_Answer_. First, upon the size of its grate and fire-box, because more fuel can be burned in a large fire-place than in a small one; second, on the amount of heating surface to which the products of combustion are exposed, and third, on the draft produced by the blast or exhaust steam. Of course the amount of steam generated is also dependent upon a great variety of other circumstances, such as the nature of the combustion, the firing, the arrangement of the fire-box, grates, etc., and the condition of the heating surfaces; but these have nothing to do with the proportions or size of the boiler. QUESTION 419. _What are the proportions of boilers used in locomotives like that which has been illustrated in these articles and represented in Plates I and II?_ _Answer_. The area of the grate is about 2,100 square inches and the total heating surface about 800 square feet, and the water capacity about 5,000 lbs., and the total weight of the boiler, including all the boiler attachments and appliances for promoting combustion, about 30,000 lbs. QUESTION 420. _At what speed are such engines usually run?_ _Answer_. The speed varies so much under different circumstances, that it is impossible to give even approximately the average speed of such engines. QUESTION 421. _How then can we determine the proper proportions of a boiler for a locomotive intended for any given service?_ _Answer_. As stated before, this can only be done empirically. The safest method is to select a locomotive which is doing the best service, and learn the average speed at which it runs, the size of its grate and the amount of its heating surface, and its adhesive weight. NOW MULTIPLY THE ABOVE SPEED IN MILES PER HOUR BY THE ADHESIVE WEIGHT OF THE LOCOMOTIVE IN TONS (of 2,000 lbs.) AND DIVIDE THE PRODUCT INTO THE AREA OF THE GRATE IN SQUARE INCHES. THEN MULTIPLY THE ADHESIVE WEIGHT OF THE LOCOMOTIVE FOR WHICH THE BOILER IS TO BE PROVIDED BY ITS SPEED, IN MILES PER HOUR, AND BY THE QUOTIENT OBTAINED ABOVE: THE PRODUCT WILL BE THE AREA OF THE GRATE IN SQUARE INCHES FOR THE NEW ENGINE. To illustrate this, suppose an engine of the dimensions given to run at an average speed of 20 miles per hour. Now, multiplying that speed by the number of tons of adhesive weight and dividing the product into the area of the grate, we have 20 × 20 = 400 and 2100 ÷ 400 = 5.25. We now want to determine the size of a grate for the boiler of a locomotive with 30 tons adhesive weight and to run at a speed of 15 miles per hour. We therefore multiply 15 by 30 and the product by the above quotient, or 15 × 30 × 5.25 = 2,362.5 = square inches of grate surface for the boiler. The required heating surface can be obtained in a similar way, by substituting it instead of the grate surface in the calculations. QUESTION 422. _How is the size of locomotive boilers usually limited?_