The causes are dirty water; trying to evaporate more water than the size and construction of the boiler is intended for; taking the steam too low down; insufficient steam room; imperfect construction of boiler, and too small a steam pipe.

Take a kettle of dirty water and place it on a fire and allow it to boil and watch it foam, and it will be the same in a boiler.

Too little attention is paid to boilers with regard to their evaporating power. Where the boiler is large enough for the water to circulate, and there is surface enough to give off the steam, foaming never occurs. As the particles of steam have to escape to the surface of the water in the boiler, unless that is in proportion to the amount of steam to be generated, it will be delivered with such violence that the water will be mixed with it and cause what is called foaming.

A high pressure insures tranquility at the sur

face, and the steam itself being more dense it comes away in a more compact form, and the ebullition at the surface is no greater than at a lower pressure. When a boiler foams we close the throttle to check the flow, and that keeps up the pressure and lessens the sudden delivery.

Too many tubes in a boiler obstruct the passage of the steam from the lower part of the boiler on its way to the surface; this is a fault in construction; but nearly all foaming arises from dirty water, or from trying to evaporate too much water without heating surface or steam room enough.

This table gives the value of all properties of saturated steam required in calculations connected with steam boilers.

COLLAPSE OF FURNACES.

"In marine boilers one of the most frequent, most annoying, and at the same time most expensive accidents (?) is the collapse of the furnaces. Those interested may possibly be pleased to learn that extensive experiments have recently been made by M. Hirsch on the causes which lead to the burning out of furnace plates, and as a result he confirms the fact, long known to experts, that under some conditions oil in the interior of the boiler is highly dangerous. The interior of a boiler to be examined was painted with oil before being filled with water and firing in the usual way, and it was found that some oils so diminished the efficiency of contact between the water and the plates that in one case the furnace plates rose to a temperature exceeding 680°, or the melting point of zinc, when only evaporating 35 lbs. of water per square foot of grate surface."

The combustion chamber should be larger where bituminous coal is used.

In piping boilers avoid too rigid connections, and thus prevent leakage and accidents.

The transverse strain in a cylinder boiler is double the longitudinal strain, and in a tubular boiler a much larger ratio exists, because of the support given the heads by the tubes.

The smallest point in a suction pipe is practically the measure of ability of the whole; hence

where a foot valve is used, care should be taken that it has as large an opening as the pipe itself.

In careful engine tests a counter can be set up and connected with the reducing motion and the speed of the engine thus noted. The reading can be divided by the exact length of test and an accurate average thus obtained.

INCRUSTATION OF STEAM BOILERS.

One of the greatest difficulties to be contended against in steam engineering is the incrustation of the boiler walls, arising from impure water. This crust is a poor conductor of heat, and causes increased fuel consumption, as well as the oxidizing or "burning" of the plates, owing to their increased temperature. A plate of iron 371⁄2 inches thick conducts heat as well as a "crust" of one inch. A boiler bearing scale only inch thick requires 15 per cent. more fuel, with 4 inch 60 per cent. more, 1⁄2 inch 150 per cent. more. If the plates be clean, 90 pounds of steam require a plate temperature of only 325° F., that is, about 5° above the steam temperature. But if there be a 1⁄2 inch scale or crust, the plate must be heated to about 700°, or nearly "low red" heat. Now, above 600° iron soon gets granular and brittle, hence, such a scale is dangerous in its results. Crust also retards the circulation of the water. Two very common ingredients in boiler scale are carbonate