S18 LECTURE xviir. 



zontal portions of the fcirces may be equal, the more inclined to the horizon 

 must be the greater: the upper spring will, therefore, be a httle depressed, 

 and the carriage will remain more nearly horizontal than if the springs were 

 parallel. The reason for dividing the springs into separate plates has already 

 been explained: the beam of the carriage, that unites the whee Is, supplies the 

 strength necessary for forming the communication between the axles: if the 

 body of the carriage itself were to perform this office, the springs would re- 

 quire to be so strong that they could have little or no effect in equalising the 

 motion, and we should have a waggon instead of a coach. The ease with 

 which a carriage moves, depends not only on the elasticity of the springs, 

 but also on the small degree of stability of the equilibrium, of which we 

 may judge in some measure, by tracing the path which the centre of gravity 

 must describe, when the carriage swings. (Plate XVIII. Fig. 229.) 



The modes of attaching horses and oxen to carriages are different in different 

 countries, nor is it easy to determine the most eligible method. When horses 

 are harnessed to draw side by side, they are usually attached to the opposite 

 ends of a bar or lever; and if their strength is very unequal, the bar is some- 

 times unequally divided by the fulcrum, the weaker horse being made to act 

 on the longer bar, and being thus enabled to counteract the greater force of 

 his companion. But even without this inequality, a compensation takes 

 place, for the centre on which the bar moves is always considerably behind 

 the points of attachment of the horses; and when one of them falls back a 

 little, the effective arm of the lever becomes more perpendicular to the direc- 

 tion of his force, and gives him a greater power, while the opposite arm be- 

 comes more oblique, and causes the other horse to act at a disadvantage: so 

 that there is a kind of stability in the equilibrium. If the fulcrum were 

 further forwards than the extremity of the bar, the two horses could never 

 draw together with convenience. (Plate XVIII. Fig. 230.) 



In mining countries, and in collieries, it is usual, for facilitating the mo- 

 tion of the carriages employed in moving the ore or the coals, to lay wheel- 

 ways of wood or iron along the road on wliich they are to pass; and this 

 practice has of late been extended in some cases as a substitute for the con- 

 struction of navigable canals. Where there is a turning, the carriages are 

 usually received on a frame, supported by a pivot, which allows them to be 



