TRANSPORTATION ON LAND 



293 



Secure a spark coil and connect it with three dry cells and 

 a switch, as shown in the illustration. Attach the high 

 tension or secondary wires, one to the spark plug and the 

 other so that it touches some part of the can. (Be cautious 

 in handling high voltage wires.) Now spray a little gaso- 

 line into the can with an atomizer or Flit spray. Place the 

 lid on securely and close the switch. Stand away from the 

 can and be cautious as the switch is closed. 



FIG. 485 



Answer the following questions about the experiment. 



Is the voltage of the batteries low or high? Is the volt- 

 age required to make the current flow across the gap in 

 the spark plug low or high? What does the spark coil do? 

 Explain how the energy of the exploding gas might be 

 harnessed. 



READINGS WHICH WILL HELP ANSWER THE 

 PROBLEM QUESTIONS 



How have highways been improved since early 

 times, and what are the causes for the change? Roads 

 have long aided man in transportation. Among the 

 most lasting of the monuments to the Roman Em- 

 pire are the roads built to connect Rome with its 

 provinces. Many of these roads, built from 1,500 to 

 2,000 years ago, are still in actual use or are being used 

 as foundations for modern highways. Europe has long 

 been noted for its fine roa3s and today has a greater 

 proportion of its roads improved than North America. 



In the United States road building was given little 

 or no attention until the close of the Revolutionary 

 War. Between 1790 and 1812 over three hundred per- 

 mits for the construction of turnpikes were given to 

 private companies. These turnpikes, built either of 

 plank or gravel, comprised nearly 5,000 miles of road 

 and an investment of more than six millions of dollars. 

 Toll gates were placed about every ten miles on these 

 to enable the owners to collect on their investment. 

 The modern toll bridge is a remnant of this early cus- 

 tom. The turnpikes, while an improvement over the 

 earlier trails, were very rough and difficult to keep 

 up. One of these early roads, the Lancaster Turnpike, 

 was built between Philadelphia and Lancaster, Penn- 

 sylvania, a distance of sixty-two miles. Other roads 

 of this period which became famous are the Mohawk 

 Trail, built from Albany to Buffalo ; the Pennsylvania 

 State Road built from Philadelphia to Pittsburgh ; the 

 Oregon Trail from Independence, Missouri, to the 



Columbia River in Oregon, a distance of two thousand 

 miles; the Santa Fe Trail from Independence to 

 Santa Fe, New Mexico; the National Trail, built by 

 the government in 1830 from Hagerstown, Maryland, 

 to the Pacific Coast. 



The development of the automobile has brought 

 our modern roads. The rapid rise of the automobile 

 and bus as means of transportation has created a de- 

 mand for wider and smoother roads. Many improved 

 roads were formerly made of water-bound macadam. 

 This type of road was named for John McAdam, a 

 Scotch engineer, who first introduced the method. 

 Small crushed stones were held together by a natural 

 -tar binder- 



f'me ro.ck ..<& sand 



^?&&Q& 

 rriedium roc 



FIG. 486. CROSS SECTIONS OF MACADAMIZED AND CEMENT ROADS 



cement of rock dust and water. This type of pave- 

 ment, while a marked improvement over the older 

 gravel roads, had certain limitations. It was very 

 dusty when dry and could not withstand the heavy 

 traffic of the automo- 

 bile. This older form of 

 macadam is now being 

 replaced by a more 

 modern type called the 

 bituminous-bound mac- 

 adam. An excavation is 

 made and a layer of 

 coarse rock is placed in 

 the bottom. This is cov- 

 ered with finer crushed 

 rock and this with fine 

 rock and sand. These 

 are bound with a layer 

 of tar or asphalt ap- 

 plied hot and rolled 

 to smoothness. These 

 roads are sometimes 

 hard surfaced by mix- 

 ing fine gravel or gran- 

 ite with the tar or 

 asphalt while it is 



still warm. Figure 486shows a cross section of this 

 type of road. 



Another type of road surface which has become 

 widely used in the past few years is reenforced con- 

 crete. These roads are practically wear-resistant and 

 require less upkeep than almost any other type of road 

 surface. Many miles of concrete highway are being 





A 



Portland 



FIG. 487. ROAD BEFORE AND AFTER 

 CEMKNTING 



