A.—­I did not state that the resistance to traction was 1/500th of the weight upon an average—­to which condition the answer given to a previous question must be understood to apply—­but I stated that the average traction was about 1/300th of the load, which nearly agrees with my former statement.  If the total friction be 1/300th of the load, and the rolling friction be 1/1000th of the load, then the friction of attrition must be 1/429th of the load; and if the diameter of the wheels be 36 in., and the diameter of the axles be 3 in., which are common proportions, the friction of attrition must be increased in the proportion of 36 to 3, or 12 times, to represent the friction of the rubbing surface when moving with the velocity of the carriage, 12/429ths are about 1/35th of the load, which does not differ much from the proportion of 1/33d as previously determined.

495. Q.—­What is the amount of adhesion of the wheels upon the rails?

A.—­The adhesion of the wheels upon the rails is about 1/5th of the weight when the rails are clean, or either perfectly wet or perfectly dry; but when the rails are half wet or greasy, the adhesion is not more than 1/10th or 1/12th of the weight or pressure upon the wheels.  The weight of a locomotive of modern construction varies from 20 to 25 tons.

496. Q.—­And what is its cost and average performance?

A.—­The cost of a common narrow gauge locomotive, of average power, varies from L1,900 to L2,200; it will run on an average 130 miles per day, at a cost for repairs of 2-1/2d. per mile; and the cost of locomotive power, including repairs, wages, oil, and coke, does not much exceed 6d. per mile run, on economically managed railways.  This does not include a sinking fund for the renewal of the engines when worn out, which may be taken as equivalent to 10 per cent. on their original cost.

497. Q.—­Does the expense of traction increase much with an increased speed?

A.—­Yes; it increases very rapidly, partly from the undulation of the earth when a heavy train passes over it at a high velocity, but chiefly from the resistance of the atmosphere and blast pipe, which constitute the greatest of the impediments to motion at high speeds.  At a speed of 30 miles an hour, the atmospheric resistance has been found in some cases to amount to about 12 lbs. a ton; and in side winds the resistance even exceeds this amount, partly in consequence of the additional friction caused from the flanges of the wheels being forced against the rails, and partly because the wind catches to a certain extent the front of every carriage, whereby the efficient breadth of each carriage, in giving motion to the air in the direction of the train, is very much increased.  At a speed of 30 miles an hour, an engine evaporating 200 cubic feet of water in the hour, and therefore exerting about 200 horses power, will draw a load of 110 tons.  Taking the friction