. Railroad construction : theory and practice : a textbook for the use of students in colleges and technical schools . 1 same draw-bar pull. At A its ^Welocity head is that due to 30miles per hour, or feet. At B it has gained 40 feet more,and its velocity is that due to a velocity head of feet, orslightly over 45 miles per hour. At J5 its velocity is again 30miles per hour and velocity head feet. At C the velocityhead is but feet and the velocity about miles per hour. 504 RAILROAD CONSTRUCTION. §432. As the train runs from C to D its velocity increases t
. Railroad construction : theory and practice : a textbook for the use of students in colleges and technical schools . 1 same draw-bar pull. At A its ^Welocity head is that due to 30miles per hour, or feet. At B it has gained 40 feet more,and its velocity is that due to a velocity head of feet, orslightly over 45 miles per hour. At J5 its velocity is again 30miles per hour and velocity head feet. At C the velocityhead is but feet and the velocity about miles per hour. 504 RAILROAD CONSTRUCTION. §432. As the train runs from C to D its velocity increases to 30 miles atC and to over 45 miles per hour at D. At E the velocity againbecomes 30 miles per hour. Although there will be some slightmodifications of the above figures in actual practice, yet the aboveis not a fanciful theoretical sketch. Thousands of just suchundulations of grade are daily operated in such a way, withoutdisturbing the throttle or applying brakes, and the , if measured by a dynamometer, would be found to bepractically constant. Of course the above case assumes that VTrtuaf profTle. D„,_^-^ Fig. are no stoppages and that the speed through the sags isnot so great that safety requires the application of that the virtual profile is here a straight line—as italways is when the draw-bar pull is constant. The virtualprofile (in this case as well as in every other case, illustrationsof which will follow) is found by adding to the actual profileat any point an ordinate which represents the velocity headdue to the velocity of the train at that point. As another case, assume that a train is climbing the grade AEand exerting a pull just sufficient to maintain a constant velocity up that grade. Then AB (parallelto AB) is the virtual profile, AA^representing the velocity head. Astop being required at C, steam isshut off and brakes are appliedat B, and the velocity head BB^reduces to zero at C. The trainstarts from C, and at D atta
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