258 THE LEVER AND WHEELWORK. 



and then multiplying together the numbers which express the circumferences 

 or diameters of the several axles. The proportion of the two products will 

 express the power of the machine. Thus, if the circumferences or diameters 

 be as the numbers 10, 14, and 15, their product will be 2,100 ; and if the cir- 

 cumferences or diameters of the axles be expressed by the numbers 3, 4, and 

 5, their product will be 60, and the power of the machine will be expressed by 

 the proportion of 2,100 and 60, or 35 to 1. 



The manner in which the circumferences of the axles act upon the circum- 

 ferences of the wheels in compound wheelwork is various. Sometimes a strap 

 or cord is applied to a groove in the circumference of the axle, and carried 

 round a similar groove in the circumference of the succeeding wheel. The 

 friction of this cord or strap with the groove is sufficient to prevent its sliding, 

 and to communicate the force from the axle to the wheel, or vice versa. This 

 method of connecting wheelwork is represented in fig. 22. 



Fig. 22. 



Numerous examples of wheels and axles driven by straps or cords occur in 

 machinery, applied to almost every department of the arts and manufactures. 

 In the turning-lathe, the wheel worked by the treddle is connected with the 

 mandrel by a catgut cord passing through grooves in the wheel and axle.- In 

 all great factories revolving shafts are carried along the apartments, on which, 

 at certain intervals, straps are attached, passing round their circumferences, 

 and carried round the wheels which give motion to the several machines. If 

 the wheels, connected by straps or cords, are required to revolve in the same 

 direction, these cords are arranged as in fig. 22 ; but if they are required to re- 

 volve in contrary directions, they are applied as in fig. 23. 



Fig. 23. 



One of the chief advantages of the method of transmitting motion between 

 wheels and axles by straps or cords is, that the wheel and axle may be placed 

 at any distance from each other which may be found convenient, and may be 

 made to turn either in the same or contrary directions. 



When the circumference of the wheel acts immediately on the circumfer- 

 ence of the succeeding axle, some means must necessarily be adopted to pre- 

 vent the wheel from moving in contact with the axle without compelling the 

 latter 4o turn. If the surfaces of both were perfectly smooth, so that all fric- 

 tion were removed, it is obvious that either would slide over the surface of the 

 other without communicating motion to it. But, on the other hand, if there 

 were any asperities, however small, upon these surfaces, they would become 

 mutually inserted among each other, and neither the wheel nor axle could 

 move without causing the asperities with which its edge is studded to encoun- 



