The four-way cock is liable to some practical objections. The quantity of steam which fills the tubes between the cock and the cylinder, is wasted every stroke. This objection, however, also applies to the sliding valve (figs. 22, 23.), and to the sliding tube or D valves (figs. 24, 25, 26, 27.). In fact, it is applicable to every contrivance in which means of shutting off the steam are not placed at both top and bottom of the cylinder. Besides this, however, the various passages and tubes cannot be conveniently made large enough to supply steam in sufficient abundance; and consequently it becomes necessary to produce steam in the boiler of a more than ordinary strength to bear the attenuation which it suffers in its passage through so many narrow tubes. [Illustration: Pl. VII.] One of the greatest objections, however, to the use of the four-way cock, particularly in large engines, is its unequal wear. The parts of it near the passages having smaller surfaces, become more affected by the friction, and in a short time the steam leaks between the cock and its case, and becomes wasted, and tends to vitiate the vacuum. These cocks are seldom used in condensing engines, except they be small engines, but are frequently adopted in high-pressure steam-engines; for in these the leakage is not of so much consequence, as will appear hereafter. CHAPTER VIII. BOILER AND ITS APPENDAGES.--FURNACE. The Boiler and its Appendages. -- Level Gauges. -- Feeding Apparatus. -- Steam Gauge. -- Barometer Gauge. -- Safety Valves. -- Self-regulating Damper. -- Edelcrantz's Valve. -- Furnace. -- Self-consuming Furnace. -- Brunton's Self-regulating Furnace. -- Oldham's Modification. (65.) The regular action of a steam engine, as well as the economy of fuel, depends in a great degree on the construction of the boiler or apparatus for generating the steam. The boiler may be conceived as a great magazine of steam for the use of the engine; and care must be taken not only that a sufficient quantity be always ready for the supply of the machine, but also that it shall be of the proper quality; that is, that its pressure shall not exceed that which is required, nor fall short of it. Precautions should, therefore, be taken that the production of steam should be exactly proportioned to the work to be done, and that the steam so produced shall be admitted to the cylinder in the same proportion. To accomplish this, various contrivances, eminently remarkable for their ingenuity, have been resorted to, and which we shall now proceed to describe.