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when it comes in connection with an article commencing with | Italian nouns: I. with and without an article; II., with some 1, it is optional to contract it; it being equally correct to say important words frequently preceding them. These tables are con lo' or col-lo scét-tro, with the sceptre; col or con l' in-gán-no, so important that they must be committed to memory. But let with the deceit; con la or col-la si-gno-ra, with the lady ; con le me first remark, that it will be sufficient for our present purpose or col-le brác-cia, with the arms.
to lay down this fundamental rule with regard to the formation Once for all, being obliged for the greatest part to divide the of the plural of Italian nouns:syllables as they are divided in Italian spelling, I must em AU Italian nouns, masculine and feminine, change their final phatically warn the reader not to read the combination ce (when vowel into i in the plural ; as, il pó-dre, the father; i på-dri
, not followed by h) as though the first c was a k (the Englishman the fathers ; el po-é-ta, the poet; i po-e-ti, the poets; il cér-vo, would naturally do so), but to read the whole combination as the stag; i cêr-vi, the stags; la má-dre, the mother; le má-dri though it was ttch, gliding with great rapidity from one syllable the mothers; la má-no, the hand ; le má-ni, the hands. to the other. I must refer, on this point, to my remarks and The most important exceptions from this rule are feminine tables on pronunciation.
nouns terminating in A, which form their plural by changing a Two important prepositions, tra and fra, between, among, can into E; as, la so-rel-la, the sister; le so-rel-le, the sisters. likewise be contracted with the article, but in a special way, and with modifications which must be stated separately.
1.--NOUNS DECLINED WITH AND WITHOUT AN ARTICLE. If tra and fra are to be contracted with an article com.
li-bro mencing with 1, the letter I must be doubled, U; as, for example,
il U-bro, the book.
Gen. di l-bro frál-le mon-ta-gne, between the mountains; trál-le due so-rei-le,
del l-bro, of the book. l-bro
al l-bro, to the book. between the two sisters; frál-lo scri-gno e la sê-lia, between the
il b-bro, the book. chest and the chair.
Abl. da U-bro
dal U-bro, from the book. Whenever tra or fra is to be joined to the article i, the latter
nel 1-bro, in the book, is omitted, and an apostrophe placed in its stead. For example:
col ll-bro, with the book. fra' cu-gi-ni, between the cousins; tra' fra-têl-li, between the
pel U-bro, for the book, brothers.
sul l-bro, on the book. The words tra and fra are never contracted with the article
Plural. gli. For example: fra gli a-mi-ci, between the friends; tra gť
Nom. I-bri i K-bri, the books. in-fe-li-ci, between the unfortunate.
Gen. di U-bri
dei (de') li-bri,t of the books. When tra or fra stands before il, the letter i of the article is
ai (a') i-bri, to the books. U-bri
i l-bri, the books. commonly not heard in pronunciation, and in writing the
Abl. da 14-bri dai (da') U-bri, from the books. apostrophe is used in its place. For example: fra 'l són-no,
noi (ne') U-bri, in the books, during the sleep; tra 'l si e'l no, between yes and no, hesi
coi (co') U-bri, with the books. tating.
poi (pe') 6-bri, for the books, The so-called indefinite article uno, masculine, and una,
sui (su') Il-bri, on the books. feminine, will be hereafter explained.
Singular. In Italian, as in English, the nouns have no terminational
schióp-po lo schióp-po, the gun. alteration in either number; that is to say, all cases are alike.
di schióp-po de-lo schióp-po, of the gun. Strictly speaking, therefore, they cannot be said to have any
schióp-po al-lo schióp-po, to the gun. declensions. All changes in Italian nouns denote only a clif
schióp-po lo schióp-po, the gun. ference in gender or in number. For example: pás-se-ro, sparrow,
Abl. da schióp-po dal-lo schióp-po, from the gun,
in not only denotes the object sparrow, but also that it is a male;
schióp-po nel-lo schióp-po, in the gun.
