MECHANICS. 
65 ? 
tt 4. The wood of the north, fide of all trees which grow 
sn our European climates is the weakeft, and that of the 
Pouth-eaft fide is the ftrongeit; the difference being moft 
remarkable in hedge-row trees, and fuch as grow fingiy. 
The heart of a tree is never in its centre, but always nearer 
to the north fide, and the annual coats of wood are thinner 
on that fide. In conformity with this, it is a general opi¬ 
nion of carpenters, that.timber is llronger whofe annual 
plates are thicker. The trachea, or air-vellels, are weaker 
than the firnple ligneous fibres. Thefe air-veffels are the 
fame in diameter and number of rows in trees of the fame 
fpecies, and they make the vifible feparation between the 
annual plates. Therefore, when thefe are thicker, they 
contain a greater proportion of the firnple ligneous fibres. 
“ 5. All woods are more tenacious while green, and 
lofe very confiderably by drying after the trees are felled.” 
The only author who has put it in our power to judge 
of the propriety of his experiments is Mufchenbroek. He 
has defcribed his method of trial minutely ; and it feems 
unexceptionable. The woods were all formed into flips 
fit for his apparatus, and part of the flip was cut away to a 
parallelopiptd of-J-th of an inch fquare, and therefore - 2 -Vth 
of a fquare inch in feflion. The abfolute ftrength* ot a 
fquare inch were as follow : 
Locull-tree 
20,ioolbs. 
Pomegranate 
9 , 75 °lbs 
Jujeb 
18,500 
Lemon 
9,250 
Beech, oak - 
O 
O 
CO 
S'- 
Tamarind 
8,750 
Orange - 
I 5 , 5 °° 
Fir 
8,330 
Alder 
1 3 ; 9 °° 
Walnut 
8,130 
Elm 
I 3,200 
Pitch-pine 
7,650 
Mulberry 
12,500 
Quince 
6,750 
Willow - \ 
12,500 
Cyprefs 
6,000 
Afli - 
I 2,000 
Poplar 
5 . 5 oo 
Plum 
Elder - 
1 1,800 
10,000 
Cedar - 
4,880 
Mufchenbroek has given a very minute detail of the ex¬ 
periments on the afh and the walnut, Hating the weights 
which were required to tear afunder flips taken from the 
four fides of the tree, and on each fide, in a regular pro- 
greffion from th® centre to the circumference. The num¬ 
bers of this table correfponding to thefe two timbers may 
therefore be confidered as the average of more than fifty 
trials made of each ; and he fays that all the others were 
made with the fame care. YVe cannot therefore fee any 
teafon for not confiding in the refiilts ; yet they are con¬ 
fiderably higher than thofe given by fome other writers. 
Mr. Pitot lays, on the authority of his own experiments, 
and of thofe of Mr. Parent, that 6olbs. will juft tear afunder 
a fquare line of found oak, and that it will bear 50 with 
fafety. This gives 8640 for the utmolf ftrength of a fquare 
•inch, which is much inferior to Mufchenbroek’s valuation. 
We may add to thefe, 
Ivory - i6,27olbs. 
Bone - - - - - 5,250 
Horn - 2)750 
Whalebone - 7,500 
Tooth of fea-calf - - 4>°75 
The reader will furely obferve, that thefe numbers exprefs 
Something more than the utmolt cohefion ; for the weights 
ere fuch as will very quickly, that is, in a minute or two, 
tear the rods afunder. It may be faid in general, that two 
thirds of thefe weights will fenfibly impair the ftrength af¬ 
ter a confiderable while, and that one-half is the utmoft 
that tan remain fufpendedat them, without rifk, forever; 
and it is this ball allotment that the engineer fhould rec¬ 
kon upon in his conftruilions. There is, however, con¬ 
fiderable difference in this refpeft. Woods of a very 
ftraight fibre, fuch as fir, will be lefs impaired by any load 
which is not fufficient to break them immediately. 
According to Mr. Emerfon, the load which may be 
fafely fufpended to an inch fquare is as follows: 
Iron .... 76,4oolbs. 
