erating from water at a temperature which gives it the same pressure as the atmosphere, an additional temperature of 38° will give it the pressure of two atmospheres; a still further addition of 42° gives it the tension of four atmospheres; and with each successive addition of temperature, of between 40° and 50°, the pressure becomes doubled. It is well for the student of the steam engine to know the reason of this effect, and we will endeavor to explain it. We have already said that there is no cohesion among the particles of fluids, but there is, however, an attraction between all matter in nature. The action of heat in generating steam has to overcome this attraction among the particles of the water, and likewise the gravity of the water itself. As the water becomes rarefied by heat, and, either in its natural state or as steam, occupies a greater volume, this attraction is diminished, and also the weight or gravity of the water; hence an additional rate of temperature does not have to contend with the same resistance as the temperature which preceded it, and is, therefore, enabled to produce greater effects in the generation of steam. Among a variety of facts and notes relative to the nature of steam, we select the following: If water be boiled in an open vessel, no temperature greater than that for the boiling point, which for fresh water is 212°, can be produced in it. All surplus heat which may be applied passes off in the steam. If the vessel be closed, and the steam as it is formed be retained within it, the temperature may be raised, and retained in the steam. If the steam, as

it is formed, is allowed to accumulate in the boiler, its pressure on the water-level makes an increased temperature necess ry to continue its production. Steam in itself is invisible, and becomes visible only upon condensation, as when a jet is discharged in the open air; its loss of temperature causes it to condense, and ́ we see it in the form of a vapory cloud. In treating of steam, the term heat is understood as expressing its sensible heat, while the term caloric provides for the expression of every conceivable existence of temperature. To explain the theory of ebullition, or boiling liquids, we will observe that in metals, heat is communicated by the conducting property they possess; but in liquids it is communicated by a circulation of particles. If heat be applied to the bottom and sides of a vessel containing water, that portion of the water in contact with the heated metal becomes heated and rarefied, and consequently lighter than the rest, whereby it ascends to the surface, gives off its vapor, be comes cooled, and in consequence of becoming heavier, descends again, to become heated, rise, and descend as before, and to maintain these operations in a constant succession so long as the heat is applied. This action is performed in vertical planes, and if the heat be applied above the bottom of the vessel, the water below that point will receive but little heat, and can never be made to boil. An established relation must exist between the temperature and elasticity of steam; in other words, water at 212° must be under the pressure of the steam naturally resulting from that

temperature; and so at any other temperature. If this natural pressure on the surface of the water be removed without a corresponding reduction in the temperature, a violent ebullition at the water-level is the immediate result. Thus, suppose the entire steamroom in a boiler to be six cubic feet, and the contents of the cylinder.which it supplies to be two cubic feet; at each stroke of the piston one-third of all the steam in the boiler is discharged, and the surface of the water is consequently relieved from one-third of the pressure upon it before that stroke. The temperature

remains the same, but as it does not bear the natural relation to this diminished pressure, it causes the water to boil violently, and produces foaming. Foaming is the cause of which priming, or working water along with the steam into the cylinders, is the effect. Provision must therefore be made in all boilers, that they may have a large extent of steam-room compared with the cylinders which they supply. Another result attending the formation of steam is, that when an engine is in operation and working off a proper supply of steam, the water-level in the boiler artificially rises, and shows by the gauge-cocks a supply greater than that which really exists. This is owing to the steam forming in the water and rising in bubbles to the surface, and displacing by its bulk the amount of water indicated by the rise at the gaugecocks. As the production of steam under the same temperature cannot continue under an increased pressure, it follows that when the discharge of steam is

stopped, and its entire pressure is thrown on the surface of the water, steam is no longer generated, and the water takes its natural level. At whatever point in a boiler steam be taken, there is a determination of water to that point, which is occasioned by the sudden reduction in the pressure, owing to the withdrawal of the steam. This is the case with all boilers having steam domes with throttle in the same; and it was for this reason that, on certain new engines lately constructed, the steam-dome was omitted, and in its stead a steampipe, perforated on its upper side, was extended the whole length of the boiler, occupying the position usually given to the steam-pipe in ordinary locomotives. The object of this was to take the steam alike from all parts of the steam-room of the boiler, so that no rise of water should result at any one point.

CHAPTER I.

STEAM ENGINEERING.

THE observation and experience of the writer* have confirmed him in the opinion that there exists among so-called "practical men," a great need of "scientific education" on subjects directly pertaining to the business they may be engaged in.

The very foundation of science is the faculty of calling things by their right names; for by such a method only, can one be perfectly understood and answered accordingly.

These lines are written in the exacting light of Webster's dictionary, and it is suggested to all who may read them with the view of knowing just what they mean, to read with a dictionary close at hand, and if its occasional use will start readers in a habit of thoroughly comprehending whatever they give their attention to, an important step towards a scientific education will have been taken.

Science is simply "a positive statement of truth, established by observation and experiments," and can as well be applied to the operations of every-day work, as to the most intricate branches of human inquiry; in other words, there is one best way to perform every