926

THE COLLIERY GUARDIAN

November 5, 1915.

so does the floor help sustain the roof so long as the
former is unbroken.

One of the patented features of the Luten bridge
construction is its holoid structure. Luten builds not
one bridge, but two—one for vehicles to pass on, and
one to bind the feet of the piers together so as to form
a perfect holoid. Destroy the lower bridge and the
upper or travelling bridge is in a precarious condition.
(See fig. 15.)

The holoid structure gives the strongest of roofs; the
pyoid is less strong; the cumoid is still weaker. Every
horizontal shear makes the roof progressively less stable.
Strange to say, vertical cracks permit the replacement in
many eases of a less stable by a more stable structure.
Fractures in a vertical direction, which would ordinarily
be thought more detrimental than those in any other
direction, prove not unimportant, it is true, yet in a
way strengthening.

When a cumoid of great depth finds itself inadequate
to its own support as a cumoid, and begins to rend
vertically from the surface downward to its supports
and upward from the span centres to the surface, then
the cumoid beam becomes an arch, and the cumoid
plate becomes a dome, and until it fails in its new
structural relations, it cannot continue to extend those
rents which threatened its stability. Let us say that
the cumoid has become a conchoid (from conchus, a
shell), the lines of stress forming figures resembling the
shell of a bivalve. As the elements of which the roof
consists crowd each other into the opening it is impos-
sible for the roof to rupture or fall until the various
strata cease to act as a unit, and begin to slide past
each other. (See fig. 16.) If we imagine an element
of rock stretching from a rib to the half-span a distance,
let us say, of 100 ft.; if we further suppose the excavated
coal or other mineral to be 10 ft. thick, and regard the
rib, floor, and falling rock element to be so adamant
than none of the three will crush, heave, or bend, then
the inclination of the element when one corner reaches
to the floor will be 10 per cent.

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FIG.I8

If the excavation is 1,000 ft. below the surface, the
displacement of the block at the surface would have to
be 100 ft. (See fig. 17.) At a greater depth the dis-
placement would be more. It is needless to say such a
displacement cannot occur even where the crop, broken
roof, or the sags in near by workings make a certain
amount of lateral adjustment possible. But horizontal
shear along weak planes, such as beds of slay or coal or
stratification planes, will permit such a number of
readjustments in the element of rock itself that it will
be able to come down. It will, in fact, rather tend to
tilt than actually upset forward. (See fig. 18.)

This, then, is how the end comes unless the rock is
too strong to shear horizontally. Subsidence ceases
when the required deformation has taken place. The
choking up of the falls probably has but little to do
with the final result, though it may have its effect in
some cases. In fact, the theory as given is comforting
to the conservationist, as the beds above the one
extracted suffer but little, being exposed only to the
horizontal shearing process, which is by no means
destructive of the integrity of the measures.

However, the rupture at the half-span may often
result in falls of moderate size at that point, for when
the monolithic roof is weakening by successive hori-
zontal shears, we have a series of superposed cantilever
cumoids entirely unequal to the task of self support. If
the action which forms these cumoids does not let them
all down to the floor suddenly, they will be sure to frac-
ture successively from the bottom upward, and such
seams of mineral as partake of this independent action
will be destroyed.

Hull Coal Exports.—The official return of the exports of
coal from Hull to foreign countries for the week ending
October 26, is as follows :—Amsterdam, 711 tons; Aalesund,
25; Arendal, 969; Bergen, 1; Copenhagen, 1,000; Calair,
1,396; Dunkirk, 243; G-efle, 1,703; Guernsey, 971; Genoa,
1,375 ; Gothenburg, 1,157 ; Harlingen, 1,262 ; Honfleur, 612 ;
Jersey, 108; Malmo, 1,180; Rotterdam, 2,400; Rouen,
14,760; Stockholm, 1,251; St. Nazaire, 1,291; Treport, 609;
Naples, 491; total, 33,515 tons. The above figures do not
include bunker coal, shipments for the British Admiralty,
nor the Allies’ Governments. Corresponding period October
1914, total 43,201 tons. Corresponding period October 1913,
total, 83,988 tons.

Development in Mechanical Ventilation Hygienica’ly, Above
Ground, Under Ground, and Under Deck.*

By JAMES KEITH.

