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to DF ; and therefore a jet from B will have the same ranee as that from D. Of all
the perpendlcnhirs. OG, drawn from the middle point C, la the greatest; therefore,
the ]et from C has the longest possible rauge.

The area of the orifice and the velocity of the flow being known, it is easy to cal-
culate the quantity of water discharged in a given time. Thus, suppose the ntvh to
be 1 square inch, and the velocity SO feet a second, it is evident that there isi«ucs> in a
second a cylinder or a prism of water 1 square inch in section and 90 feet l0%, the
content of which is 1 x S40 = MO cubic inches. In any given time, then, as Uiree
minutes ( = 160 seconds), the disdiarge is 840 x 180 = 43,900 cubic inches.

It has as yet been assumed that the water in tho vessel or reservoir Is kept con-
stantiv at the same height, and that thus the velocitv is constant. We have now to
consider the case of a vessel allowed to empty itself through an orifice at the bott )m.
As the surface of the water sinks, the velocity of the discharge diminishes or Is re-
tarded ; and wiien the veasi'l is of the same area from top to bottom. It can be proved
that the velocity is uniformly retarded. Its motion follows the same law as thatof a
body projected vertically upwards. Now, when a motion uniformly retarded comes
to an end, the space described is lust half what the body would have passed over, had
it gone on uniformly with tlte velocity it had at the outset Therefore, when the ves-
sel has emptied itself in the way supposed, the quantity discharged is half what would
have been discharged had the velocity been uniform from tlie'begUming.

Th€^'Ckmtraetionqfth$ rWn.'*— Wlien, by means of the area of the opening
and the velocity thus determined, we calculate the number of cubic feet or of gaf
ions that oitght to flow ont in n given time, and then meafure tho qnantlty that act-
■ally does flow, we find that the actual flow falls short of the theoretical by at least
a third. In fact, it is only the central port of tlie jet. which approaches the opening
diri ctly, that lias the velocity above stated. Ttie onter particles approach from ajl
aides, with less velocity; they jostle one another, as It were, and thus the flow is re-
tarded. In consequence of this want of uniformity in velocity and direction among
the component layers of the jet, ns they enter the orifice, there tukt^ place what ia
called a ** contraction of the vein " {vena eontraeta) ; that Is, the let, after leaving the
orifice, tapers, and becomes narrower. The greatest contraction is at a distance
from the orifice equal to half its diameter; and tlH>re the section of tho stream is
about two-thirds the area of the opening. It is. In fact, the section of the con-
tracted vein that is to be taken as the real area of the orifice, In calculating by tho
theory the quantity of water discharged. If the wall of the vessel has considerable
thickness, and the orifice Is made to widen gradually inwards, in the proportions of
the contracted vein, tho stream does not suffer contraction, and the area of the
orifice where It is narrowest may be taken as the actual area of discbarge.

Adjutages,— li has as yet been supposed that the issue \b by moans of a simpio
opening or hole in the side or bottom of the vessel: but if the fiow takes piaco
throni;h a short tube, the rate of discharge la remarkably effected. Through a sim-
pie opening. In a thin phite. tho actual discharge Is only alxnit 64 per ci-ut of the
theoretical; through a cylindrical conducting-tube, or adjutage^ us it Is ca]lt>d, of
like diameter, and whose length is four times its diameter, the dit>cliartre Is 84 per
cent. The effect is still greater if the discharge-tube is made conical both ways,
first contracting like the contracted vein, and then widening. 1'he effei't of a con-
ductiug-tnhe in iucreasiug the discharge is accounted for by the adhesion of the
water to iu sides, which widens out the column to a greater area than it would nat-
urally have. It has thus a tendency to form a vacuum In the iul)e, which acts Ilko
suction on the water iu the roi<«rvo]r, and increases the quantity dischai^ged. 'ilio
flow is more free if the orifice is iu the bottom of the vessel, than In the side on a
level with the bottom. If the dischurge-tube is made to project inwards beyond the



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Hfdrogen '^^

tbickncM of the walls of the yeeeel, the velocity is mnch Impeded, owing to the op-
posiug curreuts prodnoed by the water approaching the opetihig.

