462 
PNE 
POD 
also stand upon the same plane with the re- 
c iver-plate, and this plane is raised high 
enough to admit the common gage of 32 or 
33 inches- to stand under it without incon- 
venience in working the pump ; as the winch 
moves through a less portion of an arch at 
each stroke, than it would do if the pistons 
moved through the whole length of the bar- 
rels. 
A g.ige for measuring the degree of con- 
densation, having a free communication with 
tiie valves, cock, See. is placed between the 
barrels in this pump; and the gage is so 
constructed that it will also serve to measure 
the rarefaction above the valves when the air 
is worked off by the valve-pump. It consists 
of a pedestal, the die of which is made of 
glass, which forms a cistern for the mercury ; 
a hollow brass pillar; and glass tube hermeti- 
cally sealed at one end, which moves up 
and down in the pillar through a collar of 
leathers. When the pump is used as a con- 
denser, the degree of condensation is shewn 
by a scale marked on one edge of the pillar; 
when it is used as an exhauster, the degree of 
rarefaction of the air above the valves, is 
shewn by a scale on the other edge of the 
pillar, 'This gage will also shew, when the 
valves have done playing, either with the 
weight of the atmosphere on them or taken 
off, in the manner which the author has de- 
scribed. The degree of condensation may 
be also measured by the number of strokes 
of the winch. For the purposes of great con- 
densation, Mr. Prince has litted a condenser 
of a smaller bore than the barrel of the great 
pump to the cistern of the valve-pump, to be 
screwed on occasionally. Or, without this 
condenser, the valve-pump may be adapted 
to the purpose by being made a little larger, 
and by having a plate made to screw into the 
bottom of the cylinder, with a valve on it 
opening into the cistern ; a hole must be 
made to be opened on the same occasion 
near the top of the cylinder, to let ab in be- 
low the piston when this is drawn up above 
it. 
The common gage, which is generally 
placed under the receiver-plate, is placed in 
the front of this pump, that it may be seen by 
the person who works it, and that the plate 
may be left free for other uses. The plate is 
so fixed- to the pipe leading to the cock, that 
it may be taken off at pleasure, and used as 
a transferer ; and it may also serve for other 
purposes. 
The head of this pump is made whole, ex- 
cept a small piece on the back, where the 
wheel is let in; and the wheel is freed from 
the piston-rods by pushing it into the back 
part of the head, and it is kept in its place by 
.a button screwed into the socket of the axis 
behind. By this apparatus, the piston-rods 
are dislodged from the wheel, and let down 
into the cisterns, when the pump is not used ; 
and in these cisterns they may also have the 
advantage of being covered with oik The 
principal joints of this pump are sunk into 
sockets, that the leathers which close them 
vgay be covered with oil to prevent leaking. 
The lower part of the pump is fitted with 
drawers to contain the necessary apparatus. 
We shall close our account of the two 
pumps of Prince and Cuthbertson, with the 
following judicious remarks of Mr. Nicholson 
on their respective merits and imperfections: 
There is no provision to open the upper 
PNE 
fixed valve of Prince’s greater barrel, except 
the difference between the pressures of the 
elastic fluid on each side of the strip of blad- 
der; and this may reasonably be inferred to 
limit the power of his small pump. In Cuth- 
bertson’s pump, the same valve is exposed to 
the action of the atmosphere, together with 
that of a column of oil in the oil-vessel, d he 
mischief in either instrument is probably tri- 
lling, but in both the valve might have been 
opened mechanically. If this were done, the 
small pump of Prince might, perhaps, be un- 
necessary in most states ot the atmosphere. 
With regard to the lower valves, Cuthbertson, 
by an admirable display of talents as a work- 
man, has ensured their action. Prince, on 
the other hand, has, by the process of reason- 
ing, so far improved the instrument, that no 
valves are wanted. In this respect, he has 
the advantage of simplici y and cheapness 
with equal effect. r l he mechanical co;“ibi- 
nation of Cuthbertson’s pump, reduces the 
operation to one simple act ot the handle ; 
but Prince’s engine requires some manipula- 
tion with regard to the play of the small 
pump, though this might have been remedied 
by a more skilful disposition ot the first 
mover. 
