198 OUR PHYSICAL WORLD 



on the transmission shaft may one at a time be brought into such 

 position that their teeth interlock with the teeth on the cogwheels 

 on the secondary shaft, and the transmission shaft is set rotating. 

 Through a flexible joint it conveys the rotary- motion to the 

 rear wheels. If a large cogwheel on the transmission shaft is 

 geared into a small one on the secondary shaft, it will take several 

 turns of the latter to turn the former once and the transmission 

 shaft and the rear wheels will turn slowly. If, on the contrary, 

 a small wheel on the transmission shaft is geared into a large 

 one on the secondary shaft, then the rear wheels will turn rapidly 

 and the car will run fast. When the gear shaft is set " at neutral, " 

 no wheel on the transmission shaft is playing into the wheel on 

 the secondary shaft. 



There are many bearings in an automobile engine that need 

 constant lubrication. Thus the crank shaft may make 1,000 

 or more revolutions a minute when the machine is running 

 rapidly. This would create much friction unless the bearings 

 were well oiled. The oiling is partly accomplished by having 

 below the engine a pan of oil, which splashes up and keeps the 

 moving parts lubricated. In addition an oil pump forces oil along 

 small tubes bored in the center of the shafting and out of tiny 

 holes in the bearings. Frequently an oil gauge is put on the 

 instrument board connected with this system to show that the 

 oil is moving properly. 



An electric motor is connected with the storage battery 

 so that when a current is sent into it, it turns a cogwheel that 

 plays into cogs on the circumference of the flywheel and the 

 engine is "turned over" to start it. You step on the starter or 

 press a button to accomplish this. Just as soon as the cylinders 

 have drawn gas and air into themselves and the mixture is set 

 of! by the sparking of the plugs, the engine begins to run of itself, 

 the starter is disconnected, and the electric motor stopped. 

 Sometimes the engine is turned over by hand by means of a 

 crank temporarily fitted on to the forward end of the crank shaft. 

 But the self-starter is in quite general use. 