con schióp-po col-lo schiop-po, with the gun. and pás-se-re (female), sparrows, not only denotes the feminine,
per schióp-po per lo schióp-po, for the gun. but the plurality of number. The article in Italian, as in
schióp-po súl-lo schióp-po, on the gun. French, Spanish, and English, does not in itself denote the case,
Plural. but is a word that distinguishes one noun as a determined
Nom. schióp-pi gli schióp-pi, the guns. object from another noun of the same class. It is on this Gen.
schióp-pi dé-gli schióp-pi, of the guns. account a fixed principle of the language never to place the Dat.
schióp-pi á-gli schióp-pi, to the guns. article before a noun, when the latter is used in its general and Aco.
schidp-pi gli schiốp-pi, the guns. indeterminate signification. The articles il, lo, and la, are in Abl. .
da schióp-pi dá-gli schióp-pi, from the guno. themselves as indeclinable as the neun itself. They only change
in schióp-pi né-gli schióp-pi, in the guns. according to the gender and number of the noun; and when the
con schióp-pi có-gli schióp-pi, with the guns. Italians desire to denote cases, they must, on this account, like
per schióp-pi pé-gli schióp-pi, for the guas. the English, place before the articles certain words, which are
schióp-pi sú-gli schióp-pi, on the guns. the substitutes of those inflections by which, in the Greek,
Nom. Latin, and German languages, the cases are expressed. The
a-nel-lo l'a-nél-lo, the ring.
a-nel-lo English have only two such signs of cases-the words of and
dell' a-nel-lo, of the ring.
Dat. ad a-nel-lof all' a-nel-lo, to the ring. to. The Italians have three: di, for the second case, or genitive; a, for the third case, or dative; and da, for the sixth case,
Instead of the plurals i, dei, ai, dai, some old writers used the or ablative. These three words, di, a, and da, are used in the plurals li, delli, alli, dalli; but this is no longer usual. singular as well as in the plural, before masculine nouns as well + The plurals dei, ai, dai, nei, coi, pei, sui, are frequently marked as feminine. In the first case, or nominative, and in the fourth with the apostrophe for the sake of harmony, thus: de', a', da', tad case, or accusative, the Italian noun has, as well as the English, co', pe', su'; especially when coming before several words all of which no case sign before it, and both these cases are sufficiently dis- terminate in i. For example, a ca-gió-ne do mól-ti sub-i peo-cá-ti, en tinguishable by the place which they take before or after the account of his many sins. verb, for which reason they require no special distinguishing and pronunciation of Italian words, frequently requires that to the
# Harmony, which has had so much influence on the formation mark. For example :A-les-sán-dro vin-se Dá-rio, Alexander conquered Darius ; added; as, ad o-no-re, to honour; ad a-ml-co, to the friend; for e
case-sign a, when it comes before a vowel, the letter d should be Cár-lo per-cuô-te il cá-ne, Charles strikes the dog ; il prín-ci-pe onors, and a amico. á-ma la các-cia, the prince likes the chase ; Pie-tro lêg-ge le The lawe of harmony, likewise, frequently require the mark of the gas-zét-te, Peter reads the newspapers.
apostrophe on the case-sign di, when it comes before words com. I must once for all, and emphatically, warn the reader, mencing with a vowel; as, cá-po d' 6-pe-ra, masterpiece; sé-gno di because I am obliged, in the case of the double g (99), to place u-mil-td, sign of humility: the first g in one syllable, and the second g in the next, not to
The case-sign da, on the other hand, is never marked with the read (when the gg is not followed
by h) the first
g like g in the apostrophe, but always written in full, in order to avoid the inevitable English word get, to which mistake many readers will be ambiguity
, of confounding the case-sign di with it whenever it is naturally liable ; but I must refer, with regard to the pronun- case coming together must be tolerated ; because, as I have already ciation of the g (og), to the lessons on pronunciation.
remarked, perspicuity is a more urgent law than harmony in these I shall now subjoin two tables illustrating the declensions of contractions.
per l' a-nél-lo, for the ring, As a useful art, as well as a popular pastime, rowing deserves a su a-nel-lo sull' a-nél-lo, on the ring. very high place among physical exercises. To be able to perPlural.
form an efficient part in the management of a boat may Nom. e-nalgli a-ne-li, the rings.
frequently be of the greatest service; and even if no such Gen. di a-nél-li de-gli a-nél-li, of the rings.
