_Answer_. The priming of a boiler can be known by the white appearance of the steam which escapes from the smoke-stack and the cylinder cocks. Dry steam always has a bluish color. When an engine primes or works water into the cylinders, it is usually indicated by a peculiar dead sound of the exhaust, which from this cause loses its distinctly defined and sharp sound. This can be observed best when the furnace door is opened. It is also indicated by the discharge from the gauge-cock, as the water which then escapes from the lower cocks is mixed with steam, or, as runners say, is not “solid,” and the steam from the upper cocks is not clear, but mixed with water. To use a phrase employed by practical men, the priming or foaming of the boiler may be known by the “flutter” of the gauge-cocks. As soon as there are any indications of priming, foaming, or that water is working into the cylinders, the cylinder cocks should be opened at once, otherwise the cylinders, cylinder heads or pistons may be broken. The throttle-valve should be either partly or entirely closed. When the latter is done the foaming will in most cases cease for the time, so that the runner can tell how much solid water there is in the boiler. If he finds that the boiler has too much water in it? it is best to shut off the pumps, and in many cases the blow-off cock is opened. The latter is, however, attended with some danger, because if any obstruction should get into the blow-off cock, or it should stick fast, so that it could not be closed, all the water would escape from the boiler, and with a heavy fire in the fire-box there would be great danger of overheating, and thus injuring the boiler or of “burning” it, as it is ordinarily termed. A much better method of affording relief in such cases is to place what is called a _surface-cock_ in the back end of the fire-box, about half way between the upper and lower gauge-cocks. With such a cock, the water can be blown off from the surface instead of from the bottom. As foaming or priming is often caused by oil, or other floating impurities on the surface, they can be blown out of the boiler with this arrangement, whereas, if the water escapes from the bottom of the boiler, the floating impurities will always remain after it is blown off. A perforated pipe, which extends for some distance along the surface of the water inside the boiler, is sometimes attached to the surface-cock, so that the water which is blown off will be drawn from a number of points along the surface.