Encyclopaedia Britannica > Volume 15, PLA-RAM

Itamp.
70
Button
valves
7i
Very ufe-
ful;
71
Though
fomewhat
imperfed;
irvthe wa-
ter-wiy.
73
A very
funple
valve de-
fcribed.
74
A valve
by Belidor
uniting e-
very requi*
file.
P U M [663
branches form the fupport for the four clacks. We
have feen a valve of this form in a pump of fix feet di¬
ameter, which difcharged 20 hogfheads of water every
Itroke, and made 12 ilrokes in a minute, railing the wa¬
ter above 22 feet.
There is another form of valve, called the button or
tail valvs. It confifts of a plate of metal AB (fig. 34.)
turned conical, fo as exactly to fit the conical cavity
a £ of its box. A tail CD projects from the under
fide, which palfes through a crofs bar EF in the bot¬
tom of the box, and has a little knob at the end, to hin¬
der the valve from rifing too high.
This valve, when nicely made, is unexceptionable.
It has great ftrength, and is therefore proper for all fe-
vere Itrains, and it may be made perfe6lly tight by
grinding. Accordingly it is ufed in all cafes where this
is of indifpenfable confequence. It is molt durable,
and the only kind that will do for paflages where fleam
or hot water is to go through. Its only imperfec¬
tion is a frnall water-way ; which, from what has been
faid, cannot exceed, nor indeed equal, one-half of the
area of the pipe.
If we endeavour to enlarge the water-way, by giving
the cone very little taper, the valve frequently Hicks
fo fall in the feat that no force can detach them.—
And this fometimes happens during the working of the
machine; and the jolts and blows given to the machine
in taking it to pieces, in order to difcover what has
been the reafon that it has difcharged no water, fre¬
quently detaches the valve, and we find it quite loofe,
and cannot tell what has deranged the pump. When
this is guarded againll, and the diminution of the wa¬
ter-way is not of very great confequence, this is the bell
form of a valve.
Analogous to this is the fimplell of all valves, repre-
fented in fig. 33. It is nothing more than a fphere of
metal A, to which is fitted a feat with a fmall portion
BC of a fpherical cavity- Nothing can be more effec¬
tual than this valve ; it always falls into its proper
place, and in every pofition fits it exaftly. Its only
imperfection is-the great diminution of the water-way.
If the diameter of the fphere does not conliderably ex¬
ceed that of the hole, the. touching parts have very
little taper, and it is very apt to Hick fall. It oppofes
much lefs refinance to the paffage of the water than the
fiat under-furface of the button-valve. N. B. It would
be an improvement of that valve to give it a taper-fhape
below like a boy’s top. The fpherical valve muff not
be made too light, otherwife it will be hurried up by
the water, and much may go back while it is returning
to its place..
1
P U M
Belidor defcribes with great minutenefs (vol. ii. p.
221, &c.) a valve which unites every requiiite. But
it is of fuch nice and delicate conHruClion, and its de-
feCls are fo great when this exaftnefs is not attained, or
is impaired by ufe, that we think it hazardous to in¬
troduce it into a machine in a fituation where an intel¬
ligent and accurate artiff is not at hand. For this
reafon we have omitted the defcription,. which cannot
be given in few words, nor without many figures; and
defire our curious readers to confult that author, or per-
ufe Dr Defagulier’s tranflation of this paffage. Its
principle is precifely the fame with the following rude
contrivance, with which we fhallconcludethe'defcrip-
tive part, of this article.
Suppofe ABCD (fig. 36.) to be a fquare wooden Pump,
trunk. EF is a piece of oak-board, exaftly fitted to
the trunk in an oblique pofition, and fupported by anAnojh5er
iron pin which goes through it at I, one-third of its valve on
length from its lower extremity E. The two ends of cb? fame
this board are bevelled, fo as to apply exa&ly to thePr*nci^e‘
fides of the trunk. It is evident, that if a Hream of
water comes in the diredlion BA, its preffure on the
part IF of this board will be greater than that upon
El. It will therefore force it up and rulh through,
making it Hand almoll parallel to the fides of the trunk.
To prevent its rifing fo far, a pin muH be put in its
way. When this current of water changes its direc¬
tion, the preffure on the upper fide of the board being
again greateff on the portion IF, it is forced back again
to its former fituation ; and its two extremities reffing
on the oppofite fides of the trunk, the paffage is com¬
pletely Hopped. This board therefore performs the
office of a valve j and this valve is the moll perfeft that
can be, becaufe it offers the freed paffage to the water,
and it allows very little to get back while itis (hutting ; •-
for the part IE brings up half as much water as IF al¬
lows to go down. It may be made extremely tight,
by fixing two thin fillets H and Cx to the fides of the
trunk, and covering thofe parts of the board with lea¬
ther which applies to them ; and in this Hate it perfect¬
ly refembles Belidor’s fine valve. ^5
And thb conHruClion of the valve fuggefls, by theDefcriptioa
way, a form of an occafional pump, which may be °fan ^
quickly fet up by any common carpenter, and will beca^ >nal
very effeClual in fmall heights. Let a b c d e (fig. 36.) be jil^con-*
a fquare box made to Hide along this wooden trunk with-ftrudUd.
out (hake, having; two of its fides projecting upwards,
terminating like the gable-ends of a houfe. A piece of
wood e is mortifed into thefe two fides, and to this the
piHon-rod is fixed. This box being furniffied with a
valve fimilar to the one below, will perform the office
of a piHon. If this pump be immerfed fo deep in the
water that the pifion (hall alfo be under water, we
fcruple not to fay that its performance will be equal to
any. The pifion may be made abundantly tight by co¬
vering its outfide neatly with foft leather. And as no -
pipe can be bored with greater accuracy than a very or¬
dinary workman can make a fquare trunk, we prefume
that this pump will not be very deficient even for a con-
fiderable fuCtion.
We now proceed to the lafi part of the fubjeCl, to p|ie
77
confider the motion of water in pumps, in reference totionofwa*-
the force which muH be employed. What we haveter irl
hitherto faid with refpeCl to the force which muH be?11111?8
applied to a pifion, related only to the fufiaining the
water at a certain height: but irv aCfual fervice we
muft not only do this, but we mufi difeharge it at the
place of delivery in a certain quantity ; and this mufi
require a force fuperadded to what is neceffary for its
mere fupport at this height.
This is an extremely intricate and difficult
and very imperfeCUy underfiood even by profeffed en- cate fub-
gineers. The principles on which this knowledge mutl jedt-
be founded are of a much more abfirufe nature than the
ordinary laws of hydroftatics ; and all the genius of
Newton was employed in laying the foundation of this
part of phyfical fcience. It has been much cultivated
in the courfe of this century by the firfi mathematicians
of Europe. Daniel and John Bernoulli have written
very
fubjea,An I’f,;.