TRANSACTIONS OF THE SECTIONS. 197 



The measure of brake-power developed in the engine may be illustrated by the 

 following' experiments which we made : — 



A North-Eastern Company's engine, 40 tons weight, with tender 27 tons 

 (altogether 67 tons), and travelling at 59 miles an hour, was brought up by 

 .Smith's Vacuum Brake applied to the engine only in 617 yards, giving a retarding 

 force of 4"22 tons or 10'50 per cent, of the weight of the engine, an important 

 factor when it is considered how large a portion of the gross weight of a train is 

 represented by its engine. 



On several of the continental lines, on a few of our railways here, and notably 

 on the Highland railway, the Chatelier or Counterpressure Brake has been applied 

 to the engines with considerable advantage, and this may in cases of emergency 

 prove a valuable auxiliary to the engine-brake. 



The Locomotive Superintendent of the Highland Railway Company has favoured 

 me with some details of its working on that line, where it has been applied to 60 

 out of their 67 engines. " Generally speaking, it is not used on that line every daj r , 

 and never in stopping at stations if approached at the ordinary speed ; nor by the 

 majority of drivers is it used to control the trains down the banks in ordinary 

 working, but each driver can refer to cases in which it was of great use, especially 

 by the service it has rendered going down banks in snow, when the ordinary brakes 

 had lost their effectiveness." 



When the reversing lever is pulled back to cut off at 40 per cent., the retarding 

 force produced by back-pressure steam appears to be just sufficient to prevent 

 acceleration in the speed of a train of 180 tons weight, inclusive of engine and 

 tender, on a descending gradient of 1 in 70. 



The question of the best material for brake-blocks has of late received a good 

 deal of consideration, and it would seem that cast-iron and even steel blocks are fast 

 superseding wood. The majority of the trains sent to Lincoln for the trials were 

 litted with cast-iron blocks, the only important exception being the Lancashire and 

 Yorkshire Company's train (Fay's Brake), which was furnished exclusively with 

 wooden blocks. 



We were unable to discover any clear indication of the one being superior to the 

 other in skidding power ; but doubtless the wear of iron blocks is much less rapid. 



We had occasion to observe that with none of the Continuous Brakes used did 

 the wheels become skidded until a very considerable reduction in the speed of 

 the train had taken place. 



When the rails are in a damp and greasy state it is likely that cast iron or 

 steel blocks displace sooner than wooden blocks the film of slimy matter which 

 forms on the tyres of the wheels, a film which may become impressed and 

 embedded into the soft substance of the wood, preventing, until removed by 

 prolonged pressure, a full and powerful bite on to the wheels. 



It is to be remarked that the utmost brake-power obtained scarcely exceeded 

 10 per cent, of the weight of the braked vehicles ; whereas, from the experiments 

 of Morin, Rennie, and others, it has been shown that the coefficient of friction 

 of iron sliding upon iron, the surfaces being clean, is from l-5th to l-6th, say 16 

 to 20 per cent, of the insistent pressure. 



The discrepancy is to be accounted for by the circumstance that dm-ing the 

 larger portion of the space traversed in effecting the stop, only few of the wheels 

 become skidded, and for the remaining portion not all the wheels come into that 

 condition. 



Nor does it seem desirable that such should be the case ; for without doubt the 

 wear and tear of wheel-tyres and of rails is sensibly increased by skidding. 



For ordinary stops, therefore, the brake-pressure should be adjusted to act just 

 short of skidding the wheels, whilst the full skidding power should only be applied 

 in cases of imminent danger. 



A difference of opinion exists on the question of the relative effects of skidded 

 and unskidded wheels in effecting the stop ; but this we considered was set at rest 

 by two experiments with a train of six of the Midland Railway Company's four- 

 wheeled vans coupled together and formed into a single train, each van having its 

 own guard to apply the brake-power by hand. All the wheels of this train had 

 brakes. 