opportunity should ever arise, the time is well spent that gives Dat. Q-nél-li d-gli a-nél-li, to the rings.
power to enjoy so cheerful and healthy a recreation. Acc. a-nelli gli a-nel-li, the rings,
Before teaching the learner how to row, we must describe the Abl. da a-ne-li dá-gli a-nél-li, from the rings. instruments of rowing. These are, of course, the boat, and the in G-nelli né-gli a-nél-li, in the rings.
oars or sculls by which it is propelled. A scull is much the con anelli có-gli a-nel-li, with the rings. same thing as an oar, the difference being that it is shorter and per a-nelli pé-gli a-nél-li, for the rings.
lighter, and therefore more manageable in the hands of the a-nelli sú-gli a-nél-li, on the rings.
rower. When he takes an implement in each hand, he uses a Singular.
pair of sculls; but when he rows with one implement only, Nom. cá-sa la ca-sa, the house.
some one else taking one on the opposite side, he uses an oar. Gen. di cd-sQ del-la ca-sa, of the house.
The description of one will apply equally to the other. Dat. cá-sa ál-la cá-sa, to the house.
The oar consists of three principal parts—the handle, the Acc. cá-sa
la cá-sa, the house. Abl. da cá-sa dál-la cá-sa, from the house.
shank, and the blade. The blade is the wider portion, which is in cá-sa nél-la ca-sa, in the house.
dipped into the water; the shank is the middle part, divided con cá-sa
cól-la ca-sa, with the house. from the handle by a small piece of leather, which is fixed to the per cá-sa per la cá-sa, for the house.
undermost side, and known as the button. The handle is again 84 cá-sa súl-la cá-sa, on the house.
divided technically into two portions—the extremity, which is Plural.
grasped by the hands, and the part between the grasp and the Nom. cả-se le cá-se, the houses.
button, which is called the loom. Gon. di co-80 da-lo cá-se, of the houses.
Boats are of several kinds, according to the variety of purDat. cá-se ál-le cá-se, to the houses.
poses to which they are devoted. The different parts of all Acc. cá-se le ca-so, the houses.
boats, however, go by the same names, so far as they correspond Abl. cá-se
dál-le cá-se, from the houses. with each other. For convenience of description of these parts, in cá-sa nél-le cd-se, in the houses.
we will take as the representative of its kind the boat depicted Con cá.se col-le cd-se, with the houses.
in the illustration. per ca-s8
per le cá-se, for the houses. ซีน cd-se
Here again we have three principal parts—the bows, the midsúl-le cá-se, on the houses.
ships, and the stern. The bows are the narrowest and foremost Singular.
portion, just behind the stem or cutwater, the position of which Nom. ár-te Pár-te, the art.
is sufficiently designated by the name. The stern is the hindGen. ár-te
dell' ár-te, of the art. Dat.
most portion of the boat, and the midships are the intervening ár-te
all' ár-te, to the art.
space between the stern and the bows. Immediately under the Abl. da ár-to dall' ar-te, from the art.
stern is the rudder by which the boat is guided, and the part of
the boat to which it is affixed is called the stern post.
The boards of which the sides of the boat are composed are
known as the strakes, and the lower strakes are called garboards. su ár-te sull' ár-te, on the art.
Immediately under the garboards, and projecting from the Plural.
bottom of the boat, is the keel. The pieces of wood which cross Nom. le ár-ti, the arts.
the boat between the strakes are known as the timbers. Those Gen. ár-ti da-lo ár-ti, of the arts.
immediately at the bottom of the boat, in the midships, are the Dat, ad al-le dr-ti, to the arts.
bottom-boards; those in the bow are the bow-sheets; and the Acc. dr-ti ls ár-ti, the arts.
planks in the stern are the stern-sheets. Abl. da ár-ti dal-le ár-ti, from the arts.
