748 
MECHANICS. 
.or .may be .made to convey water to every part of the 
bqtife; and this without hurting or obftrufting its com¬ 
mon uj'es. All that is neceffary is to have a large cock 
on the upper part of the working-barrel oppofite to the 
Jatcral pipe in this figure. This cock ferves fora fpo.ut 
when the pump is tiled for common purppfss ; and the 
merely (hutting this cock converts the whole into an en¬ 
gine for extinguiftiing fire or for fupplying difiant places 
"with water. It is fcarcely neceffary to add, that for thefe 
Cervices it will he proper to connect an air-veffel with 
fome convenient part of the riling pipe, in order that the 
current of the water may be continual. 
In all great works a confiderable degree of uniformity 
is produced by the manner of difpoling the aftion of 
different pumps; for it is very rarely that a machine 
works but one pump. In order to maintain fome unifor¬ 
mity in the refinance, that it may not all be oppofed at 
once to the moving power with intervals of total ina&ion, 
which would produce a very hobbling motion, it is ufual 
to dilfribute the work into portions, which fucceed al¬ 
ternately; and thus both diminifh the (train, and give 
greater uniformity of action, and frequently enable a na¬ 
tural power which we can command, to perform a piece 
of work, which would be impofiible if the whole refiltar.ee 
were oppofed at once. In all pttmp-machines, there¬ 
fore, we are obvioufly directed to conftruct them fo that 
they may give motion to at leaf! two pumps, which work 
alternately. By this means a much greater uniformity of 
•current is produced'in the main pipe. It will be rendered 
Hill more uniform if four are employed, fucceeding each 
other at the interval of one quarter of the time of a com¬ 
plete (troke. 
But ingenious men have attempted the fame thing with 
a fingle pump, and many different confirmations for this 
purpofe have been propofed and executed. The thing 
is not of much importance, nor of great refearch. We 
(hall content ourfelves, therefore, with the defeription of 
one that appears to us the moft perfect, both in refpefl of 
limplicity and effect. It confifts of a working-barrel A B, 
(fig. 65.) clofe at both ends. The pifion C is folid, and 
the rod O P paffes through a collar of leathers in tIte 
plate, which clofes the upper end of the working-barrel. 
This barrel communicates laterally with two pipes H, K ; 
the communications m and n being as near to the top and 
bottom of the barrel as poffible. Adjoining to the paffage 
m are two valves, F and G, opening upwards. Similar 
valves accompany the paflage n. The two pipes H and 
K unite in a larger rifing pipe L. They are all repre- 
lented as in the fame plane; but the upper ends mufi be 
bent backwards, to give room for the motion of the pilton- 
rod OP. Suppofe the pifion clofe to the entry of the la¬ 
teral pipe n, and that it is drawn up: it compreffes the 
air above it, and drives it through the-valve G, where it 
efcapes along the rifing pipe ; at the fame time it rarefies 
the air in the fpace below it. Therefore the weight of 
the atmofphere (huts the valve E, and caufes the water 
of the cifiern to rife through the valve D, and fill the 
lower part of the pump. When the pifion is pufhed down 
again, this water is firft driven through the valve E, be- 
caufe D immediately (huts; and then moft of the air 
which was in this part of tire pump at the beginning 
goes up through it, fome of the water coming back in 
its Head. In the mean time, the air which remained in 
the upper part of the pump after the afeent of the pifion 
is rarefied by its defeent; becaufe the valve G (huts as 
foon as the pifion begins to defeend, the valve F opens, 
the air in this fuclion-pipe Fy r expands into the barrel, 
and the water riles into the pipes by the preffure of the 
atmofphere. The next rife of the pifion muft bring more 
water into the lower part oi the barrel, and mult drive a 
little more air through the valve G, namely, part of that 
which had come out of the fudlion-pipe F f\ and the 
next defeent of the pifion muft drive more water into the 
rifing-pipe H, and along with it molt, if not all, of the air 
which remained below' the pifion, and mufi rarefy (till 
more the air remaining above the pifion 5 and more water 
will come in through the pipe F/, and get into the bar¬ 
rel. It is evident, that a few repetition's will at laft fill 
the barrel on both fides of the pifion with water. When 
this is accomplifhed, there is no difficulty in perceiving 
how, at every rife of the pifion, the water of the cifiern 
will come in by the valve D, and the water in the upper 
part of the barrel will be driven through the valve G ; 
and, in every defeent of the pifion, the water of the cif- 
tern will come into the barrel by the valve F, and the 
water below the pifton will be driven through the valve 
E : and thus there will he a continual influx into the 
barrel through the valves D and F, and a continual dif- 
charge along the rifing-pipe L, through the valves E 
and G. 