Pipes.— yfhen a conduit pipe Ib of any con^ldemble leiieth, the walo>iMifie9froin
It at a velocHy Iom thau thnt one to the hend of wntor in the niservolr, owId^ to ilie
rewistauceof friction. With a pipe for inetatice, of 1H Inch in dinmetcr, nndao
feet long, the dii>cha»re i^ only one hnlC what it would be from a simple oiillc' o(
the same diameter. The mte of reduction depen Js upon the diameter of tlie tube,
its leusctb, the ijendinjrs it nndergows &c. The re^lptance to the flow of water tu
pipes oocajiot arise properly from friction, as nuderetood of solids, but from the
adfhesion of the viatcr to the sides of tlie pipe, and from the cohesion of the watery
particles amou^ themselves; it makes little difference, therefore, whether an earth-
enware pipe, for instance, be glazed or not. Large projections fonn an obstacle;
hut mere roughness of surface is flUcd np by an adhering film of water, which h»ss



Sood as a glaze. The resistance increases greatly with the narrowness of tlu* pipes.
Inglnccrs liavo formulas, dc<luccd In great part from experiment, for caknhttlpgthe
disclml^ through pipes of given length ana diameter, and witli a given bead ; but



the subject is too complicated for introduction here. If water flowed in a conduit
pipe wiUiout friction or other olwtrnction, so that its velocity were always equal to
that due to the head of water, there would be no lateral or bursting preasnre on the
walls of the pipe ; aud if the pl^>e were pierced the water woula not squirt out
Accordin^y, with a short tube or adjutage, which, Instead of obstructing, increases
the flow, there Is not ouly no lateral outward pressure on the walls of the tatie, bat.
there is actually a pressure inwards. If a hole is made in the wall of a cjUndriou
adjutasre, aud the one end of a small bent tube is inserted In the hole, while its other
end Is dipped in a vessel of water, the water will be sucked up the tube, shewing the
tendency that the adjutage has to form a vacuum. But when tlie velocity of d^
charge is diminished by the friction of a long pipe, or by any narrowing, bending or
other obstruction in thQpipe, then that portion of the pressure of the head of water
that is not carried off in the dischai^Ee, becomes a bursting pressure ou Uie walls of
the pipe. This pressure is unequal at different parts of the pipe. At U>e end
where the water issues free aud unobstructed, it is next to nothing, aud gradually
increases towards the reservoir, wl»ere it is equal to the difference between ilie bead
of water in the cistern, aud the head duo to the velocity with which tl»e water is actu-
ally flowing in the pipe. Tlie principle now explained accounts for the fact, that
pipes often burst or begin to leak on the motion of the water in them being checked
or stopped.

RetMtaauiB of Water to Bodien moving through tt— This is greatly affected by the
shape of the body, which ought to have all its surfaces ol>li(^ae to the direction of
the motion. When a cylinder terminates In front in a hemisphere, the reaistanGe
is only one-half what it is when the cvlinder terminates in a plane surface at right
angles to the axis; aud if instead of a hemisphere, tlie termination is an equilat-
eral cone, the resistance is only one-fourth. If a globe Is cut in halves, and a cylin-
der, whose length and the diameter of whose I>a8e are each equal to the diameter of
the globe, is fixed between them; this cylinder with hemlsplierkrai ends experiences
less resistance than the giobe alone, the diminution l)cing about oue-flftb of the re-
sistance to the globe. Also the resistance increases in alilgher ratio tlian the sim-
ple one of tlie velocity. One part of the resistance arises f«»m the momentnm that
the body has to give to the water It displaces. Moving at a ccrt^iln rate, it displaces
a certain quantity ; moving at twice that rate, it displaces twice the quantity in the
aamo time. But not only does it displace twice the number of particies of water;
it also has to displace them with twice the velocity : the pressure of the reslelance
is tlins not merely doubled, bat qiiadmpk*d or squared. SImilariy, when the velo-
city is tripled, the resistance aiistng from the simple dlsplaceiifeut of water becomes
nine times as great. Another paii of the resistance of liquids to bodies moving
in them is owing to the cohesion of the particles, which have not to be thrown asi<Ie
merely as separate grains, but to be torn asunder. In addition to this, when the
velocity is considenible, the water becomes heaped up in front, and depressed at
the other end from not having time to close in behind, thus causing an excess at
Iiydrostatic pressure amilnst the direction of the motion. Owing to tlie oombhui'
tion of these causes, the real law of the Increase of resistance Is difficult to lev ~
tigate, and the results of experiments are not a IlUle discordant See Wat



