. Railroad construction. Theory and practice . o thelower temperature of the journal-boxes, is very marked andcostly. It has been suggested that a jacketing of the journal-boxes, which would prevent rapid radiation of heat and enablethem to retain some of the heat developed by friction, wouldresult in a saving amply repaying the cost of the device. Roller journals for cars have been frequently suggested, andexperiments have been made with them. It is found that theyare very effective at low velocities, greatly reducing the start-ing resistance, which is very high with the ordinary forms ofjour
. Railroad construction. Theory and practice . o thelower temperature of the journal-boxes, is very marked andcostly. It has been suggested that a jacketing of the journal-boxes, which would prevent rapid radiation of heat and enablethem to retain some of the heat developed by friction, wouldresult in a saving amply repaying the cost of the device. Roller journals for cars have been frequently suggested, andexperiments have been made with them. It is found that theyare very effective at low velocities, greatly reducing the start-ing resistance, which is very high with the ordinary forms ofjournals. But the advantages disappear as the velocity in-creases. The advantages also decrease as the load is increased,so that with heavily loaded cars the gain is small. The excessof cost for construction and maintena-nce has been found to bemore than the gain from power saved. 344. Grade resistance. The amount of this may be com-puted with mathematical exactness. Assume that the ballor cylinder (see Fig. 206) is being drawn up the plane. If W. Fig. 206. is the weight, N the normal pressure against the rail, and G the force required to hold it or to draw it up the plane with uniform velocity, the roUing resistances being considered zero or considered as provided for by other forces, then WhG:W:h:d, or G==-^; but for all ordinary railroad grades, d = c to within a tenth of 1%, , G = = WX rate of grade. In order that the student (/ may appreciate the exact amoimt of this approximation the per-centage of slope distance to its horizontal projection is given inthe following tabular form: §344. TRAIN RESISTANCE, 383 Grade in per cent. 1 2 3 4 5 Slope cUst.^jOO nor. dist. Grade in per cent. 6 7 8 9 10 Slope dist. ^jQQ hor. dist. This shows also the error on various grades of measuring withthe tape on the ground rather than held horizontally. Sincealmost all railroad grades are less than 2% (where the e
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