It must not, however, be hastily supposed from what has been said that the total pressure against the axle can be greater than its resistance to the pressure. As soon as the one exceeds the other it will move. But supposing that it requires a force equal to 3,333 pounds to draw a train coupled to the engine, as soon as the difference between the force exerted against the axle by the piston to move it forward and that which presses it back exceeds 3,333 pounds, the locomotive will move the train. If it continues to exceed it the speed of the train will be accelerated and thus the resistance which holds the engine back and that which pushes it forward will always be equal. QUESTION 313. _Does the fact that the piston is working from the end of a longer lever E G B, fig. 192, when the crank-pin is above the axle, enable the locomotive to start a heavier train than when the crank-pin is below the axle and the piston is working against a shorter lever, G E B, fig. 193?_ _Answer._ No; because, as has already been shown, the pressure against the axle is the same in both cases. It is in fact only during the forward stroke that the pressure on the crank-pin moves the engine forward. The forward pressure which is exerted by the crank-pin at the axle is then greater than that exerted against the latter in the opposite direction by the cylinder-head and frames. It is this excess of crank pressure which moves the engine and which is the tractive force during the forward stroke. During the backward stroke the piston is pushing the axle backward, and the pressure against the front cylinder-head is pulling it forward. The latter then exceeds the former, and the difference between the two is the force which moves the engine forward. As has been shown, this difference is the same in both positions of the crank, and therefore the locomotive can not from this cause pull more when the crank is above the axle than when it is below. PART XIV. INTERNAL DISTURBING FORCES IN THE LOCOMOTIVE. QUESTION 314. _What are the internal disturbing forces in a locomotive?_ _Answer._ They are: 1, the momentum of the parts which have a reciprocating motion; 2, those due to the varying pressure of the steam in the cylinder-heads; 3, those caused by the thrust of the connecting-rods against the guide-bars; and 4, those produced by unbalanced revolving parts. QUESTION 315. _How can the effects of these disturbing forces be neutralized?_ _Answer._ By putting counterweights, _A_, _A_, fig. 161, in the driving-wheels opposite the crank-pins. The motion of these will then be in the reverse direction to that of the weight of the parts attached to the crank-pins, and the motion of the one will thus, to some extent, at least, neutralize the disturbing influence of the other. QUESTION 316. _Can the weight of these counterweights be calculated for any locomotive?_