MECHANICS. 
below, the horfe will bear no part of the load on level 
ground. In fome fituatioris the animal will be lifted from 
the ground when there is the greateft neceflity for his 
being preffed to it, and he will fometimes bear a great 
proportion of the load when he (hould rather be relieved 
of it. 
The only way of remedying thefe evils, is to aflign fuch 
a pofition to the centre of gravity, that the horfe may bear 
fome portion of the weight when he mud exert great force 
againft the load, that is, in level ground, and when he is 
afcending deep roads ; for no animal can pull with its 
greateft effort unlefs it is preffed to the ground. Now 
this may be in fome meafure effected in the following man¬ 
ner. Let BCN, fig. r, be the wheel of a cart, A D one 
of the (hafts, D that part of it where the cart is fufpended 
on the back of the horfe, and A the axle-tree; then, if 
the centre of gravity of the load is placed at m, a point 
equidiftant from the two wheels, but below the line D A, 
and before the axle-tree, the horfe will bear a certain 
weight on level ground, a greater weight when he is going 
up hill, and has more occafion for it ; and lefs weight 
when he is going down hill, and does not require to be 
prefled to the ground. All this will be evident from the 
figure. When we recolleft that the (haft D A is horizon¬ 
tal, the centre of gravity will prefs more upon the point 
of fufper.fion D the nearer it comes to it; or the preffure 
upon D, or the horfe’s back, will be proportional to the 
dillance of the centre of gravity from A. If m, there¬ 
fore, be the centre of gravity, b A will reprefent its pref¬ 
fure upon D, when the (haft D A is horizontal. When 
the cart is afcending a deep road, AH will be the pofi¬ 
tion of the (haft, the centre of gravity will be raifed to a , 
and a A will be the preffure upon D. But, if the cart is 
going down hill, AC will be the pofition of the (liafr, 
the centre of gravity will be depreffed to n, and c A will 
reprefent the preffure upon the horfe’s back. The weight 
fuftained by the horfe, therefore, is properly regulated 
by placing the centre of gravity at m. We have (fill, how¬ 
ever, to determine the proper length of ba and b m, the 
diftance of the centre of gravity from the axle, and from 
the horizontal line DA; but, as thefe depend upon the 
nature and inclination of the roads, upon the length of 
the (haft DA, which depends on the fize of the horfe, on 
the magnitude of the load, and on other variable circum- 
ftances, it would be impoliible to fix their value. If the 
load, along with the cart, weighs 4.00 pounds; if the dif¬ 
tance D A be eight feet, and if the horfe (liould bear 50 
pounds of the weight; then b A fhould be one foot, which, 
being one-eighth of DA, will make the preffure upon D 
exaiftly 50 pounds. If the road (lopes four inches in a foot, 
bm muft be four inches, or the angle b A m (liould be 
equal to the inclination of the road ; for then the point m 
will rife to a when afcending fuch a road, and will prefs 
with its greateft force on the back of the horfe. 
When carts are not made in this manner, we may, in 
fome degree, obtain the fame end by judicioufly difpofing 
the load. Let us fuppofe that the centre of gravity is at O 
when the cart is loaded with homogeneous materials, fuch 
as fand, lime, Sec. then if the load is toeonfift of heteroge¬ 
neous fubftances, or bodies of different weights, we (hould 
place the heaviest at the bottom and neareft the front, 
which will not only lower the point O, but will bring it 
forward, and nearer the proper pofition m. Part of the 
load, too, might be fufpended below the fore-part of the 
carriage in dry weather, and the centre of gravity would 
approach (till nearer the point m. When the point m is 
thus depreffed, the weight on the horfe is not only judi¬ 
cioufly regulated, but the cart would be prevented from 
overturning ; and in rugged roads the weight fuftained 
by each wheel would be in a great degree equalized. 
Experiments by Richard Lovell Edgeworth, Efq. M.R.I.A, 
and F.R.S.—From the Tranfactions of the Royal Irilh 
Academy, Dublin; 1788. 