Now as to the various portions of the midships, in which the in dr-ti nel-le ár-te, in the arts,
rowers sit and perform their office. The seats themselves are con er ti col-le ár-ti, with the arts.
called the thwarts, and the wooden fastenings by which they are per ár-ti per le ár-ti, for the arts.
fixed to the sides of the boat are the knees. The thwarts in su ár-ti súl-le ár-ti, on the arts.
river boats are usually provided with small mats for the conSingular.
venience of the rower, which are tied underneath the seat to Løn-dra, London. Nom, Al-bér-to, Albert. Gon, đi Lớn-dra, of London.
prevent them from slipping. In front of each rower, at the Gen. di Al-bér-to, of Albert. Dat.
bottom of the boat, is a movable plank, which is fixed in a Lán-dra, to London.
Dat. ad Al-bar-to, to Albert.
vertical or slanting position, so as to form a resting-place for Abl. da Lón-dra, frem London.
da Al-bêr-to, from Albert.
his feet. On the upper side or wale of the boat, in front of it Lón-dra, in London,
Al-bér-to, in Albert.
each rower, are projecting pieces of wood called the rowlocks, con Lón-dra, with London
con Al-bêr-to, with Albert. the oars or sculls being passed between them. The piece PỘT LÁT-ara, for London.
per Al-bêr-to, for Albert, nearest to the rower, when his oar is placed in the water, is Singular.
known as the thole, and the other as the after-thole, or stopper. Vit-tó-ria, Victoria. Nom. Gib-ue, Jupiter.
The two together keep the oar in its place on the side of the Gen. di Vit-td-ria, of Victoria. Gen. Gio-ve, of Jupiter.
boat, and form a fulcrum for the leverage by which the boat Vit-td-ria, to Victoria.
is propelled. Between the two pieces of wood which form the Vil-lô-ria, Victoria. Acc. Gio-te, Jupiter. rowlock, a piece of leather, called the filling, is inserted. This Abl. da Vit-tó-ria, from Victoria. Abl. da Gib-ve, from Jupiter. gives a resting-place for the oar, the button of which is placed in Vit-tó-ria, in Victoria.
in Gib-ve, in Jupiter. against the thole when in position. con Vit-tó-ria, with Victoria. con Gio-ve, with Jupiter.
The coæswain, or man who steers the boat, when such aid is per Vit-tó-ria, for Victoria. per Gib-ve, for Jupiter.
required, is seated in the stern-sheets, with his face towards the Dí-o, God.
Abl. da Di-o, from God. Gen. di Dio, of God.
rowers. He guides the rudder by means of the yoke-lines, or
in Di-o, in God. DC-o, to God.
cords which pass one on each side of the stern from the yoke,
con DC-o, with God. Dlo, God.
which is a piece of wood crossing the top of the rudder. The per Di-o, for God.
rope at the bows, used to fasten the boat, is the painter. It is obvious that proper names of gods, persons, towns, and Such are the various parts and appliances of a boat, although other localities, require no article in the singular, because their every kind of boat does not possess them all, like the boat in individual signification renders any other more precise deter- our illustration. But it will be necessary for the reader to bear mination or distinction by means of the article superfluous. these names in mind, or to become familiar with them by
Dat. a Aee.
Dat. C Ace.
frequent reference, as when he commences rowing he will con. | difference that they are out-rigged with iron rowlocks. In this stantly hear them referred to.
form they are usually called out-rigged gigs or wager-boats. We have next to mention the principal varieties of boat, as In boats larger than pair-oar, the rower who is seated nearest found upon our rivers. These are chiefly wherries and skiffs, the bow is called either“ bow" or "No. 1;" the next to him gigs and out-riggers. The two former are old-fashioned boats, is “No. 2,” and so on according to the number of rowers. The used by watermen for the carriage of passengers and goods, and oarsman nearest the stern of the boat is known by the name do not require detailed mention. Gigs and out-riggers are the of stroke, as it is by his stroke, or movement of the car vessels chiefly
through the used by ama
water, that teurs. One
the others reform of the
gulate theirs. gig is seen in
5 He should the centre il
therefore be lustration; the 3
the most skilchief charac
ful oarsman of teristics of all
the party, able gigs being the THE Boat.–1, the stem ; 2, the bows; 3, the midships; 4, the stern; 5, the rudder; 6, the rowlocks; not only to straight wale 7, the thwarts, or seats; 8, the stern-sheets; 9, the bow-sheets.
