The Express.
London, England, 1851-02-15 00:00:00

Mt. AII;IOTT'S Alit PVIIIPP.

Dr. Arnott has invented two forms of air-pump---the
barrel and piston pump, and the swing-flap or pendulum
pump. In both the very great improvement upon all for-
mer pumps is in the nature of the valv, s, which are so light
that no valvular obstruction is offered to the passage of air,
and their size is unlimited. They arc called curtain valves,
and consist simply of pieces of oiled silk, dreadnought,
painted canvass not too thick, or cloth of a close texture,
hanging across an opening, through which they are pre-
vented from passing by were lattice-work. If the pump
be very large, the valve must be formed of several pieces of
cloth or dreadnought, so filStelled as to overlap each other,
like the slates of a house or the scales of • fish.
It is plain that a draught of air blowing against a piece of
cloth so fixed would blow it open, while a draught in the
opposite direction would close it. If the opening were
grated with wire the cloth could not be blown through it,
and the uir, theretbre, could not escape. Thus, by nailing
these curtain valves within or without the upper part of
openings in a box, air may be drawn in in one direction
and forced out in another by strokes of the same piston or
flap.
The barrel and piston pump is simply a square box, closed
on all sides except where the valves are, and where tbo
piston-rod passes through. The piston is merely a piece of
wood tinily; closely to the sides of the box, but still loose
enough to move up and down in the (sex, with but little
friction. It is mole so that it can only move in the part of
the boa between the upper and lower valvular openings, so
that it may not interfere with the action of the valves. One
side of the box must communicate with the open air, either
directly or by means of a tube ; and the other side with the
tube which is to distribute the air throughout the ship. It
is thus made a supply-pump of fresh air; but, by reversing
the action, it may be made a discharge-pump of impure air.
It is not necessary, however, to keep two pumps in action,
for if a tube communicate with the upper part of each

compartment, the impure air will escape without pumping,
when fresh air is freely supplied, especially if a lamp be
kept burning in the discharge tube.
The action of this pump is very easily understood. It is
nothing more than a square bellows. As the piston moves
downwards, air enters through the upper outside opening.
It cannot be forced outwards again, because the lower out-
side valves close the opening. It must, therefore, pass
through under the lower inside salves. fro when the piston
moves upwards, air enters through the lower outside
opening, and is forced through beneath the upper inside
valves. Thus, every motion of the piston upwards and
downwards, draws in air on one side and pumps it out on
the other, as will be seen by comparing the two cuts. It
may be worked by a lever, handle, and sling, like a com-
mon pump ; or, by fi crank and fly-wheel, or a rope and
pulley. A pump of this kind, with the space in which the
piston works three diet square, allowing a three feet-stroke
of the piston, will easily distribute 1,000 cubic feet of air
per minute. This is an allowance of four cubic feet per
minute for every person in a crew of 250, a quantity
sufficient to afford pure air fur breathing, as well as fur the
dilution and removal of all impure exhalations.
The swing-flap or pendulum pump is also a cubical box,
divided into two equal parts by a hanging flap or partition.
The flap is hung so that it may swing backwards and for-
wards, like a pendulum, from one lower corner of the box
to the other. The bottom of the box must be concave, to
correspond with the sweep of the pendulum, which is made
to fix the box all round to within the twentieth elan inch.
This flap forces the air before it just like the sweep of a fan,
or the stroke of the piston in the box just described. The
valves also act in the same manner. It may either be
worked by a handle connected with the axis of the pen-
dulum, or by pulling ropes acting upon it, just as in the act
of tolling a bell. It should not be worked more quickly than
the usual rate of a (lock pendulum, unless springs were
fixed inside near each lower corner, to assist recoil, and pre-
vent injury to the apparatus from hard striking.
This pump, like the other, is both a forcing and drawing
pump, and acts either as one or the other, according to the
arrangement of the valves and air channel.. What renders
it peculiarly adapted fur ship ventilation, is the facility':
with which it may be fixed in any part of a ship, and the
certainty with which it would serve the double purpose, at
the saint, time, of supplying pure and removing impure air.
For instance, one might be fixed in every cabin of a large
steamer, and the whole kept constantly working, night and
day, by a crank from the engine at kss than half-horse
power expenditure. The lower half of a pump of this rise
a foot square, projecting through the tap in one of the
corners of each cabin, would not occupy useful space. The
upper half would communicate with air admitted by open-
ings in the deck or bulwarks, or would open into an air
tube. Nothing would be easier than to carry a double tube
all round the ship between the timbers. Into the one cum-
partrnent of this tube, the pump would discharge the impure
air from the top of the cabins, while the other compartment
would supply pure air to be distributed by the pump round
the bottom of the cabin. When hatches or ports remain
open, there would be no necessity for the supply by the
pump, fur if the impure air were effectually removed, the
pure air would find its way in to supply its place. In very
cold weather it would be very easy to warm the air before
it was admitted into the compartment of the tube destined
for its supply by the pump.
Either of these forms of pump would be also exceedingly
useful in steamers, to keep up a current of air upon the fires.
At present the draught of the funnel is absolutely necessary
to keep the fires burning, and if the funnel were injured
by a shot or otherwise, so that the draught were prevented,
the fires would go out and the steamer be rendered power-
less. A very small expenditure of power from the engine,
employed upon one of those pumps, would ensure &constant
draught under all circumstances, and render combustion of
the smoke a matter of the greatest possible ease. Great
economy of fuel might thus be ensured, with increased
efficiency of the vessel, particularly when it is desirable to
avoid observation, and much less liability to serious injury
in action.