Brafs - - 35.600 
Vol.XIV. No. 1002, 
Hempen rope - - - 
Ivory - 
Oak, box, yew, plum-tree 
Elm, afh, beech . - 4 
Walnut, plum - 
Red fir, holly, elder, plane, crab apple 
Cherry, hazel ... 
Alder, afp, birch, willow, white fir 
Lead - 
Fine freeftone ... 
i9,6oolbs. 
15,700 
7,850 
6,070 
5 > 3 , 6 o 
5>oco 
4,760 
4,290 
430 
914 
He give's us a practical rule, that a cylinder whofe diamefct 
is d inches, loaded to one-fourth of its abfolute ftrength, 
will carry as follows :—Iron, 135 cwt. good rope, 22 ; oak, 
14 ; fir, 9 cwt, The rank which the different woods hold 
in this lift of Mr. Emerfon’s is very different from what 
we find in Mufchenbroek’s. But precil'e meafuies muff: 
not be expedited in this matter. 
The next confideration is, in what fituation the ma¬ 
terial is moft likely to break, or to endure.—When a long 
cylindrical or prifmatic body, fuch as a rod of wood 01’ 
metal, or a rope, is drawn by one end, it mult be refilled, 
at the other, in order to bring its cohefion into action. 
When it is fattened at one end, we cannot conceive jt 
any other way than as equally ltretched in all its parts 
for all our obfervations and experiments on natural bo¬ 
dies concur in fliowing us, chat the forces which connect: 
their particles, in any way whatever, are equal and oppo- 
fite. This is called the third law of motion ; and we ad¬ 
mit its univerfality, while we affirm that it is purely ex¬ 
perimental. 
Since all parts are thus equally ftretched, it follows, 
that the ftrain in any tranfverfe fisftion is the fame, as 
alfo in every point of that fedion. If therefore the body 
be fuppofed of a homogeneous texture, the cohefion of the 
parts is equable; and, fince every part is equally ftretch¬ 
ed, the particles are drawn to equal diftances from their 
quiefeent pofitions, and the forces which are thus excited, 
and now exerted in oppofition to the (training force, are 
equal. This external force may be increafed by degrees, 
which will gradually feparate the parts of the body more- 
and more from each other, and the connecting forces in- 
creafe with this increafe of diftance, till at lalt the cohe¬ 
fion of fome particles is overcome. This mult be imme¬ 
diately followed by a rupture, becaufe the remaining forces 
are now weaker than before. 
It is the united force of cohefion, immediately before 
the difunion of the firft particles, that we call the Jlrtngtk 
of the fettion. It may alfo be properly called its abfolute 
Jlrength, being exerted in the fimpleft form, and not mo¬ 
dified by any relation to other circumltances. 
If the external force has not produced any permanent 
change on the body, and it therefore recovers its former 
dimenfions when the force is withdrawn, it is plain that 
this ftrain may he repeated as often as we pleafe, and the 
body which withftands it once will always withffand it. 
It is evident alfo, that this fhould be attended to in all 
conftruttions ; and that, in all our invefiigations on this 
fubjefl, it fhould be kept ttriftly in view. When we 
treat a piece of loft clay in this manner, and with this 
precaution, the force employed mult be very fmall. If 
we exceed this, we produce a permanent change. The 
rod of clay is not indeed torn afunder; but it has become 
foinewhat more (lender; the number of particles in a crofs 
feftion is now (mailer ; and therefore, although it will 
again, in this new form, Puffer or allow an endlefs repe¬ 
tition of a certain ltrain without any farther permanent 
change, this ftrain is fmaller than the former. 
Something of the fame kind happens in all bodies which 
receive a fet by the ftrain to which they are expofed. All 
ductile bodies are of this kind. But there are many bo¬ 
dies which are not ductile. Such bodies break completely 
whenever they are ftretched beyond the limit of then’ 
perfeff elafticity. Bodies of a fibrous ftrufture exhibit 
very great varieties in their cohefion. In fome the fibres 
have no lateral cohefion, as in the cafe of a rope. The 
8 E only 
