_Answer._ The condition of the track and the force and direction of the wind. On a rough track the resistance is very much greater than on a smooth one, and a strong head-wind makes it much more difficult to pull a train than it is in calm weather. PART XXI. PROPORTIONS OF LOCOMOTIVES. QUESTION 412. _In proportioning a locomotive to any given kind of work, what are the first facts which should be known?_ _Answer._ We should first know the weight of the train which the locomotive must draw; second, the speed at which it must run; and third, the steepest grades and the shortest curves of the road on which it must work. From these data the resistance of the train which the locomotive must overcome can be at least approximately determined. QUESTION 413. _When the greatest resistance of the train is known, what is the next thing to be determined?_ _Answer._ As was stated in answer to Question 66, if the wheels revolve and their adhesion is greater than the resistance opposed to the movement of the locomotive, the latter will overcome the resistance; but if the latter is greater than the friction, the wheels will slip. It therefore follows that the adhesion must be somewhat greater than the resistance. As the adhesion is equal to about one-fifth[100] of the adhesive weight or pressure of the driving-wheels on the rails, obviously this weight should be five times the resistance. Thus, if we have a train weighing 400 tons which we want to take up a grade of 40 feet per mile at a speed of 20 miles per hour, its resistance, calculated from the table given in the previous part, would be 9,360 lbs. Therefore, 9,360 × 5 = 46,800 lbs. = the required adhesive weight. [100] See answer to Question 309. QUESTION 414. _What considerations determine the manner of distributing this weight on the wheels?_ _Answer._ It is found by experience that if too much weight is placed upon one wheel, the material of which the rails are made is partly crushed and injured, and they then wear out much more rapidly than they would if the weight was distributed on more wheels, and thus a smaller amount of weight rested on each point of contact with the rails. The amount of weight which can be carried on a single wheel depends upon the material of which the rails are made, and to some extent on their form and size, or as the latter is usually expressed, on their weight per yard. QUESTION 415. _When the adhesive weight and the number of driving-wheels are known, how is the size of the latter and of the cylinders determined?_