298 



TRANSPORTATION 



of the most important parts of the car. This is called 

 the differential. When a car turns a corner or goes 

 around a curve in the road the rear wheel on the out- 

 side of the curve must travel faster than the wheel 

 on the inside. This is made possible by placing a 

 complicated system of gears in the center of the rear 



While some manufacturers have gone in for more 

 complete streamlining than others, most cars are now 

 showing models with slanting windshields, pointed 



. LOW SPEED 

 HIGH POWER 

 DELIVERED TO 

 REAP. WHEELS 



MODERATE SPEED 



MODERATE POWER 



DELIVERED TO 



REAR WHEELS 





HIGH SPEED 

 LOW POWER 

 DEUVEREPTO 

 REAR WHEELS 



I 



Courtesy Standard Oil Co. of New York 



FIG. 495. TRANSMISSION GEAR SYSTEM 



axle. When the car is moving along a straight road 

 both rear wheels turn at the same rate. Have you 

 ever observed one rear wheel turning while the other 

 is still? This may be seen sometimes when one wheel 

 is lifted off the ground. This is made possible by the 

 differential. Figure 497 shows a picture of the gear 

 system which makes up the differential and how it 

 works. See if you can explain how each rear wheel 

 is enabled to turn independently of the other. 



Exercise. Carefully study Figure 497 and explain 

 how the encircled inset of a column of soldiers marching 

 around a corner explains the operating principle of the 

 differential of an automobile. 



One of the important advances made in the design 

 of automobiles and buses in recent years is an effort 

 to cut down air resistance by streamlining. This is im- 

 portant because modern cars travel faster, and air re- 

 sistance increases rapidly at higher speeds. 



Courtesy Greylwund Lines 



FIG. 496. A STREAMLINED BUS 



radiators, sharply tapered backs, and fenders designed 

 to create as little air resistance as possible. Figure 

 496 shows a cut of a modern streamlined bus. It is 

 claimed that streamlining has brought about con- 

 siderable savings in fuel costs. 



How has transportation by rail been improved? 

 The history of the steam engine has been told in Unit 

 VII, "Man's Use of Machines." James Watt, an in- 

 strument maker of Scotland, is given credit for the 

 perfection of the steam engine in its modern form. 

 From an experiment at the beginning of this topic you 

 have again seen that steam, when confined, has energy 

 because of its expansive powers. You have also re- 

 viewed the construction and operation of the modern 

 steam engine. 



By again referring to Figure 484, check your under- 

 standing of the operation of the steam engine. Steam 

 from the boiler enters the steam chest. From the 

 steam chest into the cylinder there are two openings 

 or ports. These are controlled, that is, opened and 

 closed, by a slide valve which operates back and forth 

 over them. When one port is open, live steam gets into 

 the cylinder on one side of the piston and drives the 

 piston with its expansive force. At the same time 

 the other side of the cylinder is connected to the ex- 

 haust through the other port. As the piston is driven 

 back by expanding steam the slide valve moves back, 

 connecting the first port with the exhaust and allow- 

 ing steam to enter the cylinder on the other side of the 

 piston. This piston is attached to a fly wheel through 

 a connecting shaft which is so constructed that it can 

 change the to-and-fro motion of the piston into the 

 rotary motion of the fly wheel. 



The earliest application of steam in an engine to run 

 on rails was made by Richard Trevithick in England. 

 His engine was not successful. About 1813 another 

 Englishman, William Hedley, designed and built a 

 locomotive for use in the coal mines, which had very 

 little success because it was too cumbersome. In 1814 

 George Stephenson, an English inventor, built his 

 first steam locomotive. For fifteen years he improved 