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*7QQ Hfdro

*^^ Hjrdrogea

nYDRO-FLUCRIC ACID. Sec Fluobini.

UY'DROGEN (symbol H, eqniv. 1), so called from the Greek irordB Aj^d9r, \iiiter,
and r^enmlo, to geDerato, is au elementary (>nb9t:iiice, whicb exiHfl lu the form of a
pennan<'iit, coloi'leeflf aiid {uodoroan gas. One of its most iiriklnir pecaliuritiea is
its specific gravity, it beiug the lightest of all known bodies. Assumiug the weight
of a giyeii volnnic of tttmo9|>iicric air to be 1. the weiglit of the same volume of hy-
drogen under similar conditions is 0*0682 ; lience liydrogeu isl4>^ limes lighter than
atmoeplieric air; while, ou the other hand, it is 241*578 times lighter than platiunm,
the heariewt body kitown. Its refmctive power is greater than that of any other
gas, and is more tlian 6 times as great as that of atmospheric air. It is comi)a9tible ;
timt is to say, it is capable of comliiiiing with oxygen, und developing light and lieat.
When a lighti*d taper is passed np Into mi inverted jar of hydrogen, tlie gas bams

anietly with a palfvblae, scarcely visible flame, aitd the taper is extingaished. Tlie
ame only occnrs at the line of junction of tlie hydrogen and the external air. If
tlie liydrogen he mixed witli air or oxygen prior to the application of the taper, tb«
whole mixture Is simultaneously inflamed, and there is a loud exploBion, which is
most violent when 2 volumes of hydrogen are mixed with 1 volnme of oxygen, or
with 6 volumes of atmosplieric air. The hydrogen and oxygen in these cases com-
bine to form watery vapor or nteam, which suddenly expands from the high tem-
perature attendant ou the combustion, hut immediately afterwards becomes con-
dei<c>fd; this condensation caoses a pania) vacuum, into wiiich the snrronnding air
rushes, and by the collision of Its particles, produces the report. At ordinarv tem-
peratures, water dieeohres rather lees tiian 2 per cent, of its volume of liydrogen.



It Is one of the few gjises wiilcii liave never yet been liquefied. Pure hydrogen,
though it cannot snttport life, is not poisonous, and when mixed with a sufficient
quantity of ntmoepheric air or oxygen, may be breathed for some time without in-



Ilydrogeti does not possess very marked chemical properties. The only substances
with wliicii It combines directly at ordinary temporaturtis are chlorine and oxygen.
Hvdrogen nud clilorlue, mixed together, and exposed to direct sunlight, couiuhie
With explosion; in diifuMxi daylight, they gradually unite; but In the dark do not
act on one anotticr. Hydrogen and oxygen do not combine Rpontaneonsly even in
direct sunlight, but reqaire tlie presence of a red-hot solid, of flame, or of spongy
platiuum.

