STB
473
STB
! . 'Och square, by the last given note. Mr. Emerson says, a
a^i 6 S° od oa ^’ an * nc '' square and a yard long, supported33ft 0t ^ ends > W >U bear in the middle, for a very little time,^ , lb. avoirdupoise, but will break with more than thatf ^ ei ght: now, upon the supposition that the strength isj Cl Procally as the length, the section being the same,Ion !: : ^ or ^ ie strength of a piece of oak one foot
as I and an * nc h square, which is nearly 1000 lb., but' 3 greater than 3, and almost equal to 4, we have* 4 = 1320, instead of 1000; so that Emerson’s timberWronger than Buffon’s.
Table XII.
It,
4In.sq.
5 In. sq.
6 In.sq.
7 In.sq.
8 In. sq.
5621 —
10978 +
18970+
30125+
44968+
J*i
4756—
9290 +
16054+
25494+
38055+
4105—
8018 +
13856—
22003+
32845—
105
3599+
7029 +
12146—
19288+
28792—
3
2865+
5596 +
9671—
15357+
22925—
145
4615 +
7976—
12665+
18908—
16|
3906 +
6750—
10715+
16000—
3371 +
5826—
9251 +
13810—
205
2955 +
5107 +
8109+
12105—
22}
2623 +
24}
2353 +
28}
1939 +
Th
' *bo v 6 su cceeding Table contains the differences between thet|i e j.® an< l the Table of experiments : most of them are withinba rs tee ntli part of the truth, excepting those of the five-inch*Ppe Eaar ^ ed with a star: that column, as has been observed,ar s to have much stronger timber then the others.
Table XIII.
L.
7
8
9
"hT
12
14
16
18
20
~22
24
4 In. sq.
5 In. sq.
6 In. sq.
7 In. sq.
8 In. sq.
— 281
+ 592
+ 44
+ 2163
+ 2796
— 174
+ 548
— 455
+ 656
+ 1826
— 43
+ 347
— 623
+ 460
+ 103
+ 54
+ 160
— 804
+ 312
+ 878
+ 171
+ *556
— 460
+ 969
+ 722
+ *774
— 372
+ 736
+ 1097
+ ‘546
— 240
+ 486
+ 638
+ *444
— 98
+ 220
— 314
+ *398
+ 23
+ 517
— 290
+ *493
+ 37
+ 15
6 E
M. De Buffon uniformly found, that two-thirds of the 'weight necessary to break a piece of timber, in a few minutesimpaired its strength, and frequently broke it at the end oftwo or three months : the half of that weight brought it toa certain degree of curvature, which did not increase after jthe first minute or two ; and this, he found, might be bornefor any length of time ; but when the weight was removed,the timber had acquired a curvilinear figure. One-third of ^the weight appeared to have no effect ; and, when properlyseasoned, the timber recovered its rectilinear figure, afterbeing loaded several months. Therefore, when the strengthof a piece of timber is known, according to any of the prece-ding methods, and if it is applied to any mechanical purpose,in bearing or supporting a heavy body, it ought to havesufficient strength to bear thrice, or rather four times, theweight to be applied.
Several reasons may be assigned for the irregularity ofexperiments ; some pieces, even from the same tree, might 'be much stronger than others. M. De Buffon says, thathealthy trees are always strongest at the root. The ligneouscoats might not be disposed in the same way, and theirresistance is always greater when perpendicular to the forceapplied, than when in an horizontal direction. The trunk of ja tree is composed of many hollow cylinders, each formed jin the annual growth; these are only united by a soft jmedullary substance; and when the tree is brought to a ! Jsquare, or cut into beams, the portions of the ligneous cylin- jdric coats at the angles, present but a very small resistance.
A bar of small size, such as an inch square, cut out of alarge tree, may have its annual plates nearly in planes, dis-posed perpendicularly to one of its sides, and, consequently,it will be as much stronger, or weaker, than the largestsquare beam cut out of a tree of the same size and texture,as the force is applied to the side, or edges, of the plates.
M. De Buffon found, from repeated trials, that the strengthof a bar, with its plates in the direction of the breaking force,was to that of a bar with its plates perpendicular to it, as 8to 7, nearly.
From what has been said, and exhibited in the fore-going Tables, it appears, that the resistance of timberincreases in a ratio somewhat less than the square of j
the depth, and somewhat greater than in the reciprocal jratio of the length. j
The cohesion of timber is probably of a nature different |
from that of metal, and is subject to greater inequalities ; j
which inequalities are occasioned either by the soil in which jthe tree was reared, the growth of the tree, or the part from jwhich it is cut.
To what has already been said, the following practicalremarks may be added. \
Mr. Petit, on the authority of his own experiments, and ithose of M. Parent, declares that the utmost strength of asquare inch of oak, by pulling in the direction of its length,does not exceed 86401b., whereas Muschenbroeck makesit 17,3001b., but by experiments tried at the Koyal Military jAcademy, Woolwich, the strength of oak has been found butlittle exceeding. 9,0001b. its specific gravity being 774. Thewriter of the article Strength of Materials , in Rees’sCyclopcedia, observes, that “ We have not this datum in eitherof the above cases ; yet we conceive it to be a very import- jant one, as we have always found the strength of wood of !the same kind, to depend a great deal upon the weight, orspecific gravity. The same experiments give for the strengthof ash 17,0001b. and fir from 10,0001b. to 13,0001b., bothconsiderably different from Muschenbroeck’s tabular results.”
The same writer, or experimenter, adds, that “ an oak rodof an inch surface requires a weight of about 9,0001b. to