278 Introduction to the Study of Science 



The rod may be made of a square piece of wood six or eight feet 

 long and marked off in feet and tenths of a foot. The rodman- may use 

 a white card as an indicator, which is to be moved up and down on 

 the rod until the top of it is at the right level, as is indicated by the 

 signals of the one who is sighting across the top of the " T." The rod- 

 man takes and records the reading. After each reading is taken, the 

 instrument is moved to the position of the rod, which is advanced to 

 another suitable position. 



The distance A is always known ; the distance B is shown by the 

 rodman's reading and record. The distance C is to be measured 

 with tape (a steel tape is best). The difference between the distance 

 A and the distance B gives the elevation for the distance C. The per 

 cent grade is found by determining the number of feet rise in a dis- 

 tance of 100 feet. 



SUMMARY 



The improvement of roads must accompany the improvement of 

 vehicles of travel and transportation. 



Roads are built of various materials, such as dirt, stone, asphaltum, 

 concrete, and brick. 



The harder and smoother and more nearly level the road, the 

 heavier the load a horse or other motive power can pull. 



Formerly railroads and highways were built with many curves 

 and high grades to save cost of construction. To-day they are built 

 with few curves and low grades to save cost of operation. 



A long grade to reach a certain elevation reduces the tractive force 

 needed, but increases by extra friction the amount of work done. 



Work is expressed in foot-pounds. A force of one pound acting 

 through a distance of one foot does one foot-pound of work. 



In spite of the added friction, light grades are to be preferred. 



The resistance of the rail or road to the passing of the wheel is 

 called rolling friction. 



According to the principle of inertia a body in motion tends to move 

 in a straight line. This tendency is the tangential force of the body. 



As a train rounds a curve, the tangential force tends to tip the train 

 outward. This tendency is counterbalanced by gravity, which is 

 brought into action by the elevation of the outside rail. 



REVIEW QUESTIONS 



1. In what ways do transportation and travel depend on roads? 

 2. Mention examples from ancient times of good roads. 3. Describe 

 the form, construction, and materials used in city streets. In a good 