It i? generally staled that hydrogen coee not exist naturally In a pure or nncom-
bIno<1 fittite, but Bunseii recogniseirits presence in variablu proportions in the gases
evolved from ttie enlfatarns of Iceland, and It will probably oe detected iu other
localities where simihir geological relations hold pooa. In combination witli oxy-
gen, as water, it not only forins a very considerable part of the earth, and of the
atmosphere, liut enters lawcly Into the structure of every animal and vegetable
organism. It is an essentialingredietit of many inflammable minerula, such as coal,
an)t>er, and petroleum ; and of certain gases, such as marsh gas, ammonia, and
hydrosulphnric acid (or sulphuretted hydrogen). It likewise enters Into the
composition of a Isrge iiumb<»r of manufartnred substances and products used iu
tho nrti<, medicine, &c., as for instance, sal-ammoniac, starch, sugar, vinegar, alcohol,
oleflant gas, aniline, indigo, morphia, strychnia, hydrocyanic add, &c

There are numerous ways In which hydrogen may be prepared, but the upual
and most convenient process is by tlie action of diluted sulphmlc acid on sine.
About half an ounce of granulated zinc is placed in a retort, and a dilute acid, pre-
pared by gradually mixing an ounce of oil of vitrol with eix ounces of cold water,
Is (^K)nred on the sine. liydrogen gas is rapidly evolved in great abundanc«>, Imt the
fiifit portions should not be collected, since they are mixed witli the atmosplieric air
which was contained hi the retort. Tlie rest of the gas may be collected in the or-
dinary way over water. Iu tills process the xinc takes oxygen from tlio water, and
forms oxide of zinc, which combines with tiie sulphuric acid, forming sulphate of
zinc, which remains in solution, while the hydrogen of the decomposed water cs-
dipes. The reaction Is rhewn in the formula, Zii + H0,S08 = Zn6,SOa -^ H. A

Secisely similar reaction eueoea if we use iron iu phice of zinc, bat iu tlds case
e gas IS generally lesa pure.



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51'5S.y 800

Uydrogen m, under the name of combnstibl^ air, was obtained !n the l€th c
t»y Furaoelsns Dj treating certain metals with dilute acids, and was more or less
kiiowu to Bojie and olliera ; bnt Cavendish, In bis paper on ** Paetitioiis Alrs,''pQb-
lislied iu the ^ Trauaactions o( the Kojal Society " for 1766, was the tiret to describe
ncciirately the propertlea of thia gas, and the methods of obtaining it ; iience he ia
a:»niillT mentioned aa its discoverer.

UYDROOK^, Binozide of (symb. H0„ eqniv. 17), is a colorless liquid of asympy
consistence, with a specific gravity of 1*45 (water being 1), and a peculiar odor,
bonietblng lllte that Off very dilute cblorlae. It bleaches vt^elable coUxs, and when
apiriled to the tongue or tlie skin, produces a white spot, and excites considemble
num. From tlie readiness with which it gives off its ozygini, it is a powerfol oxidia-
ittg agent. The method of preparing it is complicated aud dtfBcnlt. This substnuce
was cUscoversd in 1818 by Tbeunrd, who termed it oxidised water. Dr B. W. Rich-
ardson, an eminent London phvsiciau, lias lately examined its value (in eolu-
tlou) as a ther^wvtic ageut, and has found it to be of extreme use In hooping-
cough, and in certain forms of rlieomatism, and (as a paUi«tkve) in the lust atagesof
eousamptioo.

UYDIUyORAPHY (Qr. Aj^dr, water, gropk-, to write) is a description of the
surface waters o< the earth, particulariy of the bearings of coasts, of currenra, soand-
ings, islands, shoals, Jte., and of anythhig the knowledge of wliicli may be osefol
for purposes of navigation. It, conaeqnently, incUdes the constraction of charts,
maiis, ^, iu which ttiese particulars are detailed. It is, in fact, to the sea what geo-
graphy is to the huKL The first steo in the erection of hydrograpliy into a scieuco,
was made iu the 16th c by llenry the Navigator, who vras the flivt to construct a
sea-cliart worthy of the name. Among the maritiine iMiioua of Snrope, It ia now
made a matter of prime concern ; the hydrographicoflioe being an imponaut branch
of the naval admiulstnition. The head of the hydrographic department in the British
service is usually a captain iu the royal navy. The offlcen surveying in dif&srrut
ports of the world send their observations, soundings, ^tc; and It is tlie bust-
nese of the hydrographer to consolidate these into available maps. The hydro-
grapher reoeives £800 u year iu addition to his half-pay. In proof of the valoe
attached to these Admiralty charts among the marine of England and even of for^
eign nations, it may be mentioned that many thousand charts, besides books of soSh
lug directions, are sokl annually.

