RAILWAY CONSTANTS. 249 



necessarily act at their respective centres of gravity. Thus the 

 force which moves the first carriage will act at its centre, and 

 that portion of it which is expended on the friction of that par- 

 ticular carriage will act in a manner as favourable as a drawing 

 force would act. The same may be said of the force of gravi- 

 tation of the several carriages ; but that portion of the force of 

 gravitation which balances the resistance of the air is subject in 

 a modified sense to the objection. Thus, that part of the gra- 

 vitation of the second coach which is over and above the resist- 

 ance from friction, is transmitted to the first coach, and through 

 it to the air which it drives before it ; and the like may be said 

 of the gravitation of each succeeding coach. But it should also 

 be remembered that the resistance of the air to a train of coaches 

 does not act exclusively on the front of the first coach. The 

 coaches of the train are nearly four feet asunder, and the air 

 probably acts more or less on the foremost end of each coach. 

 This portion of the resistance is not acted upon with the same 

 disadvantage by the gravitation of the coaches as that resistance 

 which is produced by the end of the first coach. 



It is intended to test the force of this objection by moving a 

 train of coaches with an engine along a level, or up an inclina- 

 tion, first placing the engine in front and afterwards behind, and 

 comparing the time taken by the engine to drive the train 

 a given distance under both circumstances : but at the time of 

 making this report the committee had not had an opportunity 

 of making such an experiment. 



Whatever importance may be attached to this objection, it is 

 presumed that it cannot for a moment be supposed that the dif- 

 ference between the resistance in pushing a train from behind 

 and drawing it in front can account for the enormous dispropor- 

 tion between the common estimate of resistance, and that which 

 results from the experiments here given, the common estimate 

 being about nine pounds per ton, while that which the trains 

 exhibited moved down the Whiston plane at thirty-two miles 

 an hour, amounted to more than twenty-three pounds a ton, 

 and that even with the advantage of a favourable wind. 



A further objection, however, has been made to the effect, 

 that the trains on which the various experiments have been 

 made, especially those with which the greatest velocities were 

 attained, were lighter than trains generally are in railway prac- 

 tice, and that therefore the proportion which the atmosphei'ic 

 resistance would bear to the whole resistance would be greater 

 than in practice it is, for that if the magnitude of the train were 

 increased the resistance from the air would not be proportion- 

 ately increased. 