resists every change of ti:ntistpheie)rarintnurieit::.,2laniyfidapcd,iiirrisussiran:oe:wniarpnevhcoiryor
shrink from excessive heat, or expand from saturation.
sle‘a.h, When
nnidt its'hcaotn''Oudrerterdadthe
climates, the advantage of ships being built of a material
which combines within itself the best properties of other
woods is utterly obvious, and needs no comment; more ea-
pecially since the shippina. interests of Great Britain have
not increased in regard of British-built ships in the Sallie ratio
with insist of the mercantile navies of other countrit s.aud but
for the colonial built tonnage would have been superseded in
some important branch,: of the carrying trade. But
• where shipbuilders can still hold their ground in competi-
tion with all foreigners, is in the conotruction of first-elsos
ships for long voyages to and from India, for hdl heavy
trades, and for the conveyance of male passengers and
' emigrants, to distant colonies. This more particularly ap-
,' plies to the increaseduse and demand for steamers 01' the
' largest and first-class; and, as the engines of this country
are superior to those of foreign nations, the British ship-
owners and builders, possessing in these respects so many
advantages, they surely have a right to demand from
Lloyd's committee that the came rule should he applitol to
mahogany as to other high-class woods, as the refusal of its
admission fur all parts of 3 first class ship operates to restrict
employment in shiiimilding at hams, and tends to sustain
at an exothitaut price other high-class woodo intrinsically
not ouporior to mahogany; thus, instead of transferring the
shipbuilding trade to the colonies or to foreign powers,
there would be, with wise legislation, and judicious rules,
nothing to apprehend from the rivalry of other countri. s.
llondurat mahogny is found to work as freely and :tend
as readily as English oak, as may be ascertained by inspect-
ing NOM of the fine chips now being built, practicully de-
monstrating that it can he turned round the must difficult
curves, without any symptoms of "spelling," or splitting.
It has been shown by a aeries of e.xperimento, med., by
Messrs. Thos. and John White, shipbuilders, of Owe.. that
Honduras mahogany is stiffer than either East India
teak, Eurrlish oak, Quebec oak, American chn, Dsutoic
fir, or American yellow pine, and that •'• it is the best
wood for obipbrabling purposes." The non-verrosi in of
metal. is likewise a -highly favourable property in this
wood, sod this has been ascertained by the celebrated
chemist, Dr. Ere, who, having tested samples of
Ilonduras mahogany which bad been submitted fur ana-
lysis, reported that " A decoction of the chips or shavings
had hardly any chemical reaction, and soareely atfeettd
iron and cropper ;" and, as a case in point, it is known that
when the old Spanish frigate, Prima-se, built of ineho-
piny,. was taken to pieces, so sound and tenacious were her
Iron fastenings, that the own were paid double wages for
extracting the belts. The mahogany which is the best adapted
slop-building is that which is firm, tough, and roty, by
which latter term is meant the irregular direction, or over-
lapping of the grain of the wool; the soft ~r etolar-like
kinds being used for the inside planking, whilst the hard
and wavy are suited for the outside planking and timber-
ing. These differeut properties were well understood by the
Spanish shipbuild,srs, who, by judiciously s-deeting such
logo as were of suitable texture, succetoled iu building
ships, some of which, even at the present day. arc in a um-
mission iu our navy, or when broken up (roan age, have
hail their mahogany timbers, used in new ships,. Nor let It
be supposed that the Spaniurds eat mahogany iu their
islands alone ; their arehives attest that very much wood,
cut on the continent of America, was Font for the use of the
Spanish nary to the arsenal in Cuba. Indeed, the botanical
character of miihogany is essentially the saute, whether
grown on the continent or on the Islands of Cuba or St.
Domingo. A superficial observer may not bate skill in
discriminating bermd the formation and soundness of we'd
(which are the chief essentials in English oak, it being of
nearly uniform texture), but if the judgment of the sur-
veyor Ise not matured so us to appreciate the useful nut
vaned texture of inalio,:my, in order to apply it practically
to the purposes, for which it is best suited, irantnee is dome
to the wood and injury to the trade and best interest
of the country. The minims of eminent shipbuilders
art., that sound, hard, and well selected Ilondurao mahogany
is suited fur all parte of new " twelve years' merchant
ships," and if long and well squared, it is very ceoitomi. al,
the offal being of much more value than that of any other
wood at present mod fur shipbuilding. The specific
gravity of %esteemed llondurao mahogany being less than
than that of oak and teak, it is calculated that a vessel of
1,000 tons hurthen built of it, would displace. when afloat,
nearly 100 tons of water less than if conatructed of the latter
woods : and as buoyancy is ions of the highest qualities in a
ship, this characteristic feature ought to have much weight
with those who are desirous of producing the best specimens
of naval architecture. Its durability is so well-known, that
it scarcely requires further comment, than to °Norm,
that in household furniture, in the old Spanish 'hips of war,
and in the droghero and vessel, of the countries in which
it grows, its imperishable nature is attested by the mat
incontrovertible evidence. Instance. in which ships built of