TOW steadily, and the almost 3
but to time upright stern.
his movements In appearance
the the gig is a THE CAR.-1, the handle ; 2, the loom; 3, the button; 4, the shank ; 5, the blade.
greatest unimuch smarter
formity. vessel than the wherry, and it is gradually superseding the latter, The duty of steering, or guiding the head of the boat, is even for watermen's uses, on our chief rivers.
usually assigned to a coxswain, who faces the stroke, and, as Gigs are of all sizes, from pair-oared to eight-oared boats, the before mentioned, holds a yoke-line in each hand, by which to number of seats for the rowers in the boat giving it its particular influence the rudder. He is also useful, in practice especially, designation. Thus a boat with two seats is called a pair-oared in calling the attention of either of the men to any irregularity boat, each of the rowers taking one
in his stroke, by which the geneoar; but the wale has four rowlocks,
ral uniformity is affected. In his in pairs opposite each other, so that
absence the oarsmen must guide the rowers may, if they prefer it,
their own course by an occasional take a pair of sculls each instead of
glance over the shoulder, and a A gig with three seats is
stronger stroke, or perhaps a cessacommonly termed a randan, and is a
tion of the pull, on the one side very useful boat for a learner to
or the other, as the position may repractise in. The first and third of
quire. the rowers use each a single oar,
When taking his seat in a boat, while the occupant of the centre
the learner should first observe seat handles a pair of sculls; and
that the thwart is firmly fixed, the learner who takes his place in a
and that the mat upon it is se randan boat has thus an opportunity of becoming familiar with curely tied to that part of it which is farthest from his rowthe use of both the oar and the sculls.
lock. He has next to adjust the position of the stretcher Out-rigged boats are used chiefly for racing purposes. They according to the length of his legs, so that when seated his feet owe the name to the fact of the rowlocks being supported on an rest firmly against it, and give a purchase to assist the free iron framework which is rigged outside the boat. This altera- movement of the loins. The heels should be together, and the tion enables
toes parted; the boat to be
the knees bent, both narrower
and about a and lighter in
foot from each its construc
other. The tion than in the ordinary
be straight, method, and at
and the whole the same time
position easy, gives more le
but upright verage in pro
The oar should pelling it. The out-rigger pro
with the han per, as used by
dle in both a single sculler
hands, and for racing purposes, is de
rowlock with picted in our
the button illustration. It
against and in will be ob
side the thole. served that
The outside the dimen
hand--that is, sions of the
that farthest central part of the boat, called the “box,” are merely sufficient from the rowlock—should grasp the handle nearly, but not quite, to afford the sculler a seat; the remainder of the upper surface at the
extremity; the inside hand taking its hold two or three is covered with canvas, and the stem and stern are tapered off inches away from the other. The thumb of the outside hand to mere points. Thus the vessel altogether is calculated to cleave may be either above or below, but that of the inside may be the water with great rapidity. The length of such a boat as under the handle, and the entire grasp of the car should be firm this is usually about thirty feet, and its width amidships little without tightness. more than a foot.
We must reserve for another paper onr instructions how to Racing boats, accommodating from two to eight oars, are now proceed, and the mode of practice to secure efficiency as an 2ommonly constructed somewhat in gig fashion, but with the oarsman.
A RANDAN GIG.