I was prefent, in the year 1773, at a fet of experiments 
679 
that were tried in London, to determine the relative ad¬ 
vantage of high and low wheels for carriages. Difputes 
had arifen upon this fubje<ft between mechanics of no 
fmall eminence; and to determine them an apparatus 
W'as provided, confiding of a very long and fmooth table, 
upon which the carriages to be compared were to be drawn 
by a firing and a defeending weight. The carriages were 
conftrucled by fome of the belt workmen in London ; the 
firings were made of plaited (ilk, of fmall diameters, pafs- 
ing over a pulley nicely turned, and mounted in fuch a 
mannerasto have fcarcely any friftion. The experiments, 
however, were undecifive, each party claiming the evi¬ 
dence in favour of their own opinion. Very little differ¬ 
ence was perceptible between the carriages when they ran 
upon the fmooth table ; and, when they were drawn over 
obftacles, fometimes the high and at other times the low 
wheels had the advantage, according to the different 
heights and (hapes of the obftacles. It appears, upon a 
firlt view, that the force which drew thefe carriages was 
employed only in overcoming the friction of the axle-tree, 
or in lifting the weight over the obftacles ; but I fufpefted 
at the time, and have been fince convinced, that an ob- 
ftruftion of another fort exifted, which was more confi- 
derable than either of thole which I have mentioned, and 
which has not, to my knowledge, been taken notice of 
by any writer upon mechanics. 
The load upon a carriage, in palling over an obftacle, 
refills the power which draws it, not only by its weight, 
but by its vis inertia. After a carriage has been once fet 
in motion upon a fmooth road with any given velocity, 
its motion, fo longy as that velocity is continued, is nei¬ 
ther retarded nor promoted by its vis inertia ; bur, when¬ 
ever it paffes over any height, not only the weight of the 
carriage muft be lifted up, but the vis inertia of that weight 
muft be overcome in a new direction ; and as much velo¬ 
city muft be communicated to it, in that new direction, 
as will enable it to rife to the height of the obftacle whiKt 
it paffes over its bafe. When an obftacle is of luch a fize 
and (liape that a wheel of fix feet diameter muft ftrike the 
top of it at once, and not roll from the bottom upwards,, 
and when its (liape will permit a fmaller wheel to touch it 
during its whole afeent, as there is more time allowed for 
overcoming the vis inertia of its weight in the latter cafe 
than in the former, the fmaller wheel may be drawn for¬ 
wards by a lefs power than the larger, notwithftanding 
the advantage of lever, which is in favour of the larger 
wheel. 
To determine thefe circumftances by experiment, it 
was neceffary to conftrufl an apparatus different from that 
which I have delcribed. I at firft made ufe of an inclined 
plane, of five or fix feet long, and one foot high, placed 
upon a fmooth horizontal floor. The diftance to which 
the carriage was driven upon the floor, by the velocity 
which it acquired in its defeent down this inclined plane, 
I allumed as the meafure of its acquired force ; and the 
refiftance of any obftacle, which I placed in its way, I de¬ 
termined by the diminution of this diftance. But, though. 
I was well fatisfied with the accuracy of this mode of trial, 
I conltruffed another that may appear to others lefs liable 
to objection. This is (ltown at fig. 4. Upon a folid floor, 
A B, I (crewed a circle of iron, C D, three feet three 
inches diameter ; in the centre of this circle I ereeled an 
upright axis, or roller, E, upon two pivots, one relting 
in a locket of brafs upon the floor, f, the other in a frame 
or bridge, GgG, which was railed acrofs the machine. 
This axis, or roller, had a fmall filk cord, h k, wound 
round its circumference, which, pafiing into an adjoining 
(taircafe, had a fcale and weights, I, tied to it, which 
turned the roller with the required velocity. From the. 
roller a horizontal arm of wood, K, extended to the cir¬ 
cumference of this iron circle; and to its extremity was 
faltened a piece of (teel in the form of an axle-tree of a 
carriage, and upon this was placed a wheel, L, which by 
thefe means was carried round upon the iron circle, as the 
(tone of a tanner’s bark-mill moves round the trough which 
belongs 
