MECHANIC S. 
6;6 
cumflaitces, however, which, independent of the addi¬ 
tional weight and expenfe of large wheels, prefcribe limits 
to their fize. If the radius AC of the wheel exceeds the 
height of that part of the horfe to which the traces are 
attached, the line of fraction DA will he oblique to the 
horizon, and part of the power P will be employed in 
preffing the wheel upon the ground. A wheel exceeding 
four-and-a-half feet radius, which is the general diftance 
from the ground of that part of the horie to which the 
traces are attached, has (fill the advantage of a fuialler 
wheel; but, when we confider that the traces or poles of 
the cart will, in this cafe, rub again!! the flanks of the 
horfes, fo that the power of the wheel is diminifhed by 
the increafe of its weight, we (hall be convinced that no 
power is gained by making the radius of the w heels greater 
than four feet and a half. Even this ftze is too great, as 
fhall be ihovvn when we treat of the line of traction ; fo 
that we may fafely aflert, that the diameter of wheels 
ihould never be greater than fix feet. The fore-w-heels of 
our carriages are dill unaccountably fmall ; and it is not 
uncommon to fee carts moving upon wheels fcarcely 
fourteen inches in diameter. The convenience of turning 
is urged as the reafon for diminifliing the fore-wheels of 
carriages; and the facility of loading the cart is confidered 
ns a fufficient reafon for tiling wheels fo fmall as fourteen 
inches. 
The next thing to be determined is the (hape of the 
wheels. Now it is certainly a matter of furprife how the 
unnatural lhape which is at prefent given to them could 
ever have been brought into ufe. A cylindrical wheel, 
with the fpokes perpendicular to the naves, is undoubt¬ 
edly the form which every mechanic would give to his 
wheels, before he had heard of the pretended advantages 
of concave or difiling wheels, or thofe which have inclined 
fpokes and conical rims. It has been alleged, indeed, 
-that the form reprefented in fig. 2. where Ar, Bt, is the 
conical rim, and 0 A, pH, the inclined fpokes, renders 
the wheel ltronger than it would otherwife be ; that by 
extending the bafe of the carriage it prevents it from 
being overturned; that it hinders the fellies from rubbing 
again!! the load or the Tides of the cart; and that, when 
one wheel falls into a rut, and therefore fupports more 
than one-half.of the load, the fpokes are broaght into a 
vertical pofition, which renders them more capable of 
lultaining the additional weight. 
Now it is evident that the fecond of thefe advantages 
is very trifling, and may be obtained, when required, by 
interpofing a piece of board between the wheel and the 
load. The other two advantages exift only in very bad 
roads; and, if they are necefiary, which we much quef- 
•tion, in a country like this, where the roads are fo excel¬ 
lently made and fo regularly repaired, they can eafily be 
procured, by making the axle-tree a few inches longer, 
and increafing the ftrength of the fpokes. But it is al¬ 
lowed on all hands that perpendicular fpokes are prefer¬ 
able on level ground, The inclination of the fpokes there¬ 
fore, which renders concave wheels advantageous in rug¬ 
ged and unequal roads, renders them di!advantageous 
when the roads are in good order; and where the good 
roads are more numerous than the bad ones, as they cer¬ 
tainly are in this country, the difadvantages of concave 
wheels muft overbalance their advantages. It is true in¬ 
deed, that, in concave wheels, the fpokes are in their 
ftrongelt pofition when they are expofed to the fevered 
drains, that is, when one wheel is in a deep rut, and fuf- 
tains more than one-half of the load ; but it is equally true, 
that, on level ground, where the fpokes are in their weakeft 
pofition, a le(s fevere drain, by continuing for a much 
longer time, may be equally if not more detrimental to 
the wheel. 
