more than twice the force 

 to tear out this middle piece, than to tear the pin 

 asunder by a direct pull. A. piece of fine freestone re- 

 quired 205 pounds to pull it directly asunder, and 575 

 to break it in this way. The difference was very con- 

 stant in anyone substance, but varied from |ds to yds 

 in different kinds of matter, being smallest in bodies 

 of a fibrous texture. But indeed he could not make 

 the trial on any bodies of considerable cohesion, be- 

 cause they required such forces as his .apparatus 

 could not support. Chalk, clay baked in the sun, 

 baked sugar, brick, and free-stone, were the strongest 

 that he could examine. 



We conceive that a good reason may be given why 

 the resistance to the strain should be greater than the 

 direct cohesion. The exterior line of particles is sup- 

 ported by those behind. Before any remarkable shift 

 in the place of the exterior line takes place, the co- 

 hesion of the whole section may be called into action. 

 This will certainly be the case with the granular and 

 less compressible kinds of matter. But this is not all. 

 The lateral motion of the first particle will, to a cer- 

 tain degree, be communicated to that adjoining and 

 within it ; this will, in its turn, exert a similar resist- 

 ance, and communicate the force still farther. The 

 pressure, therefore, is not confined to the area of sec- 

 tion, but is propagated intb the body of the beam ; it 

 not only forces each particle to slide along its neigh- 

 bour, squeezing and crushing those which are in the 

 neighbourhood of the section of fracture, but like- 

 wise tearing them away from those on the side oppo- 

 site to the pressure. Fibrous matter, such as timber, 

 must especially act in this way. We cannot expect 

 the fibres to snap and fly at once, unless where the 

 motion is exceedingly rapid, as in the penetration of 

 cannon-shot. The fibre will yield gradually, and 

 bend. The bending will be propagated into the 

 beam, to a distance which we cannot well tell, but 

 which seems proportional to the tenacity of the fi- 

 bre. At length, it will yield much in the same way 

 as it does to a direct pull ; which accounts for the 

 smaller excess of strength found in fibrous bodies. 

 This case is, therefore, by no means so simple as 

 may appear. Although there be reason to believe 

 that the strength is proportional to the surface of 

 the section, we should think it advisable that the 

 beaiing surface should be as wide and flat as possible, 

 for all matter is compressible, and wood, especially, 

 is most so across the fibre. Should the part, there- 

 fore, which first receives the strain be narrow or- 

 thin, it is possible that the front fibres may be crip- 

 pled or give way ere those intended to support them 

 can well be brought into action. Tina principle 



should always be kept in view in the formation of te- Theory of. 

 nons and mortices, or joints of a similar kind, and ap- Carpentry, 

 pears well known to the intelligent carpenter. The ""^ """" 

 same thing is frequently aimed at, by making the te- 

 non long, and giving it a deep hold in the mortice. 

 Such a practice gives no additional strength what- 

 ever ; it is even injurious. For if the tenon bear up- 

 on the interior part of the mortice instead of the out- 

 er edge, the strain comes to act upon it with the ener- 

 gy or a lever, and may thus be increased in any de- 

 gree. The bearing side of the tenon being no longer 

 than sufficient to give a firm hold, the end may be 

 bevelled above, till flush with the upper side of the 

 beam. We may trust to every square inch of the sec- 

 tion of such a tenon the weight stated under the first 

 head ; for though experiments shew it to bear more, 

 it is as well that this excess of strength should not 

 be depended on. 



What we have now stated may throw some light 

 upon the resistance which a wedge or tree nail affords 

 to compression. It is obvious that this must chiefly 

 depend on the structure of the timber ; and we have 

 hardly any experiments worthy of memory upon the 

 subject. 



Respecting the resistance to splitting, and that 

 which hinders a piece of straight-grained timber from 

 sliding out in the direction of the fibre, we are equal- 

 ly at a loss for information. The strength is, with- 

 out doubt, proportional to the surface of fracture, 

 but we cannot state its amount. Straight fibred tim- 

 ber is very weak in this way, and the, reason is obvi- 

 ous ; it is the cohesion of the soft connecting matter, 

 and not of the woody fibre, that we have to resist the 

 strain. 



Emerson has said, that tough wood* such as elm 

 or ash, across the grain, is from seven to eight, or 

 even ten times weaker than when strained in the di- 

 rection of the fibre. These woods may be supposed, 

 therefore, to resist a force tending to split them, or 

 to tear out the heel of a mortice, with about 700 Ibs. 

 on the square inch of the surface of fracture. BuL 

 he says, that the wood of straight fibre which easily 

 splits, such asjir, is 16, 18, or "20 times weaker than 

 in the length of the fibres. Their resistance will, in 

 that case, be from 250 to 300 Ibs. per square inch of 

 section. 



We have now considered those strains in which the 

 cohesion of the timber is directly opposed to the 

 straining force. We have sound reason to regret, 

 that the paucity of the experiments hitherto made on 

 the direct cohesion, has prevented us from giving 

 such satisfactory information respecting the corre- 

 sponding strength as might be wished. We regret 

 this the more, as it is to some one of these simple 

 kinds that the carpenter should endeavour to reduce 

 every other : he is then sure of benefiting by tin- 

 whole cohesive strength of his materials. In the 

 others, or compound strains, he has not the same ad- 

 vantage. 



OF THE COMPOUND STRAINS. 



The variety of compound strains is inexhaustible ; Compound 

 we can only be expected here to treat of the simplest raia.<. 

 and most important. 



