July z\, 1879] 



NATURE 



317 



It will be seen that, when the adhesion equals "30 of 

 the weight, a pressure equal to i'2 of the weight would 

 skid the wheel at 7^ miles per hour, whilst a pressure 

 equal to 4' 14 times the weight would be required to do so 

 at 60 miles per hour. 



On the other hand, if the adhesion is only '15, the 

 pressure requisite to skid the wheel would be only '60 of 

 the weight at 7 J miles per hour, and 2 '08 of the weight at 

 60 miles per hour. 



Thus the efficiency of a brake depends upon the pres- 

 sure being proportioned to the speed and to the adhesion. 

 If the adhesion were always uniform, the rule would be 

 very simple ; but this is not the case. 



The adhesion of the wheels to the rails varied accord- 

 ing to the materials, that is, whether the train was 

 travelling upon iron or steel rails ; and according to the 

 state of the rail, whether dry, wet, or sanded. 



On dry rails it was found that the coefficient of adhesion 

 of the wheels was generally over "20. In some cases it 

 rose to "25, or even higher. On wet or greasy rails, with- 

 out sand, it fell as low as •15 in one experiment, but 

 averaged about '18. With the use of sand on wet rails it 

 was above '20 at all times ; and when the sand was 

 applied at the moment of starting, so that the wind of the 

 rotating wheels did not blow it away, it rose up to '35, and 

 even above •40. Consequently, the retarding effect of 

 the _ brakes would be greatly increased, were means 

 devised for placing sand under every wheel to which a 

 brake is applied, during the progress of a stop. 



The effect in stopping a train is greatest when the 

 friction between the brake-blocks and the wheels amounts 

 to a quantity just short of the resistance caused by the 

 adhesion, because as soon as the brake-block friction 

 exceeds the adhesion, the wheel becomes fixed and begins 

 to slide. In order, however, to secure the best results in 

 stopping, it is obviously necessary that the brake-block 

 pressure should be regulated to give a friction about equal 

 to the adhesion of the wheels at every stage during the 

 progress of a stop. 



There is no reason why, in the progress of mecha- 

 nical science, these conditions should not be regulated by 

 a self-acting arrangement. 



Mr. Westinghouse has devised a valve to regulate the 

 pressure between the blocks and the wheels. The prin- 

 ciple of the valve is, in the first place, to prevent the 

 actual friction from exceeding the adhesion at any point ; 

 and in the next place, whilst allowing the fullest amount 

 of pressure necessary to produce the maximum friction 

 to be applied to the brake blocks when the brakes are 



first put on, that is to say, when the speed is high at the 

 commencement of a stop, to reduce that pressure gradu- 

 ally during the progress of the stop, so as to maintain as 

 nearly as possible a uniform amount of friction. 



As the adhesion varies it is necessary to consider what 

 amount of adhesion for purposes of retardation can be 

 safely calculated upon. 



The following table shows the distances required to 

 stop a train on a level line from a speed of fifty miles 

 per hour, with a retarding force of from 5 to 30 per cent, 

 of the total weight of the train : — ■ 



Percentage of 

 retardation. 



s 



10 



12 

 IS 



Yards run at fifty 

 miles per hour. 



SSSt 



277* 

 231! 



18s 



Percentage of 

 retardation. 



18 

 20 



2S 

 30 



Yards run at fifty 

 miles per hour. 



i54i 

 139 

 III 

 921 



If the brakes act upon each wheel, then a retardation 

 of 10 per cent, of the load carried by each wheel — count- 

 ing the rotating momentum as part of the weight — %vill 

 stop a train in 277J yards. 



If the brakes act upon only half of the weight of a train, 

 a retardation of 20 per cent, would have to be exerted 

 upon the braked half to produce the same result. As 

 pointed out, 20 per cent, adhesion is rather above the 

 average obtainable, while 25 per cent, is the highest 

 result obtained under the most favourable circumstances 

 at any considerable speed, or except when sand was 

 applied to wheels moving slowly. 



The above table should be carefully noted, for it will 

 be seen that, even if brakes act upon all wheels, 2S per 

 per cent, retardation will only give twenty-eight yards 

 better result than 20 per cent., or if half of the train only 

 be braked, it will give fifty-nine yards advantage. 



A consideration of this feature of the brake problem 

 points out (i) that the advantage to be gained by trying 

 to obtain above 20 per cent, retardation on each wheel is 

 greatly overbalanced by the risk of " skidding ; " and (2) 

 that it is far easier and safer to make a stop in 250 yards 

 from fifty miles per hour with the whole train braked, 

 than with brakes upon only half of the train. 



The following are some of the results of the earlier ex- 

 periments, obtained with Mr. Westinghouse' s pressure- 

 regulator : — 



In the second experiment the pressure was not reduced 

 with sufficient rapidity to prevent the skidding, and in the 



fourth and fifth experiments the pressure was insufficient 

 at the beginning of the experiment. From these and 



