_Answer._ In the absence of any special instruments for the purpose, BY COUNTING THE NUMBER OF REVOLUTIONS OF THE DRIVING-WHEELS PER MINUTE, THEN MULTIPLYING THE LENGTH OF THEIR CIRCUMFERENCE IN INCHES BY THE NUMBER OF THEIR REVOLUTIONS PER MINUTE AND THE PRODUCT BY 60, AND DIVIDING THE LAST PRODUCT BY 63,360. THE QUOTIENT WILL BE THE SPEED IN MILES PER HOUR. Thus, supposing driving-wheels which are 61¹⁄₂ in. in diameter, and whose circumference is therefore 193.2 in., should make 164 revolutions per minute, then 193.2 × 164 × 60 ÷ 63,360 = 30, (nearly) miles per hour. PART XXV. WATER-TANKS AND TURN-TABLES. QUESTION 449. _How are locomotive tenders or tanks supplied with water?_ [Illustration: Fig. 225. Scale ¹⁄₁₆ in. = 1 foot.] _Answer._ At suitable points, called water stations, along the line of the road, large tanks or reservoirs are located, which are filled either from a natural stream which is higher than the tank and thus flows into the latter, or else the water is pumped in, either by hand or by horse, wind, water or steam power. These tanks are usually, when there is room for them, located near the track, as shown in fig. 225, so that the water can be conducted by a spout, _a_, direct from the tank to the man-hole of the tender. Communication to and from this spout is opened and closed by a valve, _b_, inside of the tank. The spout is usually attached to the tank by a hinged joint, so that it can be lowered to the tender and then raised up out of the way of the engine and train. It is generally balanced by a counterweight, suspended to one end of a rope, which passes over a pulley and is fastened to the spout at the other end. Such tanks are now generally made of wooden staves like a tub or pail, and supported on a heavy frame, _c c c_, made of wood, as shown in the engraving, or on stone or brick masonry. [Illustration: Fig. 226. Scale ¹⁄₁₆ in. = 1 foot.] When there is no room for the tank or reservoir near the track, it is placed in any convenient position at some distance from it, and the water is then conveyed by an underground pipe to the place where the locomotive must take water. At the end of this pipe what is called a _water-crane_, fig. 226, is located. This consists of a vertical pipe, _A_, with a horizontal arm, _B_, which is made so as to swing around over the man-hole of the tender when the latter is to be filled with water. In some cases the horizontal arm alone swings around, but in others the vertical pipe turns with the horizontal one in a joint, _C_, underneath the surface of the ground. The latter plan is thought to be preferable to the first, as the pipe is less liable to freeze fast in the joint when the latter is underground than when it is exposed above. A suitable valve, _D_, is also attached to the pipe below ground, so that the stream of water can be turned off or on at pleasure by the wheel _E_.