MECHANICS. 
6©0 
view has very jtldiclo'ufly obfervecl, that all thofe intel- 
lethial a net mechanical contrivances which diminifh the 
charges of producing bread and butchers-meat, have an 
effect precifely fimilar to the inventions which economize 
the labour and capital of the manufacturer. In both 
cafes, indeed, fame inconvenience is felt for a time by 
thofe whole labour is difpenfed with ; but in both cafes 
too, the community is benefited, by the greater abundance 
and confequent cheapnefs of the feveral articles. Befides, 
as this cheapnefs has a powerful effect in augmenting 
confumption, the capital laved in the firft inltance, or the 
greater part of it, is ufually reinvelted in the undertaking ; 
and the fame, or even a greater, number of hands is em¬ 
ployed than before. It is not probable that the cotton- 
jnanufa&ure, for example, would now employ fo many 
people, if the mechanical inventions of Arkwright, Cromp¬ 
ton, and others, had not brought its fabrics into general 
ufe by their cheapnefs. The thrafning-mill, in like man¬ 
ner, has pot diminilhed the number of farm-labourers, as 
-was fo much dreaded; and wages have increafed inltead 
of diminifhing,- becaufe the capital faved by this machine 
is now employed in extending agricultural improvements, 
which the ufe of it has rendered more profitable. 
The giand objeft of all anechanifin, or machinery, is to 
convey and modify the motion of the firft mover of the 
machine, and communicate it in a proper manner to the 
fubjett to be operated upon : thus, the flow rotative mo¬ 
tion of a water-wheel is, by the machinery of cranks, le¬ 
vers, and toothed wheels, converted into a rapid recipro¬ 
cating motion for workirig fawing-machines ; and the 
velocity of the motion is increafed or diminilhed, as the 
-occafion requires either great power or great fpeed. In 
like manner, the reCtilinear motion of the pifton-rod of a 
Iteam-engine is, by the machinery of parallel levers, work¬ 
ing-beam, connedting-rod, crank, and fly-wheel, converted 
into a rotative motion ; and this motion can again, by the 
machinery of wheel-work, be adapted, either in velocity 
or power, to work grinding-ftones, circular faws, thralh- 
ing-mills, and other fimilar machines which require great 
velocity; or flatting-mills, boring-machines, rafping-ma- 
chines for logwood, lead-pipe drawing machines, &c. 
which require great power to give them motion, and are 
therefore, performed with a lei's velocity. Machinery is, 
therefore, the organs by which motion is altered in its 
velocity, its period, and diredlion, and thus adapted to 
any purpofe. All machinery will he found, upon minute 
invelligation, to be only modifications of the fix mecha¬ 
nical potvers ; and the greateft number will be found to 
■confift chiefly of parts which have a motion of rotation 
round fixed axes, and derive all their energy from levers 
virtually contained in them : thus the pulleys, wheel, and 
axle, are only modifications of the lever; and the ferew 
is compounded of the lever with a variety of the inclined 
plane or wedge; fo that the number <?f mechanical powers 
may be reduced to two, which alfurfie an infinite variety 
of forms and motions. The theory and manner of calcu¬ 
lating their effects will be found in the preceding part of 
this article. 
In contriving any machinery,the engineer fliould always 
remember that nothing contributes more to the perfection 
of a machine, ef'pecially if it is maflive and ponderous, 
than great uniformity of motion. Every irregularity of 
motion wades fomc of the impelling power; and it is only 
the greatelt of the varying velocity which is equal to that 
which the machine would acquire if moving uniformly 
throughout; for, while the motion accelerates, the impel¬ 
ling force is greater than what balances the reliftance then 
actually oppoftd to it, and the velocity is lefs than what 
the machine would acquire if moving uniformly ; and, 
when the machine attains its greateft velocity, it attains 
it becaufe the power is then not ailing againft the whole 
refinance. In both of thefe fituations, therefore, the per¬ 
formance of the machine is lefs than if the power and re¬ 
finance conftantly bore the fame relation to each other, 
in which cafe it would move uniformly. 
