FALL OF BOILER-TEMPERATURE NOT INDICATED BY THE STEAM-GAUGE.
If an engine, of the class represented as pulling our train, passes over the top of the grade with half an inch of water in the glass, there will be about 700 gallons in the boiler. Now, suppose it runs down the hill without using steam, and keeps pumping till the water rises six inches in the glass, there will be about 200 gallons more water in the boiler. It is no unusual thing to do this with a mild fire, and yet have no diminished tension of steam shown by the gauge, although 200 gallons of water of about 60 degrees have been injected amongst 700 gallons at 361 degrees, the temperature due to a steam-pressure of 140 pounds. This ought to reduce the mean temperature below 300 degrees, yet the pointer of the steam-gauge keeps indicating 140. That the pressure of steam and the temperature of the water do not accord, is shown directly the throttle is opened to perform work. The brisk circulation due to the rush of steam through the dry pipe now brings the temperature of water and steam to equilibrium, and backward the index of the steam-gauge travels. The steam-pressure goes back faster than is due to the supply drawn for the cylinders; because the latent heat of the steam passes into the water, helping to bring the whole contents of the boiler to an even temperature.