Encyclopaedia Britannica, or, a Dictionary of arts, sciences, and miscellaneous literature : enlarged and improved. Illustrated with nearly six hundred engravings > Volume 13, MAT-MIC

MECHANICS.
Praftlcal over the flones lias four points of fupport, each ftone
Mechanics. a weight of two hundred weight, and there-
' fore will not be broken. But if the fame cart, with
rims only twro inches in breadth, fhould pafs the fame
way, it will cover only two of the ftones 5 and the
wheel having now only two points of fupport, each
ftone will be preffed with a -weight of four hundred
weight, and will therefore be reduced to powder.
Hence we may infer that narrow wheels are in another
point of view injurious to the roads, by pulverizing the
materials of which they are compofed.
446. As the rims of wheels wear fooneft at their
edges they fhould be made thinner in the middle, and
ought to be faftened to the fellies with nails of fuch a
kind that their heads may not rife above the furface of
the rims. In fome military waggons we have feen the
heads of thefe nails riling an inch above the rims, which
not only deftroys the pavements of ftreets, but oppofes
a continual reliftance to the motion of the wheel. If
thefe nails were eight in number, the wheel would ex¬
perience the fame reliftance, as if it had to furmount
eight obftacles, one inch high, during every revolution.
The fellies on which the rims are fixed fhould in carria¬
ges be three inches and a fourth deep, and in waggons
four inches. The naves fhould be thickeft at the place
where the fpokes are inferted } and the holes in which
the fpokes are placed Ihould not be bored quite through,
as the greafe upon the axle-tree would infinuate itfelf
between the fpoke and the naves, and prevent that
clofe adhefion which is neceflary to the ftrength of the
wheel.
On the Pofit ion of the Wheels.
447. It muft naturally occur to every perfon refleft-
ing upon this fubjeft, that the axle-trees Ihould be
ftraight and the wheels perfectly parallel, fo that they
may not be wider at their higheft than at their loweft
point, whether they are of a conical or a cylindrical
form. In this country, however, the -wheels are always*
made concave, and the ends of the axle-trees are uni-
verfally bent downwards, in order to make them fpread
at the top and approach nearer below. In fome car¬
riages which we have examined, where the wheels
were only four feet fix inches in diameter, the diftance
of the wheels at top was fully fix feet, and their diftance
below only four feet eight inches. By this foolifh
pra&ice the very advantages which may be derived from
the concavity of the wheels are completely taken away,
while many of the difadvantages remain •, more room
is taken up in the coach-houfe, and the carriage is
more liable to be overturned by the contraftion of its
bafe.
448. With fome mechanics it is a praftice to bend
the ends of the axle-trees forwards, and thus make the
wheels wider behind than before. This blunder has
been ftrenuoufly defended by Mr Henry Beighton,
who maintains that wheels in this pofition are more
favourable for turning, fince, whe "’ the wheels are paral¬
lel, the outermoft when turning would prefs againft the
Jinch pin, and the innermoft would reft againft the
fhoulder of the axle-tree. In reftilineal motions, how¬
ever, thefe converging wheels engender a great deal of
fri&ion both on the axle and the ground, and
muft therefore be more difadvantageous than parallel
,®nes.
I 27
On the Line of TraBion, and the Method by which ^ra<^lcal
tt r \ .7 ■ n 7 J Mechanics.
horjes exert their Jtrength. ■
449. M. Camus attempted to fhew that the line of trac¬
tion thould always be parallel to the ground on which
the carriage is moving, both becaule the horfe can
exert his greateft ftrength in this direction, and becaufe
the line of draught being perpendicular to the vertical
fpoke of the wheel, acts with the largeft pofiible lever.
M. Couplet, however, confidering that the roads are
never perfectly level, and that the wheels are conftant-
ly furmounting fmall eminences even in the belt of
roads, recommends the line of traftion to be oblique
to the horizon. By this means the line of draught
HA, (which is by far too much inclined in the figure) Fig. 6.
will in general be perpendicular to the lever AC
which mounts the eminence, and will therefore aft
with the longeft lever when there is the greateft nccef-
fity for it. We ought to confider alfo, that when a
horfe pulls hard againft any load, he always brings his
breaft nearer the ground, and therefore it follows, that
if a horizontal line of traftion is preferable to all others,
the direftion of the traces Ihould be inclined to the ho¬
rizon when the horfe is at reft, in order that it may be
horizontal when he lowers his breaft ^nd exerts his ut-
moft force. The particular manner, however, in
which living agents exert their ftrength againft great
loads, feems to have been unknown both to Camus and
Couplet, and to many fucceeding writers upon this
fubjeft. It is to M. Deparcieux, an excellent philo-
fopher and ingenious mechanic, that we are indebted
for the only accurate information with which we are
furnifhed-, and we are forry to fee that philofophers
who flourifiied after him have overlooked his important
inftruftions. In his memoir on the draught of horfes
he has fliewn in the moft fatisfaftory manner, that ani¬
mals draw by their weight, and not by the force of
their mufcles. In four-footed animals, the hinder feet
is the fulcrum of the lever by which their weight afts
againft the load, and when the animal pulls hard, it
deprefles its cheft and thus increafes the lever of its
weight, and diminifties the lever by which the load re¬
fills its efforts. Thus, in fig. 6. let P be the load, AD
the line of traftion, and let us fuppofe FC to be the
hinder leg of the horfe, and AE part of its body, A
its cheft or centre of gravity, and CE the level road.
Then AFC wall reprefent the crooked lever by which
the horfe afts, which is equivalent to the ftraight one
AC. But when the horfe’s weight afts downwards at
A, fo as to drag forward the rope AD and raife the
load P, CE will reprefent the power of the lever in
this pofition, or the lever of the horfe’s weight, and
CF the lever by which it is refilled by the load, or
the lever of refiftance. Now if the horfe lowers its
centre of gravity A, which it always does when it
pulls hard, it is evident that CE, the lever of its weight,
will be increafed, while CF the lever of its refiftance
will be diminilhed, for the line of traftion AD will
approach nearer to CE. Hence we fee the greet
benefit which may be derived from large horfes ; for
the lever AC neceffarily increafes with their fize, and
their power is always proportioned to the length cf
this lever, their weight remaining the fame. Large
horfes, therefore, and other animals, will draw more
than fmall ones, even though they have Ids mufcular
force.