ON RAISING AND REMOVING WEIGHTS. 167 



pending the carriage, which prevents its swinging in a parallel direction ; 

 sucli a vibration as would take place if the straps were parallel would be 

 too extensive unless they were very short, and then the motion would be 

 somewhat rougher. The obliquity of the straps tends also in some mea- 

 sure to retain the carriage in a horizontal position ; for if they were 

 parallel, both being vertical, the lower one would have to support the 

 greater portion of the weight, at least according to the common mode of 

 fixing them to the bottom of the carriage ; the spring, therefore, being 

 flexible, it would be still further depressed. But when the straps are 

 oblique, the upper one assumes always the more vertical position, and conse- 

 quently bears more of the load ; for when a body of any kind is supported 

 by two oblique forces, their horizontal thrusts must be equal, otherwise 

 the body would move laterally ; and in order that the horizontal portions 

 of the forces may be equal, the more inclined to the horizon must be the 

 greater : the upper spring will, therefore, be a little 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 wheels, 

 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 require 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 sometimes 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 per- 

 pendicular to the direction of his force, and gives him a greater power, 

 while the opposite arm becomes more oblique, and causes the other horse 

 to act at a disadvantage ; so that there is a kind of stability in the equili- 

 brium. 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 which they are to pass ; and this 

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

 construction of navigable canals. Where there is a turning, the carnages 



