POWELL] TECHNOLOGY XLI 



A wheel is a device for reducing friction, and the friction is 

 reduced inversely as the perimeter of the wheel is enlarged 

 over the perimeter of the axle. The wheel is variously modi- 

 fied for the reduction of friction. 



A pulley is a wheel or succession of wheels so geared that 

 the force applied must move over a greater space tlian the 

 weight to which it is applied; hence a larger mass may be 

 moved by a smaller, as in the case of the lever. 



These forces — tlie hammer, the lever, the wedge, the wheel, 

 and the pulley — are often combined in the same mechanism. 

 Thus, in the screw, the lever and the wedge are combined, but 

 the wedge is a spiral wedge. These fundamental mechanical 

 devices are combined in a great variety of ways in the machin- 

 ery of the industries. 



These devices for applying- power are sometimes called the 

 mechanical powers, and the powers themselves are called 

 forces. 



Again I must remind the reader that there is no such thing 

 as abstract power; it is always concrete, and its concomitants 

 must always be considered when we consider real power as 

 such. Power exists as an abstraction only in consideration. 



Having- considered the devices for applying power, we have 

 now to consider them as they are utilized in tools and machines. 

 A tool may be defined as an implement employed to utilize 

 human power. A machine may be defined as an implement 

 employed for using any other power than that of human 

 muscle. The tool is dependent on the hand and is adapted to 

 the use of the hand, while the machine is adapted to the use 

 of other powers than that of the hand, though these powers 

 may be directly or indirectly controlled by the hand. A flint 

 may be fashioned into a knife on a grindstone supported by a 

 wooden horse; the grindstone is a tool, but it may be run by 

 water power, when it becomes a machine, for it must be pro- 

 vided with the apparatus necessary to utilize the fall of water. 

 A hand hammer is a tool ; but a trip hammer is a machine, for 

 some other power than that of human muscle is used in its 

 operation. The hand dasher in a churn is a tool; a power 

 dasher in our modern dairies is a machine. The flail is a 