HY'DROMANCY. See Divination.

HYDROMA'NIA. See Pbllaora and SinoiDB.

HYDKCyMRTBR. See ARMUUsmL

UY'DROM YS, a geaos of rodent quadrupeds, of the family JTwridiB, of whtch
there are only two known species, very similar to one another, natives of Van Die-
men's Land. They liave two incisors and four molars in each jaw. They are called
BBAvmB Rats in van Diemen^ Land ; are nocturnal and very sliy ; inhabit tJie banks
both of frvsh and salt water, aud swim welL The largest species is twricc tiie siae q$
a common rat One of them has the belly white, ttie other yellow. ^

HYDRO'PATHY, or Hygiu'nic Medicine, popularly tenned the Watkb Cvmx.
Under the head of Baths and Bathing (q. v.), an account has been given of the bath
in general, as a means of preserving liealih. We have here to speak of water in its
manifold uses as an engine in the cure of disease, and as fomUng a principal element
iu that combination of hygieule appliances which goes to make up hydropathy as
at present practised. (Iu accordance with the plan followed in other cases of th^
kind, the view exhibited ia that rf an adherent of the system.)

Tlie efBcacy of water, in the ct£ie of numerous forms of disease, has looff been
recognised. Water was largely eoiplOTed by Hippocrates, the •* Father of Medidne,'
muru than 9300 vears ago, in the t/^tment of mau^ kinds of disease; and along
with a regulated diet, aud an implicit belief in the vt// rMdioatnat nmhtrm. it appears
to have formed the chief element in ids medical armory. Horace haa
enshrined the memonr of Antouins Mosa, the hydropatliic physician of the
Bmperor Augustus (Bpist 1. Ifi). Both CgIkus and ChUen— who flourished,
the one about SO years b. c., and the otiier In the Sd c—- speak favorably
iu their writings of the use of water in the cure of disease, regarding it as
of liiuli value ill the treatment of acute complaints, particularly of fevers. Through-
out the Middle Ages, Ukewise, many physicians of name, including Actios aud



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m Hydrogen

Hydropathy

Paalas ^gluetn, and tho more celebrated Pamcelsns. were adTOcaton of the remedial
virtnes of water; all of them, however, haviDg faith fu its neiea in the treatment
rutber of ncnte timn of chronic disorders. lu 1783, Nlcolo LanzanI, a Neapolitan
physician, pnblii(lie<l a learned treatise on the subject. In oar own cooniry, nbunt
the beginnint; of the ISlli c. Sir John Floyer nnd Dr Bayn:u'd made a large Q»e of
water. Tlieir conjoint work, denomiuatvd " Psychrolooeia,'' or the ••Hif'tory of
Cold Bathing, botii Ancient and Modern,'' is rejUete with quaint leitmini; and pnic-
tlcul shrewdness and siigacity. But the most able and scientific among the older
treatises that have appeared in Buglaud on tno snbject of the water treatment, is the
work of the well-known Dr Cnme (q. v.), the biographer of the poet Bnms, pub-
lished in 1797, and entitled *^ Medical Reports on the Effects of Water, Cold and
Warm, &c" In this work, Dr Cnrrie recommends the cold nffo^ion in typhns and
other fevers, and gives pracilail directions in regard to the cases and the times when
it may he nsed with advantage. EmiueDtphysichinsof the present day have admitted
that thcee views, so far as tluey went, were as scientific in pnnciple as they were novel
in their application; but the practice founded on them was considered too danger-
ous by Curric's contemporaries, and fell into si)eedy neglect. It is worthy of remark,
that Currie appears to nave limited Ids a»e of water to acute ailments exclusively.