This machine is, to be fare, equivalent to two forejnp'- 
pumps, although it has but one barrel and one pifion ; 
but it has no fort of fuperiority. It is not even more 
economical in moft cafes; becaufe we apprehend that the 
additional workman(hip will fully compenfate for the bar¬ 
rel and pifton that is faved. There is indeed a faving in 
the reft of the machinery, becaufe one lever produces 
both motions. We cannot, therefore, fay that it is infe¬ 
rior to two pumps; and we acknowledge that there is 
fome ingenuity in the contrivance. 
The forcing-pump is (ometimes of a very different form 
from that already deferibed. • Inftead of a pifton, which 
applies itfelf to the infide of tire barrel, and Hides up and 
down in it, there is a long cylinder P O Q, (fig. 66.) nicely 
turned and polifiied on the outfide, and of a diameter 
fomewbat lefs than the infide of the barrel. This cylin¬ 
der (called a plunger) Aides through a collar of leathers 
on the top of the v\ orking-barrel, and is confirudted as 
follows. The top of the barrel terminates in a flanch 
XY, pierced with four holes for receiving fcrew-bolts. 
There are tw o rings of metal, CD, EF, of the fame dia-. 
meter, and having holes correfponding to thofe in the 
flanch. Four rings of foft leather, of the fame flze, and 
fimilarly pierced with holes, are well (baked in a mixture 
of oil, tallow', and a little rofin. Two of thefe leather 
rings are laid on the pump-flanch, and one of the metal 
rings above them. The plunger is then thruft down 
through them, by which it turns their inner edges down¬ 
wards. The other two rings are then flipped on at the 
top of the plunger, and the fecond metal ring is put over 
them, and then the whole are Aid down to the metal ring. 
By this the inner edges of the laft leather rings are turned 
upwards. The three metal rings are now forced together 
by the ferewed bolts; and thus the leathern rings are 
ltrongly cotnpreffed between them, and made to grafp the 
plunger fo clofely, that no preffure can force the water 
through between. The upper metal ring juft allows the 
plunger to pafs through it, but without any play; fo that 
the turned-up edges of the leathern rings do not come 
up between the plunger and the upper metal ring, but 
are lodged in a little conical taper, which is given to the 
inner edge of the upper plate, its hole being wider belowr 
than above. It is on this trifling circumftance that the 
great tightnefs of the collar depends. To prevent the 
leathers from (hrinking by drought, there is ufually a 
little cifiern formed round the head of the pump, and 
kept full of water. The plunger is either forced down 
by a rod, V, from a working-beam, or by a fet of metal 
weights, T, laid on it. 
It is hardly neceffary to be particular in explaining the 
operation of this pump. When the plunger is at the 
bottom of the barrel, touching the fixed valve M with its 
lower extremity, it almoft completely fills it. That it may 
do it completely, there is fometimes a fmall pipe RSZ 
blanching out from the top of the barrel, and fitted with 
a cock at S. Water is admitted till the barrel is com¬ 
pletely filled, and the cock is then (hut. Now, when the 
plunger is drawn up, the valve N in the rifing-pipe muft 
remain (hut by the preffure of the atmofphere, and a void 
muft be made in the barrel. Therefore the valve M on 
4 the 