Every attention fhould, therefore, be given to tills; and 
we fliould endeavour to remove all caufe of irregularity 
through the whole machine. There are continual returns 
of ftrains and jolts from the inertia of the different parts 
ailing- in oppofite directions. Although the whole mo¬ 
menta may always balance each other, yet the general 
motion is hobbling, and the points of fupport are (trained. 
A great engine, fo conftruiled, commonly caufes the 
building to tremble; hut, when uniform motion pervades 
the whole machine, the inertia of each part tends to pre-- 
ferve this uniformity, and all goes fmoothly. It is alfo 
deferving of remark, that, when the communications are 
fo contrived, that the uniform motion of one part pro¬ 
duces uniform motion to the next, the preflures at the 
communicating points remain conftant or invariable. Now 
the accomplishing of this is generally within the reach of 
mechanics ; and the engineer fliould adapt his machinery 
to the particular cafe before him. 
As the inertia of every machine adds greatly to the re¬ 
finance to he overcome, and as the friCtion of the commu¬ 
nicating parts is proportional to the preflu re, it become^ a 
matter of great practical importance, that the different 
parts of a machine fliould be proportioned to the ftrains 
to which they are expofed. If the beam of a fleam-engine, 
for example, is larger than what is neceflary, an immenfe 
portion of the impelling power muff be deftroyed at every 
itroke of the pifton, by dragging the fuperfluous rnafs from 
a (fate of reft into motion ; the p re flu re upon the gudgeons 
will alfo be increafed, and their friCtion in their fockets 
proportionally enlarged. The engineer, therefore, fliould 
be well acquainted with the Itrength of the materials of 
which the machine is to he conftruiled, and fhould frame 
its different parts in filch a manner that they may not be 
heavier than is neceflary for refilling the forces with 
which they are urged. When the motions of the machine 
are Heceffarily irregular, and when the machine may be ex¬ 
pofed to accidental ftrains, the parts muff be made confi. 
derably ftronger than is neceflary for refilling its ordi¬ 
nary ftrains; but it is not often that fuch a precaution 
fhould be obferved. The gudgeons of water-w heels, and 
of the beams of fleam-engines, ought to be made as (liort 
and (mail as poflible, as the friCtion increafes with the 
rubbing furfaces. This is very feldom attended to in the 
conftruCtion of water-wheels. The diameter of the gud¬ 
geons is frequently thrice as large as is neceflary for iup« 
porting the weight of the wheel. 
In the conftruCtion of machinery, we muft not only 
attend to the Amplification of the parts, but alfo to the 
number of thefe parts, and the mode of connecting them. 
From the nature and quantity of the work to he performed, 
it is eafy to afeertain the velocity of the working point 
.which is moll proper for performing it. Now, this velo¬ 
city may be procured in a variety of ways, either by a per¬ 
plexing multiplicity of wheels, or by more Ample combi¬ 
nations. The choice of thefe combinations muft be left 
folely to the judgment of the engineer, as no general rules 
can be laid down to direCt him. It may be ufeful, how¬ 
ever, to remark, that the power fliould always be applied 
as near as poflible to the working point of the machine ; 
and that, when one wheel drives another, the diameter 
of the one fliould never be great when the diameter of 
the other is very fmall. The fize of wheels is often deter¬ 
mined from the ftrains to which they are expofed. If, for 
example, we are obliged to give a certain velocity to an 
axle by means of a wheel with 120 teeth, and if the force 
with which this wheel is urged requires the teeth to be at 
lead one inch tliick in order to prevent them from break¬ 
ing, we ftiall be obliged to make its diameter at leaft feven 
feet; for, fuppofing the fpaces between the teeth to be 
equal to the thicknefs of the teeth, the circumference of 
the wheel muft at leaft be equal to 120-1*110=2.40 inches, 
the fum of the teeth and their intervals, which gives a dia¬ 
meter of fix feet eight inches. 
There are Come cafes where our choice of combination 
mult be directed by the nature of the machinery. If the 
work 
