oxygen is absorbed to the exclusion of carbon in the first instance.

2. There is waste from external radiation aud conduction. M. Peclet states that the quantity of heat radiated from incandescent charcoal is 5% of the total heat of combustion, and that the heat radiated from coal somewhat exceeds that radiated from charcoal. The practical conclusion to be drawn from this statement is, that the heat radiated from the burning fuel should be carefully intercepted in every direction.

Hence the economy resulting from the use of an internally fired boiler with internal furnace tubes.

As to the heat radiated into the ash-pit; that is carried back again to the fire by the current of entering air. In respect of the loss of heat by conduction, that is obviated as much as possible by the use of fire-brick, and where the furnace is outside the boiler, the resistance to conduction is increased by double layers of brickwork with enclosed air spaces between the layers.

3. There is loss of heat by the escape of gases up the chimney at a temperature above that which is necessary for maintaining the draught.

A general idea of the value of a chimney in promoting the draught of a fire may be gathered from a statement of a law which appears to be approximately true, viz: That the velocity of air, as due to increased pressure, is that acquired in

.001293

falling down a height equal to the uniform col-
umn which gives the increased pressure.

In making any calculation on this subject it is
usual to adopt the hypothesis that air is incom- Sp
pressible and behaves as a liquid.

Let the increase of pressure support 5 inches 46. of water. We know that 29.922×13.596 inches -33.9 ft of water balance the pressure of the atmosphere which would be produced by a stratum of incompressible air 26,214 feet high.

Therefore, I inch of water will balance 64.4 feet of air.

Hence 5 inches of water balances 322 feet of air: therefore velocity due to increase of pressure

16

According to the old rule the area of the chimney should be that of the fire-grate, and there should be square foot of fire-grate for each horse-power.

The consumption of coals per horse-power being estimated, Mr. Bourne gives Boulton and Watt's rule for proportioning the dimensions of land chimneys, according to which a factory chimney 80 feet high would have a sectional area of 400 square inches, the consumption of coal being 300 pounds per hour, and the suction of the chimney being that due to a pressure of a little more than 1 inch of water.

Rankine gives formula for computing the height of a chimney in order to produce a given draught, and states that the best chimney draught takes place when the absolute temperature of the gas in the chimney is to that of the external air as 25 to 12, or when the density of the hot gas is one-half that of the external air.

For example, if the temperature of the external air be 50° Fah. the best temperature of the hot gas in the chimney will, according to this rule, be 602° Fah., which is less than that of melting lead, viz., 620° Fah.

Hence the rule that to insure the best possible draught in a chimney the temperature of the hot gas should be nearly sufficient to melt lead.

If the temperature of the furnace itself be estimated at 2,400° Fah. and that of the issuing gases at from 600° Fah. to 700° Fah. or even higher, as is often the case, we see that 25 per cent. of the heat of combustion passes up the chimney and is consumed in producing a draught of air through the furnace grate.

The loss of heat from the waste gases may be lessened by the use of an economizer for heating the feed water.

Sir W. Fairbairn, in his treatise on "Mills and Millwork," describes an economizer introduced by Mr. Green, of Wakefield, as consisting of a series of upright tubes forming a supplementary boiler placed in the main flues, and

states that the formation of soot on the pipes was the source of the ill success of previous attempts in this direction.

This difficulty has been overcome by an apparatus of scrapers or cleaners, and it is found. that when the waste gases escape at a temperature of 400° Fah. or 500° Fah. the feed-water can be heated to an average of 225° Fah., the temperature of the gases after leaving the pipes being reduced to 250° Fah.

To produce this effect 10 square feet of heating surface should be provided for each horse-power. 4. Fuel is wasted by brittleness, dust and small pieces dropping unburnt through the bars into the ash-pit.

5. The fuel is rendered less effective by the presence of earthy compounds, which form clinkers, abstract heat uselessly, and choke up the grate.

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CHAPTER III.

FORMS OF STEAM BOILERS.

THE Construction of a boiler should be regarded from two points of view. (1) There is the general form and structure adapted to support the bursting pressure of the steam. (2) There are considerations arising from the unequal action of the heat of the burning gases, and there are precautions to be taken for diminishing the waste of heat.

Considerations which influence the forms of boilers.—The early boilers were designed in simple defiance of all mechanical principles. Without doubt the safety of a boiler depends on the strength of the metal, but it is quite wrong to say that it is independent of the form of the shell, and any one who thinks for a moment on the subject will comprehend that a cylindrical tube of some kind is the proper vessel wherein to retain a supply of steam under pressure. The strongest form of a vessel for holding a gas under pressure is a sphere: that is the natural form for the purpose, as we learn in blowing a soapbubble.

But no one would recommend a spherical