“ The most perfect scheme for an air- 
pump, taking advantage of the labours of 
these judicious operators, seems to be that in 
which two pistons of the construction of 
Prince should work in one barrel, one piston 
being fixed at the lower end of the rod, and 
the other at the middle. The lower piston 
must come dear out of the barrel when down, 
and work air-tight through a diaphragm at an 
j equal distance from the effective ends ot the 
barrel. In the diaphragm must be a metallic 
valve of the form ot Cuthbertson’s lower valve, 
but with a short tail beneath, that it may be 
mechanically opened when the piston comes 
up. Above the diaphragm must work the 
other piston, similar to the first ; but as it 
cannot qi.*t the barrel when down, a small 
portion of the barrel must be enlarged just 
above the diaphragm, so that the leathers 
may he clear in that position. Lastly, the 
top of the barrel must be closed and litted 
with a valve and oil-vessel, according to the 
excellent contrivance of Cuthbertson. 
“ If we suppose the workmanship of such a 
pump to leave the space between the dia- 
phragm and lower piston, when up, equal to 
o.ie-thousandth part of the space passed 
through by the stroke of that piston, the 
rarefaction produced by this part of the en- 
gine will, in theory, bear the same proportion 
to that of the external air; and the same 
supposition applied to the upper piston, 
would increase the effect one thousand times 
more ; whence the rarefaction would be one 
million times. How far the practical effect 
might fall short of this, from the imperfection 
of workmanship, or the nature of the air, 
which, in high rarefactions, may not diffuse 
itself equally through the containing spaces, or 
j from other, yet unobserved circumstances, 
I cannot be deduced from mere reasoning, with- 
out experiment. 
PNEUMORA, a genus of insects of the 
order hemiptera. The generic character is, 
body ovate, inflated, diaphanous; head in- 
flected, armed with jaws ; thorax convex, 
carinate beneath ; wing-cases deflected, mem- 
branaceous; legs formed for running. The 
insects of this genus appear to consist of a 
mere hollow inflated membrane. By rubbing 
together their toothed legs, they make a shrill 
kind of noise morning and evening, and fol- 
low a light They are so nearly allied to the 
cricket tribe, that Fabricius lias enumerated 
them under the genus gryllus. There are 
three species. 
POA, meadow-grass, a genus of the digy- 
nia order, in the pentandna class of plants, 
and in the natural method ranking under the 
fourth order, gramma. The calyx is bivalved 
and multiflorous; the spicula or partial spike 
is ovate, with the valvules scarious, and a little 
sharp or thin on the margin. There are 71 
species, most of them grasses, and very agree- 
able food for cattle; for one species, which 
grows in marshes, the cattle will frequently 
go so deep as to endanger their lives. This 
is called tire aquatica, or water reed-grass. It 
is the largest of the British grasses, growing to 
the height of live or six feet. The leaves are 
smooth, and half an inch wide or more. The 
panicle is eight or ten inches long, great I v 
branched, and decked with numerous spicula ; 
these are of a reddish brown colour inter- 
mixed with green, of a compressed lanceolate 
form, imbricated with about six flowers for 
the most part, but varying from live to ten. 
See Husbandry, and Plates LXVill. and 
LX IX. 
POCKET, in the woollen trade, a word 
used to denote a larger sort of bag, in which 
wool is packed up to be sent from one part 
of the kingdom to another. The pocket con- 
tains usually twenty-live hundredweight of 
wool. The pocket of hops is also a small 
bag usually containing the best hops. 
P ocKET-instrumeiits, in surgery. See In- 
strument. 
PODOPHYLLUM, a genus of the mo- 
nogyma order, in the polyandna class of 
plants, and in the natural method ranking 
under the 27th order rhscsedeax The corolla 
has nine petals; the calyx is triphyllous; the 
berry unilocular, crowned with the stigma. 
There are two species, creeping plants of 
North America. 
PODU RA, a genus of insects of the order 
aptera. The generic character is, legs six, 
formed for running ; eyes two, composed of 
eight; tail forked, formed for leaping, in- 
flected; antenna: setaceous, elongated. The 
podura: are small insects which, in general, 
are found in damp places, ruyler stones, on 
the bark of trees, & c. When disturbed, they 
suddenly spring to a small distance by the 
help of a long, forked process, which is 
doubled under the abdomen, and which is 
suddenly thrown out during the act ot leap- 
ing. 
One of the most common of this genus is 
(he podura aquatica of Linnaeus, measuring 
scarcely the twelfth part of an inch in lengt h, 
and entirely of a black colour. This a gre- 
garious species, and is occasionally seen as- 
sembled in vast numbers, particularly near 
the brinks of ponds, covering the ground to 
the distance of several feet, and sometimes 
even the surface of the water itself. On the 
ground its legions, on a cursory view, have 
the appearance of scattered grains of gun- 
powder ; and, if closely examined, will be 
found in an almost continual skipping mo- 
tion. 
Podura fimetariaso perfectly resembles the 
preceding in all respects except colour, be- 
ing perfectly white, that uo other speeilic 
