VALUES OP HEATING SURFACES. 145

attainable without extra detail or expense.The safety valve and gear shown in front ofthe uptake is the ordinary kind, presentingno novelty for description or comment.

The stays are arranged between the tubesand beyond the same at given distances, rela-tive to their sectional areas and the pressureto be resisted.

This description of the arrangement beingsufficient for the present purpose, a brief com-ment on the action and effect of the flame,and values of the heating surfaces, will nextbe given.

On referring to the plan of the boiler in page115, it can be seen that the combustion cham-ber is at right angles to the grates and tubes,which also is the position of those details inthe present instance. The flame, on risingfrom the fuel, acts against the crown and sidesof the fire-boxes. Directly after passing thebridge, it forms a curve at right angles to itsformer line of progression.

Now it is due to this curve, as before stated,that the portion of the fire box beyond thebridge connecting the back of the combustionchamber—is the least affected by the flame.The ends of the combustion chamber are alsosimilarly operated on.

From the cluster of the tubes being raisedin relation to the crown of the fire boxes,doubtless the lower rows receive a purer flamethan when situated near the base line of thecombustion chamber.

The values of the heating surfaces may nextbe concluded. The fire box is of the sameratio as that of prior notice. The combustionchamber, however, is not of equal value, pro-portionate to its entire surface. This conclu-

sion is arrived at from the fact that the flame,during exit, inclines towards the uptake. Itis obvious, therefore, that the volume on pass- 'ing the bridge acts only on the crown, tubeplate, and that portion of the back above thelevel of the lower row of tubes. The ends ofthe combustion chamber are, of course, theleast in contact with the proceed from thegrate, three-fourths of the total surface beingonly worthy of attention.

The effect of the flame on the crown, back,and tubes plates is much the same as withsimilar dispositions of detail, the values, there-fore, are alike ; remembering the proportionatesurfaces in contact.

The tubes, of course, admit the passage ofthe flame through them with a given effect,due to their position ; and the flame-value isfrom that cause readily deduced. !

To render the matter under question conciseas well as practical, the following synopsis otthe proportionate ratios is added, uniformwith those previously given.

TUBES.

Total surface...

2-976

FIRE BON.

Crown

Total surface

1-000

Sides

... Above grate

•500

COMBUSTION CHAMBER.

Crown

... Total surface

1-000

Tube plate'

[Effective surface surround-),ing the tubes, and surface!(above...

•875

Back plate'

(Surface above the level of 1lthe bottom row of tubes)

•700

Ends ...

Three-fourths’ surface

•250