English oak have been tliand decayed and dry-rotten, are un-
fortunately too nuuterott.' and many merchant vescols as well
as men of war, constructed of this celebrated timber, hove
been rendered nearly valueless from the souse above men-
tinned, before they had been three years at sea ; whilst cos
the other hand, it is an undoubted fact, that tiering to the
enduring propertliii of mahogany, it not unfregently warn
in those countries ill which it grows, that the oak timbers
of a chip, from decay, are obliged to be taken out and re-
placed by that woad; and, likewise' it has been discovered
that in the same ships in which mahogany, oak and
teak had been wed, the mahogany was quite mood. whilst
the other woods alongside of it were decayed. In fact, 1111
ships aro principally employed m tropical coun-
tries, and exposed to great heat, it is only reasonable to con-
clude that the last material to build them of is the timber
produced in congenial climates—it being notorious that
hard woods in geueral, and mahogany in particular, which.
ure grown in tropical countries, arc hardly subject to dry
rot ; whilst, on the contrary, those woods which are the
production of a cold northern climate have a greater ten-
dency to decay. To shipowners the purchase of ships built
iof mahogany, n preference to other woods, Las this im-
portant consideration ; for, however ancient or worn such

;hips may bccinne, the intrinsic value- of old mahogany to
shipbreakers will be much greater than any other wood;
and cabinetmakers will frequently give very high grass
forold-seusoned mahogany. It is a curious and interesting
fact, that furniture is being made in the royal dockyanis at
this day out of the beautiful maloiganz found in breaking
up the old line of battle-ship, the Gibraltar, which was
built in the Haviumah NO years ago ! In re poet of this
sessel, it is mentioned in the " Memoirs of Admiral Mir
Jahlecl Brenton," " that when he was first lieutenant of
the Gibraltar (a huge three-decker), he narrowly escuptd,
having been driven on an iron-bound roust du: :iig a horrid
gale of wind which had forced her from her moorings, and
he suggests that all efforts might base been fruitless embus
the vessel hod been of very singular construction, v:s.,
" Spanish built, with her lower part one enurtnow, 111111.3 of
solid mahogany."

Stone's Medics! Portrait Gallery. flighty, Fleet-strco..
Mr. Stone is known in the medical world as an
assiduous and discriminating collector of portraits.
The shortcomings of art towards the talented band
who now lead the van of medical inquiry appear to
have suggested the present series of engravings.
The list of the " Medical Portrait Gallery " includes
the names of Mr. South, of St. Thomas's Hospital, the
well-known editor of Chelius's Surgery; Mr. Simon,
professor of pathology at the medieOl school of the
same institution, and medical officer of the City
Board of Health ; Professor Ferguson, whose least
praise is that he is one of the greatest living operators;
Dr. Forbes, a philosoper and author, ns well as an
industrious physician; Mr. Paget, author of the
elaborate catalogues of the llunterian and Bartholo-
mew museums; Dr. Budd; Mr. E. Forbes; Dr.
Bentley Todd; Sir W. Charles Ellis; Mr. Skcy,
of Bartholomew's, with other gentlemen, searecly
less distinguished in their various specialities.
The whole collection forms an interesting souro.ir
to the numerous body whom the requirements of ;n-
-struction or relief may at any time have brought into
communication with the originals. The portraits are
drawn by Mr. T. H. Maguire, with the exception of
that of Sir W. C. Ellis, which has been entrusted
to Mr. Aldous, and also that of Mr. Simon, ably
taken by M. Baugniet. The likenesses, taken alto-
gother,are very good, though they vary in merit.
That of Dr. Forbes certainly reduces the amount of
intellectuality which his countenance displays; whilst
those of Mr. Paget and Mr. South give the entire
character of countenance of the originals. Mr. Skey's •
is also good, and so is Mr. Simon's. In Mr. Fer-
gusson's, the resemblance is great, but the face is '
somewhat too full. The same remark will apply to
the portrait of Mr. Hancock. The least successful
portrait ; that of Sir W. C. Ellis.