(3,58 
MECHANICS. 
only way in which all the fibres can be made to unite 
their ftrength is, to twift them together. This caufes 
them to bind each other fo fart, that any one of them will 
break before it can be drawn out of the bundle. In 
other fibrous bodies, fuch as timber, the fibres, are held 
together by l'ome cement or gluten. This is feldom fo 
llrong as the fibre. Accordingly timber is much eafier 
pulled afunder in a direction tranfverfe to the fibres. 
There is, however, every poffible variety in this parti¬ 
cular. 
In ftretching and breaking fibrous bodies, the vifible 
extenfion is frequently very confiderable. This is not 
folely the increaling of the diftanceof the particles of the 
cohering fibre; the greateft part chiefly arifes from draw¬ 
ing the crooked fibre flraight. In this, too, there is great 
diverfity; and it is accompanied with important differ¬ 
ences in their power of withftanding a Itrain. In fome 
woods, fuch as fir, the fibres on which the ftrength mod 
depends are very ftraight. Such woods are commonly 
very elaftic, do not take a fet, and break abruptly when 
overflrained ; others, fuch as oak and birch, have their 
refilling fibres very undulating and crooked, and ftretch 
very fenfibly by a drain. They are very liable to take a 
let, and they do not break fo fuddenly, but give warning 
by complaining, as the carpenters call it; that is, by giving 
vifible figns of a derangement of texture. Hard bodies 
of an uniform glafiy druflure, or granulated like hones, 
are elaftic through the whole extent of their cohefion, and 
take no fet, but break at once when overloaded. 
Notwithdanding the immenfe variety which nature ex¬ 
hibits in the drudure and cohefion of bodies, there are 
certain general fads of which we may now avail ourfelves 
•with advantage. In particular, The abfolute cokejion is pro¬ 
portional to the area of the feftion. This mull be the cafe 
where the texture is perfedly uniform, as we have rea- 
fon to think it is in glals and the dudile metals. The 
cohefion of each particle being alike, the whole cohefion 
mud be proportional to their number, that is, to the area 
of the fedion. The fame mud be admitted with refped 
to bodies of a granulated texture, where the granulation 
is regular and uniform. The fame mud be admitted of 
fibrous bodies, if we fuppofe their fibres equally llrong, 
equally denfe, and fimilarly difpofed through the whole 
fedion ; and this we mud either fuppofe, or mult date the 
diverfity, and meafure the cohefion accordingly. 
We may therefore affert, as a general propofition on 
this fubjeCt, that the abfolute ITrengrh in any part of a body 
by which it redds being pulled afunder, or the force 
which mult be employed to tear it afunder in that part, is 
proportional to the area of the fedion perpendicular to 
the extending force. Therefore all cylindrical or prifroa- 
tical rods are equally llrong in every part, and will break 
alike in any part; and bodies which have unequal fedions 
will always break in the llendered part. The length of 
the cylinder or prilm has no ed'ect on the Itrength ; and 
the vulgar notion, that it is eafier to break a very long 
rope than a diort one, is a great nsidake. Alfo the ab¬ 
folute llrengths of bodies which have fimilar fedions 
are proportional to the fa uares of their diameters or homo¬ 
logous Ikies of the fedion. 
The weight of the body itfelf may be employed to drain 
it and to break it. It is evident that a rope may be fo 
long as to break by its own weight. When the rope is 
hanging perpendicularly, although it is equally llrong in 
every part, it will break towards the upper end, becaufe 
t-he drain on any part is the weight of all that is below 
it. Its relative drength in any part, or power of with- 
Itanding the drain which is adually laid on it, is inverfely 
as the quantity below that part. When the rope is 
dretched horizontally, as in towing a Ihip, the drain 
tirifing from its weight often bears a very fenfible propor¬ 
tion to its whole Itrength. 
Thefe are the chief general rules which can be fafely 
deduced from our cleared notions of the cohelion of bo¬ 
dies. In order to make any practical ufe of them, it is 
proper to have fome meafures of the cohefion of fuch bo¬ 
dies as are commonly employed in our machines, and 
other ltrudures where they are expofed to this kind of 
drain. Thefe mud be deduced folely from experiment. 
Therefore they mud be confidered as no more than gene¬ 
ral values, or as the averages of many particular trials. 
The irregularities are very great, becaufe none of the fub- 
dances are condant in their texture and firmnefs. Me¬ 
tals differ by a thoufand circumdances unknown to us, 
according to their purity, to the heat with which they 
were melted, to the moulds in which they were cad, and 
to the treatment they have afterwards received, by for ging 
wire-drawing, tempering, &c. 
It is a very curious and inexplicable fad, that, by 
forging a metal, or by frequently drawing it through a 
fmooth hole in a deel plate, its drength is greatly in- 
creafed. This operation undoubtedly deranges the na¬ 
tural fituation of the particles. They are fqueezed clofer 
together in one diredion ; but not in the diredion in which 
they refid the fradure. In that diredion they are rather 
feparated to a greater didance. The general denfity, how¬ 
ever, is augmented in all of them except lead, which grows 
rather rare by wire-drawing : but its cohefion may be more 
than tripled by this operation. Gold, filver, and brafs, 
have their cohefion nearly tripled ; copper and iron have 
it more than doubled. In this operation they alfo grow 
much harder. It is proper to heat them to rednefs after 
drawing a little. This is called nealing or annealing. It 
foftens the metal again, and renders it fufceptible of ano¬ 
ther drawing without the rifk of cracking in the opera¬ 
tion. We do not pretend to give any explanation of this 
remarkable and very important fad, which has fomething 
refembling it in woods and other fibrous bodies. 
We fhall take for the meafure of cohefion the number 
of pounds avoirdupois which arc juft fufficient to tear 
afunder a rod or bundle of one inch fquare. From this 
it will he eafy to compute the ftrength correfponding to 
any other dimenfion. 
Gold, call 
Silver, call 
("Japan 
Barbary 
Copper, call -( Hungary 
| Anglefea 
20,000 to 24,000 lbs. 
40,000 to 43,000 
- - 19,500 
Iron, caft 
Iron, bar 
Steel, bar 
Tin, caft 
^Sweden 
42,000 to 
{ Ordinary 
Styrian - 
Beit Swedifti and Ruffian 
Horfe-nails - 
C Soft - 
l Razor temper - 
f.Malacca - ’ 
| Banca - - - - 
Block - 
| Englifh block 
l- grain - - - 
31,000 
34,000 
37,000 
50,000 ■ 
68,000 
75,000 
84,000 
71,000* 
120,000 
150,000 
3,100 
3.600 
3,800 
5,200 
6,500 
860 
1,000 
2.600 
900 
Lead, caft 
Regulus of antimony 
Zinc ... 
Bifmuth 
* This was an experiment by Mufchenbroek, to examine the 
vulgar notion that iron forged from old horfe-lhoe nails was ftronger 
than all others; and /hows its faifity. 
It is very remarkable alfo, that almoft all the mixtures 
of metals are more tenacious than the metals themfelves. 
The change of tenacity depends much on the proportion 
of the ingredients ; and the proportion which produces the 
mod tenacious mixture is different in the different metals; 
We have feleded the following from the experiments of 
Mufchenbroek. The proportion of ingredients here l'e- 
leiled is that which produces the greateft ftrength. 
Two parts of gold with one of filver . - lbs. 28,000 
Five parts of gold with one of copper - - 50,000 
Five parts of iilver with one of copper- - - 481,500 
Fou? 