We Inive thus seen that up to the beginning of the present ce^Jtury, by eome of
those who employed it as a curative agent, water was used in tlie trcotmeni of acute,
and by others of chronic diseases; by some as an 1iit«nial Otfent nione, by
others as an external application in the various forms of the bath, out nevi*r in all
the manners combined. This combination wa« fin>t effected by the original genius
of Vincent Prlessuitz, a Silesiau farmer, with whom t>egau an new era for the water-
cure. It was owing. we are told, to his successful treatment of more than one bodily
injury which he hud sustained in bis own pennon that, about the year 1S80, Priess*
nilK became so fortified in his convictions as to the curative powers of the water as
to devote liimself to employ it medically in tl»e cure of others. Beginning with the '
external application of water for trifling diseases among the poor of his neighbor-
hood, he gradually undertook an extended range of cases, and multiplied the modes
of administration, introducing tlie wet compress, the douche bath, panisi baths of all
kinds, the sweating process, the wet sheet, together with copious drinking of pure
water. In addition to water in all these forms, he insisted on the value of exercise,
diet, fresh air. and mental repose, in the cure of disease ; thus practically calling to
his aid tho entire resources of hygiene, and establishing by a simple, yetthoroGghly
original combination, nothing less than a new system of medical treatment. Ab to
tile success which attended Priessuilz's practice, it is a historical fact that of 7600
patients, who had gone to Grftfenberg for advice and treatment, up to the year 1841,
Qr witttin tlie spaci of about 20 years, there had Iteen only 39 deaths, and some of
these, according to the r^istry of the Austrian police, **had died before commenc-
ing the treatment, wliile some others were reported in a forlorn state before any-
thing was attempted." It is to bt) regrett(*d^ however, that the founder of the new
system was not himself an educated physician, so that he could baTc understood
better the philosophy of his own practice, and expUiiued it more correctly. He
would not have callecl his system the " Water-cure," a name scientifically one-sided
and incomplete, and therefore misleading. It is canally to bo regretted that many
of tlie immediate followers of Priessuita, while desUtute of his remarkable sagacity
and genius, should have been no better furnished than himself with a scientific
knowledge of disease and general professional culture.

In spite of all drawbacks, however, the undoubted merits of hydropathy at length
called to its defence many men of standing in the profession, who, allowing for some
of its early extravagances, stepped forward to explain it scientifically, and pressed
it on the acceptance of their brethren ; nnd from their advocacy bus sprang up in
Bugiand a scliool of hydropathic pliysiciaus, tho philosophy of whose plan of treat-
ment we shall now briefly describe. ^

Physiology teaches us that the various organs of our bodies cannot be kepFin a
healthy state without the observance of certain regulations called the fniinary
** Laws of Bealth." When these are broken, the result to the offender is disease in
one of its many forms. Until the appearance of hydropathy, physicians attorontt^
to correct tlie evil tlius caused— and the great majority do so siill — ^by the adminis-
tration of one or other of the drugs which go to form the medical repertory known



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Hydropithj 802

aft the pbarmacopoein , and the argnment on which thin practice has been iMued fs
the very nlmple one, that experience ha« provtHl the medicine or medicines to bo
efficacious lu a large proportion of eiroiiar cases. HydropatliT proceeds According
to a very different method. Taking as his ctmtral mnxim the princlnle flr!»t pro-
ponndcd 1)y UippocrntCA, that it is nature*s own strivings after health {.vxtmedieatriM
naturo!) that renlly euro the patient when he is cured, the fnnrtion of art bfing
mainly to remove obstacles, the liydro|>athic pliysicinn avoids nsing nil mcmns with



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