M E C H 
vantages of conical rims, and the propriety of making 
them cylindrical; but we are of opinion, that he has 
af’cribed to conical rims feveral difadvantages which arife 
chiefly from the inclination of the fpokes. He infills 
much upon the injury done to the roads by the ufe of 
conical rims; yet, though we are convinced that they are 
more injurious to pavements and highways than cylin¬ 
drical rims, we are equally convinced, that this injury is 
occafioned chiefly by the oblique prefiure of the inclined 
fpokes. The defeCts of conical rims are fo numerous and 
palpable, that it is wonderful how they Ihould have been 
fo long overlooked. Every cone that is put in motion 
upon a plane furface will revolve round its vertex ; and, 
if force is employed to confine it to a ftraight line, the 
fmaller parts of the cone will be dragged along the ground, 
and the friCtion greatly increafed. Now, when a carriage 
moves upon conical wheels, one part of the cone rolls 
■while the other is dragged along; and, though confined 
to a rectilineal direction by external force, their natural 
tendency to revolve round their vertex occafions a great 
and continued friCtion upon the linch-pin, the thoulder 
of the axle-tree, and the fides of deep ruts. 
The Chape of the wheels being thus determined, we 
muft now attend to fome particular parts of their con- 
XtruCtion. The iron plates of which the rims are com- 
pofed fliould never be lefs than three inches in breadth, 
as narrow rims fink deep into the ground, and therefore 
injure the roads and fatigue the horfes. Mr. Walker, in¬ 
deed, attempts to throw ridicule upon the aCt of parlia¬ 
ment which enjoined the ufe of broad wheels ; but he 
does not aflign any fufficient reafon for his opinion, and 
ought to have known that feveral excellent and well- 
devifed experiments were lately inftituted by Boulard and 
Margueron,’which evince in the mod fatisfaCtory manner 
the great utility of broad wheels. Upon this fubjeCt an 
obfervation occurs to us, which has not been generally 
attended to, and which appears to remove all the objec¬ 
tions which can be urged againft broad rims. When any 
load is fupported upon two points, each point fupports 
one-half of the weight; if the points are increafed to 
four, each will fuftain one-fourth of the load, and fo on ; 
the prefl'ure upon each point of fupport diminifliing as 
the number of points increafes. If a weight therefore is 
fupported by a broad furface, the points of fupport are 
infinite in number, and each of them will bear an infi¬ 
nitely fmall portion of the load ; and, in the fame way, 
every finite portion of this furface will fuftain a part of 
the weight inverfely proportional to the number of fimilar 
portions which the furface contains. Let us now fup- 
pofe that a cart carrying a load of fixteen hundred weight 
is fupported upon wheels whofe rims are four inches 
in breadth ; and that one of the wheels pafies over four 
Hones, each of them an inch broad and equally high, and 
capable of being pulverized only by a prefiure of four 
hundred pounds weight. Then, as each wheel fuftains 
one-half of the load, and as the wheel which pafles over 
the Hones has four points of fupport, each flone will bear 
a weight of two hundred weight, and therefore will not 
be broken. But if the fame cart, with rims only two 
inches in breadth, fhould pafs the fame way, it will cover 
only two of the Hones; and, the wheel having now only 
two points of fupport, each flone will be prefled with a 
weight of four hundred weight, and will therefore be re¬ 
duced 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. 
As the rims of wheels wear foonefl at their edges, they 
fhould be made thinner in the middle, and ought to be 
faflened 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 deflroys 
the pavements of Hreets, but oppofes a continual refin¬ 
ance to the motion of the wheel. If thefe nails were 
eight in number, the wheel would experience the fame 
VOL. XIV, No. 1004., 
A N I C S. G77 
refinance as if it had to furmount eight obflacles, one 
inch high, during every revolution. The fellies on which 
the rims are fixed fhould in carriages be three inches and 
a fourth deep, and in waggons four inches. The na'ves 
fhould be thickefl at the place were the fpokes are inferred ; 
and the holes in which the fpokes are placed fhould not be 
bored quite through, as the greafe upon the axle-tree would 
infinuate itfelf between the fpoke and the naves, and pre¬ 
vent that clofe adhefion which is neceflary to the flrength 
of the wheel. 
It muft naturally occur to every perfon reflecting upon 
this fubjeCt, that the axle-trees fhould be Hraight and the 
wheels perfectly parallel, fo that they may not be wider 
at their higheft than at their lowed 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 univerfally bent downwards, in 
order to make them fpread at the top and approach nearer 
below. In fome carriages which we have examined, 
where the wheels were only four feet fix inches in diame¬ 
ter, the diflance of the wheels at top was fully fix feet, 
and their diflance below only four feet eight inches. By 
this foolifh practice 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 car¬ 
riage is more liable to be overturned by the contraction 
of its bafe. 
With fome mechanics it is a practice 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, when the wheels are parallel, the outermolt w hen 
turning would prefs againlt the linch-pin, and the inner- 
mod would reft againft the flioulder of the axle-tree. In 
reCtilineal motions, however, thefe converging wheels en¬ 
gender a great deal of friCtion both on the axle and the 
ground, and muft therefore be more difadvantageous than 
parallel ones. 
Of the Line of TraElion, and the Method by which Horfes exert 
their Strength. 
M. Camus attempted to fnow that the line of traClion 
fliould always be parallel to the ground on which the 
carriage is moving, both becaufe the horfe can exert his 
greateft ftreiigth in this direction, and becaufe the line of 
draught, being perpendicular to the vertical fpoke of the 
wheel, aCls with the largeft poflibie lever. M. Couplet, 
however, confidering that the roads are never perfectly 
level, and that the wheels are conftantly furmounting 
fmall eminences even in the belt of roads, recommends 
the line of traClion to be oblique to the horizon. By this 
means the line of draught H A, (which is by fartoo much 
inclined in the figure,) fig. 1, will in general be perpen¬ 
dicular to the lever A C which mounts the eminence, 
and will therefore aCt with the longed lever when there is 
the greateft neceflity for it. We ought to confider alfo, 
that, when a horfe pulls hard againft any load, he always 
brings his brealt nearer the ground, and therefore it fol¬ 
lows, that, if a horizontal line of traClion is preferable to 
all others, the direction of the traces fliould be inclined 
to the horizon when the horfe is at red, in order that it 
may be horizontal when he lowers his bread and exerts 
his utmoft force. The particular manner, however, in 
which living agents exert their flrength againft great 
loads, feems to have been unknown both to Camus and 
Couplet, and to many fucceeding writers upon this fub- 
jeCt. It is to M. Deparcieux, an excellent philofopher 
and ingenious mechanic, that we are indebted for the 
only accurate information with which we are furnifhed { 
and we are forry to fee that pliilofophers who .flourilhed 
after him have overlooked his important inftruCtions. In 
his memoir on the draught of horfes he has fhown in the 
molt fatisfaCtory manner, that animals draw by their 
8 K, weight. 
