GLOSSARY. 293 



Power, the moving- force of a macliine, as opposed to the wei;?ht, 

 load, or resistance of the substance wrought upon ; also called prime 

 mover. 



Projectile, a body thrown tliron<i:h the air. 



Pulley, one of the mechanical powers, consisting of a grooved wheel 

 called the sheave^ over which a rope passes ; the box in which the wheel 

 is set is called the block. The term is also applied to a fixed wheel over 

 which a band or rope passes. 



Pump, a hydraulic machine for raising water; or one for withdrawing 

 air. The handle is called the brake. 



Quantity of motion, the velocity of a moving body multiplied by its 

 mass. 



Rabbet, to pare down the edge of a board or timber. 



Rack, a straight bar cut with teeth or cogs, working into a correspond- 

 ing cog-wheel or pinion which drives or follows it. 



Rag-wheel, a wheel with teeth or notches, on which an endless or re- 

 volving chain usually runs. Also applied to a ratchet wheel. 



Rake-iiead, the cross-bar of a rake, which holds the teeth. 



Ram, HydRxVULIc ram, or Water-ram, a hydraulic macliine or engine 

 for raising water to a height several times greater than that of the head 

 of water, by employing the momentum of the descending current in 

 successive beats or strokes. 



Ratchet-wheel, a wheel cut with teeth like those of a saw, against 

 which a click or ratchet presses, admitting free motion to the wheel in 

 one direction, hut insuring it against reverse motion. 



Reach, the bar which connects the forward and rear axles of a wagon 

 or carriage. 



Ream, to bevel out a hole. 



Reciprocating motion, alternate motion, or a movement backwards 

 and forwards in the same path. 



Reel, the revolving frame of a reaping machine, to throw the stand- 

 ing grain towards the knives. 



Resolution of forces, dividing a force into two or more forces act- 

 ing in different directions ; rendering a compound force into its several 

 simple forces. 



Resultant, a force produced by the combination of two or more 

 forces. 



Safety valve, a valve opening outwards from a steam boiler, and 

 kept down by a weight, permitting the escape of steam when the press- 

 ure reaches a certain point, regulated by the degree of weight. The 

 terra also applies to a valve opening inwards, and similarly regulated, to 

 prevent the pressure of the atmosphere from crushing in the boiler when 

 the steam cools and leaves a vacuum. 



Scoop- WHEEL, a water-wheel with scoops or buckets around it, agaiuBt 

 which the current dashes. 



Screw-bolt, a bolt secured by a screw, or with a screw cut upon it. 



Screw-propeller, an instrument for driving a vessel, by means of 



