208 FIRST YEAR COURSE IN GENERAL SCIENCE 



countries. In the settlement of North America it took two 

 hundred years for the population to spread beyond even the 

 low ridges of the Appalachians. With the advance in the 

 science of engineering, however, and the application of 

 steam power in locomotives, railroads have been built over 

 and through very high mountains. 



The railroad often makes long detours to take advantage 

 of a valley cutting through a ridge, or winds back and forth 

 on a long slope, rising at each turn a few feet above the last 

 stretch of road. In climbing a mountain a few thousand 

 feet in elevation, the train may travel many times the 

 distance across the range, and when near the highest part, 

 passengers may look down upon several parallel stretches of 

 road over which they have passed. 



On some of the railroads crossing the Rocky Mountains 

 in the United States tunnels have been built through the 

 highest parts of the mountains. This is done to save the 

 time that would be required to haul the train over the top, 

 and also to avoid the obstruction of the track by snow 

 avalanches. The greatest tunnel in the world is over 

 twelve miles long, through the Simplon Mountain between 

 Switzerland and Italy. 



Mountains are famed as health resorts on account of the 

 purity and dryness of the air, and for summer homes because 

 of their coolness in comparison with lower regions. The 

 average decrease in temperature is about 15 F. for every 

 5,000 feet of ascent. 



Mountains act as barriers to the spread of plants and ani- 

 mals from one part of the country to another. Because of 

 cold and thin soil, trees do not grow above certain eleva- 

 tions, called the timber line. If the summit rises beyond 

 the timber line, trees cannot spread from one side of a range 

 to the other. Above the timber line the ground is covered 

 with short grass and other plants which do not require long 

 seasons. As a rule, the flowers of mountain plants are short 



