P U M
P l M
with each other. A pulse is said to be hard or soft, with regardto the artery, according as it is tense, renitent, and hard, or flaccid,soft, and lax : for the disposition of the arteries contributes greatlyto the change of the pulse ; wherefore it sometimes happens, thatthe pulse in both arms is not alike, which is very common in a liemi-plexy. Add to these a convulsive pulse, which does not proceedfrom the blood, but from the state of the artery ; and is known bya tremulous subsultory motion, and the artery seems to be drawnupwards ; this, in acute fevers, is the sign of death ; and is said tobe the pulse in dying persons which is likewise, generally unequaland intermitting. A great pulse shews a more copious afllux of theblood to the heart, and from thence into the arteries ; a little pulsethe contrary.
Pulse, is also used for the stroke with which any medium is af-fected by the motion of light, sound, &c. through it.
Pulse, in botany, a term applied to all those grains or seedswhich are gathered with the hand; in contradistinction to coin,&c. which are reaped, or mowed : or, It is the seed of the legumi-nous kind of plants, as beans, vetches, &c. but is by some used forartichokes, asparagus, &c.
PULTAUSK, a town of Poland, in Masovia , on the Nareu ; 20miles N. E. of Warsaw . Lon. 21. 47. E. Lat. 50.30. N.
PULTENiEA, a genus of the monogynia order and decandriaclass of plants. Calyx five-toothed ; corolla papilionaceous ; le-gume one-cellcd ; seeds two. There are six species natives of NewHolland.
PULTOWA, or POLTAVA , a fortified town, formerly ofPoland, in the Ukraine, but now of Russia , in Ekaterinoslaf, 84miles N. N. W. of Ekaterinoslaf, and 100 S. W. of Bielgorod .Lon. 34. 23. E. Lat. 49. 26. N.
PULVERIZATION, the act of powdering; reduction to dustor powder.
PULVINATED, in the ancient architecture, a term applied toa frieze which swells or bulges out in the manner of a pillow.
PULVIS, [Lat.] powder. See Powder.
PUMEX, the Pumice-Stone, a substance frequently thrownout of volcanoes, though there are many which never throw it out.It is very full of pores, in consequence of which it is specificallyvery light, and resembles the frothy slag produced in our iron-fur-naces. It is of two colours, black and white; the former beingthat which it has when thrown out of the volcano ; the latter, asCronstedt conjectures, being perhaps faded and bleached. M.Magellan considers it rather as a volcanic ejection than a volcanicproduction. Pumice-stone is used by silver-smiths and other me-chanics, for rubbing and smoothing the surface of metals, wood,pasteboard, and stone; for which it is well fitted by its harsh andbrittle texture; thus scouring and carrying off all the inequalities.
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PUMP, an hydraulic machine for raising water by the pressureof the atmosphere. It does not appear to have been known to theGreeks and Romans in early times; perhaps it came from Alex-andria,where physical and mathematical science was much cultivat-ed by the Greek school under the Ptolemies . The performancesof Ctesibius and Hero are spoken of by Pliny and Vitruvius as cu-rious novelties. Let AB, (Plate C LI V. fig. 1,) be the surface ofthe water in the well, and D the height where it is to be delivered.Let DC be a long wooden trunk, reaching as deep underwateras possible. Let the rope EF be fitted with its knot of hay F.V'hen it is drawn up through the trunk, it will bring up along withit all the water lying between C and A, which will begin to run outby the spout D as soon as the knot gets to G, as far below D as Cis below A. All this is very obvious; and it required but little re-flection to be assured, that if I' was let down again, or pusheddown, by a rod instead of a rope, it would again perform the sameoffice. ‘By pushing down the knot F, which we shall hencefor-ward call the piston, all the force expended in lifting up the waterbetween A and G is thrown away, because it is again let down.A valve G, at the bottom, would prevent this. But then theremust be a passage made tor the water by a lateral tube KBD, (fig. 2.)
