If the steam is rising rapidly when foaming begins, it will be well to cool the boiler off by opening the furnace door part way. This means of relief should, however, be used as little as possible, because there is always danger of causing the tubes or other parts of the boiler to leak, by either heating or cooling suddenly or rapidly. If the engine primes when there is but little water in the boiler, and at a time when the steam is rising rapidly, it may sometimes be remedied by increasing the amount of feed-water, and thus partly cooling the water inside. The use of pure water, careful firing so as to keep the steam pressure regular, feeding the boiler so that the level of the water will be nearly uniform, and then starting the engine carefully, that is, opening the throttle-valve gradually, are the most effective means in practice of preventing a locomotive boiler from priming. QUESTION 481. _What is the economical effect of priming on the consumption of fuel in locomotives?_ _Answer._ It causes a great waste of heat, first by the escape of that contained in the hot water which passes through the cylinders and which does no work, and second, when steam is mixed with a great deal of water, it will not flow either to or from the cylinders as quickly or easily as dry steam will. Consequently the initial pressure on the piston, if the engine is running even moderately fast, and is cutting off short, will not be so great as it would be if dry steam was used. Wet steam is also more difficult to exhaust from the cylinder than that which is dry, and therefore the back pressure on the piston is greater when the boiler primes than when dry steam alone is used. QUESTION 482. _When running on the open road, what should the locomotive runner observe?_