_Answer._ The size of the wheels will to a certain extent depend upon the speed, because the larger the wheels, the further will the locomotive move in one revolution; but no exact rule can be given for their size. At present there is still a great diversity of opinion among engineers regarding the best sizes of wheels and cylinders for any given service. Probably the safest plan will be to consult the best practice, and in the absence of any better reasons be guided for the present by that. In this country the most common size of locomotives used is that which we have selected for our illustrations, that is, what are called five-feet wheels, and cylinders of 16 inches diameter and 24 inches stroke. More engines of these dimensions are used than of any other. For freight service the wheels are sometimes made of smaller and for passenger trains of larger diameter; but locomotives with driving-wheels and cylinders of the dimensions given are used for both passenger and freight service. It should be stated here that what are called five-feet wheels are usually about 1¹⁄₂ in. larger in diameter than five feet. This arose from the fact that the tires which are now used are made thicker than they were on the first engines, and the practice thus established has been continued. We will therefore take the diameter of what is called a five-feet wheel at what it really is, 61¹⁄₂ in. Such locomotives also have about 40,000 lbs. of adhesive weight. Now, the circumference of such wheels is 193.2 in., and therefore in one revolution of the wheels, if they do not slip, the locomotive will move that distance on the rails. At the same time each piston will sweep through the cylinder twice, and therefore in one revolution 4 times one cylinder full of steam is used. Now a cylinder of 16 in. diameter and 24 in. stroke contains or will hold 4,825¹⁄₂ cubic inches, so that in one revolution of the wheels 4,825¹⁄₂ × 4 = 19,302 cubic inches of steam are used. As has been stated, in one revolution of the wheels, if they do not slip, the locomotive will move 193.2 in. If, now, we divide 19,302 by 193.2 it will give us the amount of steam used to move the locomotive and train one inch. Now, 19,302 ÷ 193.2 = 99.9, which, for the sake of even figures we will call 100. We thus see that a locomotive with 40,000 pounds or 20 tons (of 2,000 lbs.) of adhesive weight requires 100 cubic inches of cylinder capacity[101] to move it one inch. Now, if a locomotive had only half as much weight on the driving-wheels, it could pull only half as much load, and would therefore use only half as much steam, and consequently need only half the cylinder capacity of the other locomotive. If there was three-quarters or a third as much adhesive weight, the cylinder capacity should also be three-quarters or a third. We thus see that the cylinder capacity should be proportioned to the total adhesive weight. Now as 100 cubic inches of cylinder capacity are needed to move an