Within recent years public opinion has been much
exercised on the question of better conditions for the
workers in over-heated and ill-ventilated engine and
other working rooms on land and on board ship. The
work of physiologists has gone to show that the chief
effect of ventilation and open-air treatment depends on
the movement, temperature, and moisture of the air,
and less upon its chemical properties than was expected;
and, in order to obtain economically adequate ventila-
tion on physiological principles, without producing
uncomfortable draughts, the ventilating system
employed should be designed on certain lines embody-
ing appliances specially adapted to the cooling of over-
heated engine rooms and the like above ground, under-
ground, or under deck, by flooding them with fresh air
from outside under slight pressure. This positive venti-
lation, or a continuous change of air, also removes all
noxious gases and smells emanating from oil, etc. The
principle employed is exemplified by the following :■—

Fig. 1 illustrates part of the large electric power
house of Saint Denis, outside Paris. ■ It shows how the

SWITCH HOUSE

POWER HOUSE

Fig. 1.

long switch house (with switchboard about 500 ft. in
length) is cooled down. In this case, a series of 10
open fans, each having 25 in. diameter air inlet, and
running 700 revolutions per minute, is installed along
the length of the switch house ceiling, and delivers in
all 90,000 cu. ft. of fresh air per minute, or 5,400,000
cu. ft. per hour, and distributes it in the direction shown
by the arrows across and through the switch house into
the power house; in other words, the whole cubical air
contents of the switch house are changed once every
40 seconds, or 90 times an hour, and the inside tempera-
ture of the switch house reduced from, say, 120 degs.
Fahr, in summer, to that of nearly the normal outside
atmosphere, without production of unpleasant currents,
for a total expenditure in electrical power of 14,000
watts. In the main building or power house proper
there may be, as shown, similar fans of larger size
arranged along its length of 600 ft.

Fig. 2 shows, diagrammatieally, the delivery and dis-
persion of the fresh air into an engine room or other
underground chamber by an arrangement similar to that
adopted in the underground engine room of the Singer
Building,' Broadway, New York, in which are installed
three large open fans capable of propelling pure fresh
air into the engine room, under moderate water gauge,
to the extent of 120,000 cu. ft. per minute, thus
changing the air more than 60 times an hour, and
ensuring comfort and coolness without draughts under
all outside atmospheric conditions.

The fresh air in this case is taken from the side walk of
a street, and from the open area between the wings of
building block, and is drawn into large chambers against
and through considerable areas of moistened burlap
before it passes into the inlet conduits, and is delivered
by the fans into the engine room. This system has
superseded others hitherto in use in the engine room
of the Singer Building, and represents a satisfactory
solution of the problem how to keep such engine rooms
comfortably cool and healthy.

The change of conditions from an excessively high
and unhealthy temperature inside to that of a normal

* From a paper read before the Institute of Marine
Engineers.

outside temperature is effected (in the case of the Singer
engine room) by the expenditure of no more than 22
horse-power when the fans are running at full speed.
The air displaced mostly escapes towards the floor level
into the boiler house, thus benefiting the ventilation
of the boiler house and promoting the draught up the
chimney.

After describing the arrangements made for ventilating
the engine rooms on board a number of large Cunard
liners, the author proceeded to deal with underground
and other engine rooms having an unvarying overheated
“ windless atmosphere ” of from at least 100 degs. Fahr,
in winter to 150 degs. Fahr, in summer, notwithstanding
the presence of old-fashioned apparatus expected to
secure so-called mechanical ventilation. Until about
four years ago it was unusual to employ open fans
for ventilating engine rooms, etc., and it was difficult to
obtain open Jans capable of creating efficiently an
under pressure, in a duct or air inlet, sufficient to induce
an air current of fairly high velocity.

Fig. 3.

Fig. 4.

To meet this demand, a fan as shown in fig. 3 has
been specially developed. It will be seen that the
impeller vanes are scooped on the inlet side, and are
of such shape that they slice into the incoming air and
divert it gently from the axial into the radial direction,
as shown conventionally in fig. 4. The outer longi-
tudinal edges of the blades are inclined backwards with
reference to the direction of rotation, as shown in fig. 5.

Fig. 5.

Experience has shown that impellers so bladed do not
tend to race when employed as open fans running in
parallel, and that there is little tendency for the air
to re-enter the space between the blades and cause eddies
when the fan is running against a comparatively high
pressure. This design ensures strength and rigidity
without internal stays. Strengthened forms of this

Fig. 6.

impeller, as shown, for example, in fig. 6, driven at
high speed by steam turbines, have been adopted for
use on board the latest forms of torpedo-boat destroyers
in various navies, for giving forced draught at a static
water gauge of 6 in., and obtaining coolness in the
stokeholds.

By virtue of the vertical arrangement, air is dis-
tributed high up in the stokehold where the pressure
equalises itself, thus avoiding unequal air distribution
to the burners and formation of eddies which would
set up draughts and cause discomfort to the occupants
of the stokehold. Tests of similar fans have been pub-
lished in the Journal of the American Society of Naval
Engineers of November 1912.

Again, in certain circumstances, it is often necessary
to furnish fresh air in volume, either artificially cooled
or warmed. Fig. 7 represents a part vertical section
and a part elevation of an open fan installation capable
of furnishing fresh air, either heated or cooled, to a