WHEEL. 



in going up and down hill, tlic whole load will be fuftained b m muft be four inches, or the angle b A m (hould be equal 

 by the wheels, and will have no tendency either to prefs the to the inclination of the road, for then the point m will rife 



horfe to the ground, or to raife him from it. But if the to a when afcending fuch a road, and will prefs with its 

 . — . r :... ;. r__ .l__ - .u- ...i_ . — „. ;. „../i „„ greateft force on the back of the horfe. 



When carts are not conftrufted 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, &c. then if the load is to confift of hetero- 

 geneous fubitances, or bodies of different weight, we Ihould 

 place the heavieft 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 ftill 

 nearer the point m. When the point m is thus depreffed, the 

 weight on the horfe is not only judicioufly regulated, but the 

 cart will be prevented from overturning, and in rugged roads 

 the weight fuftained by each wheel will be in a great degree 

 equalized. 



I" loading four-wheeled carriages, great care ftiould be 

 when there is the greateft neceflity for his being preffed to taken not to throw much of the load upon the fore-wheels, 

 it, and he will fometimcs bear a great proportion of the as they would otherwife be forced deep into the ground, and 

 load when he fhould rather be relieved from it. require great force to pull them forward. In fome modern 



The only way of remedying thefe evils, fays Dr. Brewfter, carriages, this is very little attended to. The coachman's 

 is to aflign fuch a pofition to the centre of gravity, that the horfe feat is fometimes enlarged fo as to hold twoperfons, and all 

 may bear fome portion of the load when he muft exert great the baggage is generally placed in the front, direftly above 

 force againft it, that is, in level ground, and when he is afcend- the wheels. By this means the greateft part of the load is 



centre of gravity is far above the axle-tree, as it muft ne- 

 ceflarily be according to the piefent conftruftion of wheel- 

 carriages, a great part of the load will be thrown on the 

 back of the horfes from the wheels, when going down a 

 fteep road, and thus tend to accelerate the motion of the 

 carriage, which the animal is ftriving to prevent : while in 

 afcending fteep roads a part of the load will be thrown be- 

 hind the wheels, and tend to raife the horfe from the ground, 

 when there is the greateft neceflity for fome weight on his 

 back, to enable him to fix his feet on the earth, and over- 

 come the great refiftance which is occafioned by the fteep- 

 nefs of the road. On the contrary, if the centre of gra- 

 vity is below the axle, the horfe will be prefled to the ground 

 in going up-hill, and lifted from it when going down. In 

 all thefe cafes, therefore, where the centre of gravity is 

 either in the axle-tree, or diredly above or below it, the 

 horfe will bear no part of the load in level ground. In 

 fome fituations, the animal will be hfted from the ground 



ing fteep roads ; for no animal can pull with its greateft effort 

 unlefs it is preffed to the ground. Now, this may in fome 

 meafure be effetled in the following manner : — Let B C N 

 { Plate XL.^/ff. 12.) be the wheel of a cart, A D one of the 

 fhafts, D that part of it where the cart is fufpended on the 



upon the fmall wheels, and the draught becomes doubly fe- 

 vere for the poor animals, who muft thus unneceflarily fufFer 

 for the ignorance and folly of man. 



There is another great difadvantage attending fmall fore- 

 wheels ; viz. that as their axle is below the level of the 



back of the horfe, and A the axle-tree ; then if the centre horfe's breafts, the horfes not only have the loaded carriage 

 of gravity of the load is placed at m, a point equidiftant to draw along, but alfo part of its weight to bear, which 

 from the two wheels, but below the line DA, and before 



■Is 

 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 a lefs weight when he is going 

 down-hill, and does not require to be preffed to the ground. 

 All this will be evident from the figure, when we recolleft 

 that if the fhaft D A is horizontal, the centre of gravity 

 will prefs more upon the point of fufpenfion D the nearer it 

 comes to it ; or the prefTure upon D, or the horfe's back, 

 will be proportional to the diftance of the centre of gravity 

 from A. If m therefore be the centre of gravity, b A will 

 reprefent its prefTure upon D, when the fhaft D A is hori- 

 zontal. When the cart is afcending a fteep road, A H will 

 be the pofition of the fhaft, the centre of gravity will be 

 raifed to a, and a A will be the prefTure upon D. But if 

 the cart is going down hill, A C will be the pofition of the 

 fhaft, the centre of gravity will be depreffed to n, and c A 

 will reprefent the prefTure upon the horfe's back. The 



tires them fooner, and makes them grow much ftifferin their 

 hams, than they would be if they drew on a level with the 

 fore-axle ; and for this reafon, coach-horfcs foon become 

 unfit for riding. lSo that on all accounts it is plain, that 

 the fore-wheels of all carriages ought to be fo high as to 

 have their axles even with the breafls of the horfes ; which 

 would not only give them a fair draught, but likewife caufe 

 the machine to be drawn by a lefs degree of power. 



Mr. Beighton difputes the propriety of fixing the line of 

 traftion on a level with the brealt of a horfe, and fays it is 

 contrary to reafon and experience. Horfes, he fays, have 

 little or no power to draw, but what they have from their 

 weight ; otherwife they could take no hold of the ground, 

 and then they muft flip, and draw nothing. Common ex- 

 perience alfo teaches, that if a horfe is to convey a certain 

 weight, he ought, that he may draw the better, to have a 

 proportional weight on his back or fliouldcrs. Befides, 

 when a horfe draws hard, he bends forward, and brings his 



weight fuftained by the horfe, therefore, is properly regu- breaft near the ground ; and then, if the wheels are high, he 



lated by placing the centre of gravity at m. We have ffill, 

 however, to determine the proper length of b a and b m, the 

 diftance of the centre of gravity from the axle, and from 

 the horizontal line D A ; but as thefe depend upon the na- 

 ture and inclination of the roads, upon the length of the 

 fhaft D A, which varies with the fize of the horfe, on the 

 magnitude of the load, and on other variable circumftances, 

 it would be impoffibleto fix their value. If the load, along 

 with the cart, weighs four hundred pounds, if the diftance 

 D A be eight feet, and if the horfe ftiould bear fifty pounds 

 of the weight, then b A ought to be one foot, which being 

 one-eighth of D A, will make the preffure upon D exatlly 

 fifty pounds. If the road flopes four inches in one foot. 



is pulling the carriage againft the ground. A horfe tackled 

 in a waggon will draw two or three ton, becaufe the point or 

 hne of traftion is below his breaft, by reafon of the wheels 

 being low. And it is very common to fee, when one horfe is 

 drawing a heavy load, his fore-feet will rife from the ground ; 

 and he will nearly ftand on end ; in which cafe it is ufual to 

 add a weight on his back, to keep his fore-part down, by a 

 perfon mounting on him, which will enable him to draw that 

 load, without which he before could not move. The 

 great ftrefs, or main bufinefs of drawing, fays this ingenious 

 writer, is to overcome obftacles ; for on level plains the draw- 

 ing is but little, and then the horfe's back need be preffed 

 but with a fmall weight. Moft or all of thefe obftacles may be 



confidered 



