_Answer._ By conducting the exhaust steam through pipes (_e_, _e_, fig. 40) from the cylinders to the smoke-box and allowing it to escape up the smoke-stack from apertures, _f_, _f_, (Plate II, fig. 40.) called _exhaust nozzles_. The strong current of steam thus produced in the smoke-stack produces a vacuum, by which the smoke is sucked into the smoke-box with great power and forced out of the smoke-stack into the open air. QUESTION 80. _How are the water and fuel carried which must be supplied to a locomotive while it is running?_ _Answer._ The water is carried in a tank, which is constructed in the form of the letter ~U~, so as to give room for the stowage of fuel between its two branches or sides. This tank is carried on a set of wheels, and forms a separate vehicle, independent of the locomotive, called a _tender_, the construction of which will be explained in a future chapter. QUESTION 81. _What are the dimensions of the principal parts of a locomotive?_ _Answer._ There is a great variety in the plan, size and capacity of locomotives, but the type which is more generally used in this country than any other, and which has been selected for the preceding illustrations, and will be described in the succeeding chapters of the Catechism, has four driving and four truck wheels, and weighs in working condition about 60,000 lbs. The following are the dimensions of its principal parts: The driving-wheels are about 5 feet and the truck-wheels from 26 to 30 inches in diameter. The longitudinal distance between the centres of the driving-wheels is usually about 7 feet, and between the centres of the truck-wheels 5 ft. 9 in., and the total distance from the centre of the back driving-wheels to the centre of the front truck-wheels, which is called the _wheel-base_, is 21 ft. 8 in. The weight on each driving-wheel is usually about 10,000 lbs., and on each truck-wheel about 5,000 lbs. The cylinders are 16 in. in diameter and the piston has 24 in. stroke, and the connecting-rod is 7 ft. long measured between the centres of the pins to which it is attached. The centres of the cylinders are about 6 feet apart, measured across the track. The fire-box inside is 5 feet long and 2 ft. 11 in. wide, and the cylindrical part of the boiler is 4 feet in diameter measured on the outside of the smallest portion. The water spaces around the fire-box are about 3 inches wide. There are 140 tubes, which are 2 in. in diameter measured on their outside, and 11 ft. long. The inside of the smoke-stack is 16 in. in diameter, and it is 14 ft. 3 in. high measured from the top of the rails. The tender carries about 1,800 gallons of water and about 8,000 lbs. of coal. When loaded it weighs about 40,000 lbs., making the total weight of the engine and tender 100,000 lbs. The following is a list of parts designated by the letters of reference on plates I, II, III and fig. 40.