Transportation on Land 



277 



FIG. 82. Cross section of motor car 

 racing track. A car moving at a rate of 

 110 miles per hour has a position normal to 

 the track on the outside elevation. 



outside rail would be dangerous. The result of the elevation 

 of the outside rail is that the inertia of the moving train which 

 is acting toward the outside of the track, shown by the dotted 

 line C (Fig. 81), and the weight of the train acting toward the 

 inside of the track, W, bal- 

 ance each other and keep 

 the train in a position which 

 is normal to the track, N. 



The cross section of the 



curve of a typical course for |<.-2-6"->U-- 70' 



motor car racing (Fig. 82) 

 illustrates the point clearly. 

 This has an outside eleva- 

 tion of 25.6 feet over the 



inside level of the curve. It is designed to allow a motor car, 

 moving at the rate of about 100 miles per hour, to maintain 

 a position normal to the track surface at the outside of the 

 curve. The normal position is the resultant of two factors : 

 the one, the tangential force of the rapidly moving car, which is 

 acting horizontally, and the other the weight of the car, which is 



acting vertically. Make a 

 diagram and explain. 



117. Exercise : Measuring 

 grades. Grades may be meas- 

 ured with a fair degree of ac- 

 curacy by means of the simpli- 

 fied form of the surveyor's level 

 shown in Fig. 83. Cut the " T " 

 from heavy cardboard, accord- 

 ing to the dimensions given in 

 the diagram. Fasten the plumb 

 line to the " T " so that the 

 angle designated in the dia- 

 gram shall be exactly 90. If greater accuracy is desired, make two 

 readings, with the instrument reversed for the second reading. The 

 upper end of the plumb line may be fastened to a tripod or other steady 

 support. The plumb bob should just clear the ground. 



FIG. 83. A, simple level for surveying. 