Upon thefe obfervations we tnay red the opinion which 
we have been maintaining, and appeal for its truth to the 
judgment of every intelligent and unbiafted mind; but 
we fhall go a ttep farther, and endeavour to (how that 
concave difliing-wheels are more expenfive, more injurious 
to the roads, more liable to be broken by accidents, and 
lefs durable in general, than thofe wheels in which the 
fpokes are perpendicular to the naves. By infpe&ing 
fig. 2. it will appear, that the whole of the p refute 
which the wheel A B fuftains is exerted along the inclined 
fpoke p s, and therefore acts obliquely upon the level 
ground n D, whether the rims are conical or cylindrical. 
This oblique aft ion muft neceflarily injure the roads, by 
loofening the (tones more between B and D than between 
B and n ; and, if the load were diffidently great, the (tones 
would dart up between s and D. The texture of the 
roads, indeed, is diffidently firm to prevent this from 
taking place ; but, in confequence of the oblique preffure, 
the (tones between s and D will at lead be loofened, and, 
by admitting the rain, the whole of tire road will be ma¬ 
terially damaged. But, when the fpokes are perpendicular 
to the nave, as p n, and when the rims m A, n B, are cy¬ 
lindrical, or parallel to the ground, the weight fuftained 
by the wheel will aft perpendicularly upon the road ; 
and, however much that weight is increafed, its action 
can have no tendency to derange the materials of which 
it is compofed, but is rather calculated to confolidate 
them, and render the road more firm and durable. 
It was obferved that concave wheels are more expen¬ 
five than plane ones. This additional expenfe arifes from, 
the greater quantity of Wood and workmanfliip which the 
former require; for, in order that dilhing-wheels may be 
of the fame perpendicular height as plane ones, the fpokes 
of the former mult exceed in length thofe of the la lief, 
as much as the hypothenufe 0 A of the triangle 0 Am 
exceeds the fide om\ and therefore the weight and the 
refiftance of fuch wheels mult be proportionabiy great. 
The inclined fpokes, too, cannot be formed nor inferted 
with fuch facility as perpendicular ones. The extremity 
of the fpoke which is fixed into the nave is inferted at 
right angles to it, in the direftion op-, and, if the rims 
are cylindrical, the other (poke (hould be inferted in a 
fimilar manner, while the intermediate portion lias an 
inclined pofition. There are therefore two flexures or 
bendings in the fpokes of concave wheels, which require 
them to be formed out of a larger piece of wood than if 
they had no fuch flexures, and render them liable to be 
broken by any fudden (train at the points of flexure. 
We (hall now difmifs the fubjeft of concave wheels 
with one obfervation more ; and we beg the reader’s atten¬ 
tion to it, becaufe it appears to be decifive of the quel'-, 
tion. The obftacles wiiich carriages have to encounter* 
are feldom fpherical protuberances that permit the ele¬ 
vated wheel to relume by degrees its horizontal pofition r 
they are generally of fuch a nature, that the wheel is 
inftantaneoufly precipitated from their top to the level 
ground. Now, the momentum with which the wheel 
Jtrikes the ground is very great, arifing from a fucceffive 
accumulation of force. The velocity of the elevated wheel 
is confiderable when it reaches the top of the eminence 5 
and, while it is tumbling into the level ground, it is re¬ 
ceiving gradually that proportion of the load which was 
transferred to the other wheel, till, having recovered th® 
whole, it impinges againft the ground with great velocity 
and force. But in concave wheels the fpoke which then 
(trikes the ground is in its weakeft pofition, and therefore 
much more liable to be broken by the impetus of the fall, 
than the fpokes of the lowed wheel by the mere trans¬ 
ference of additional weight : whereas, if the fpokes be 
perpendicular to the nave, they receive this fudden (hock 
in their ftrongeft pofition, and are in no danger of giving 
way to the (train. 
In the preceding obfervations we have fuppofed the 
rims of the wheels to be cylindrical. In ccjicave wheels, 
however, the rims are uniformly made of a conical form, 
as A r, Bs, fig. 2. which not only increafes the dilad- 
vantages which we have aferibed to them, but adds many 
more to the number. Mr. Gumming, in a Treadle on 
Wheel-Carriages, folely devoted to the confideration of 
this Angle point, has (hown with great ability the difad¬ 
vantages 