And if this be also furnished with a valve II, to prevent its losingthe water, we have the pump of Ctesibius , as sketched in fig. 2,We shall here give a short description of the chief varieties of theseengines, considering them in their simplest form, and explain ingeneral-terms their mode of operation. There are but two sorts ofpumps which essentially ditier; and all the varieties that we see areonly modifications of these. One of these original pumps has a so-lid piston ; the other has a piston with a perforation and a valve.We usually call the first a forcing pump, and the second a lifting osucking pump.
Rump, Forcing. Fig. 2. is a sketch of the forcing pump in itsmost simple form and situation. It consists of a hollow cylinderA C c a, called the working-barrel, open at both ends, and "havinga valve G at the bottom, opening upwards. This cylinder is filledby a solid piston EF, covered externally with leather or tow, bywhich means it fits the box of the cylinder exactly, and allows nowater to escape by its sides. There is a pipe KfiD, which com-municates laterally with this cylinder, and has a valve at some con-venient place H, as near as possible to its junction with the cylin-der. Tins valve also opens upwards. This pipe, usually called therising pipe, or main, terminates at the place D, where the watermust be delivered. Now suppose this apparatus set into the wa-ter, so that the upper end of the cylinder may be under or evenwith the surface of the water AB ; the water will open the valve G,and after filling the barrel and lateral-pipe, will also open the valveH, and at last stand at an equal height within and without. Nowlet the piston be put in at the top of the working-barrel, and thrustdown to K. It will push the water before it. This will shut thevalve G, and the water will make its Vvay through the valve II,and fill a part B b of Hie rising pipe, equal to the internal capacityot the working-barrel. When this downward motion of the pistonceases, the valve H will fall down by its own weight and shut thispassage. Now let the piston be drawn up again : The valve Hhinders the water in the rising-pipe from returning into the workingbarrel. But now the valve G is opened by the pressure of the ex-ternal water, and the water enters and fills the cylinder as the pis-ton rises. When the piston has got to the top, let it be thrustdown again : the valve G will again be shut, and the water will beforced through the passage at Ii, and rise along the main, pushing-before it the water already there, and will now have its surface atL. Repeating this operation, the water must at last arrive at D,however remote, and the next stroke would raise it toe; so thatduring the next rise of the piston the water in e D will be runningoff by the spout. The effect will be the same whatever is the posi-tion of the working barrel, provided only that it be under water.It may lie horizontally or sloping, or it may be with its mouth andpiston-rod undermost. It is still the same forcing pump, and ope-rates in the same manner and by the same means, viz. the pressureof the surrounding water. The external force, which must he ap-plied to produce this effect, is opposed by the pressure exerted bytiie water on the opposite face of the piston. It is evident, fromthe common laws of hydrostatics, that this opposing pressure isequal to the weight of a pillar of water, having the face of the pis-ton for its base, and the perpendicular height d A of the place ofdelivery above the surface of the water A B in the cistern for itsheight. The form and dimensions of the rising pipe are indiffer-ent in this respect, because heavy fluids press only in the propor-tion of their perpendicular height. Observe that it is not d F, butd A, which measures this pressure, which the moving force mustbalance and surmount. The whole pressure on the undersurfaceFf of the piston is indeed equal to the weight of the pillar d Fj's ;but part of this is balanced by the water AYfa. If, indeed, thewater does not get into the upper part of the working barrel, thiscompensation does not obtain. While we draw up the piston, thispressure is removed, because all communication is cut off bv thevalve H, which now bears the whole pressure of the water in themain. Nay, the ascent of the piston is even assisted by the pres-sure of the’surrounding water. It is only during the descent ofthe piston, therefore, that the external force is necessary. Themeasure now given ot the external force is only what is necessaryfor balancing the pressure of the water in the rising pipe. Butthat the pump may perform work, it must surmount this pressure,and cause the water to issue at D with such a velocity that the re-quired quantity of water may be delivered in a given time. Thisrequires force, even although there were no opposing pressure;
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