now be taken
placed in the
any extent, the elastic force, however, increasing greatly. If we
now remove the pressure the air will expand again, so as to fill HEIGHT OF ATMOSPHERE—ELASTICITY OF AIR—BOYLE AND exactly the same space as it did before, and the piston will stand MARIOTTE'S LAW-MANOMETER — AIR-GUN-AIR-THERMO
just where it did at first. METER--EXPANSION OF GASES BY HEAT-CO-EFFICIENT OF
A familiar example of this compressibility of the air is seen if
we invert a tumbler over a piece of cork floating on the surface We have now seen the most important effects of the weight and of water. The water will rise a little way inside the tumbler, consequent pressure of the air, and also the means of measuring just as it does in a diving-bell, unless a fresh supply of air is this pressure and its variations. The question, what is the true introduced from above by means of the force-pamp. height of the atmosphere, is one which we cannot fully solve. Though the apparatus just described shows very clearly the Each sncceeding layer of air which we meet in ascending is less fact of the compressibility of the air, and the consequent increase dense than the one below it, and thus, theoretically, the limit to of its elastic pressure, yet it does not afford a very ready way of the height of the air is when the repulsive force exerted by its measuring exactly the alteration of volume, and a different plan particles on each other is exactly balanced by the earth's attrac-has therefore been adopted by experimentalists on this point. tion for those particles. We cannot calculate exactly at what the laws of this compression were studied at the same time in height this would occur, but the question is of no great practical France by M. Mariotte, and in England by Mr. Boyle,
and they importance, and we may safely say that the limit of the atmo- both arrived at the same result, which is known as Boyle and sphere is about fifty miles above the sur
Mariotte's law, and may be stated as folface of the earth. A few scientific men
lows:have, as a result of observations on the
The temperature remaining the same, refractive power of the air as shown in its
the volume of a given quantity of gas cansing twilight, stated the limit to be
varies inversely as the pressure which it much above this; but if any air does exist
bears. at a greater height, it is in a state of such
Fig. 15 represents the apparatus which rarity that it would scarcely be possible
is usually employed in the proof of this, to prove its presence even by the most
and which is known as Mariotte's tube. A delicate tests.
long glass tube is sealed at one end, and We now pass on to notice the compres
bent round not far from this end, as sibility and elasticity of the air. As we
shown in the figure. It is then fixed to al saw in our first lesson, if any gas be con
board, and a scale divided into inches and fined in a vessel, it exerts a pressure
decimals of an inchis placed against each against the sides altogether apart from
limb. Both these scales commence at its weight, and this pressure is exerted
exactly the same level, and a little meragainst the upper part of the vessel as
cury is first poured into the tube, so as to well as upon the lower side. Now, this
Fig. 19. fill the bend and stand in each tube even pressure arises from the elastic force of
with the lower end of the scale. This may the gas, and depends alone upon its com
with a little practice be easily accompression and temperature. If the volume
plished. The air in the shorter leg is now occupied by it be in any way diminished,
exposed to the pressure of the atmosphere, that is, if the same quantity of gas be
which, for simplicity, we will suppose to made to occupy a smaller space, the pres
be just equal to that of a column of mersure will be increased; or if the space oc
cury 30 inches high. Now pour mercury cupied remain the same, and the tempera
into the longer limb till it stands 30 inches ture be raised, the pressure will also be
higher than in the other limb; the air in increased.
this will then be compressed with double We have, then, to investigate the propor
the force that it was before, and, by notion which this increase of pressure bears
ticing the height on the scale, we shall to the diminution of volume. That air is
find that it now occupies 10 divisions compressible to a very great extent, and
instead of 20-that is, under double the that the pressure it exerts increases with
pressure it occupies half the space. Let the compression, is readily seen. Procure
a further quantity of mercury be now a stout glass or metal tube A, (Fig. 14),
poured into the tube, till it stands in with a stopcock, B, let into it near the lower
the long limb 60 inches above the height end, and a piston, c, fitting it air-tight.
in the other, and again notice the space When the tap, B, is open, we can place
occupied by the confined air; we shall the piston at any part of the tube, and
now find it to be 6j divisions, showing the pressure on each side of it will be the same. If now we that with three times the pressure it occupies only one-third close the stopcock, the air within will be cut off from all com- the space. Experiments have been carried on in this way munication with the external air, and therefore
no pressure will till a pressure of nearly 30 atmospheres has been obtained, and be communicated to the under side of the piston from without; this law is found to hold true with most gases. With some, and yet it does not
fall, though pressed upon by the air with a however-as, for instance, carbonic acid—it is found that when force of 15lbs. per square inch. The reason is that the elastic very high pressures are attained they suffer rather more
diminuforce of the confined air is sufficient to balance this pressure, and tion in volume than this law points ont, but the cause of this is therefore the piston remains at rest. Were we to place the whole believed to be that they are then approaching the degree of conunder the receiver of an air-pump, and thus diminish the in- densation at which
they assume the liquid state. In fact, ternal pressure, we should find that the elastic force of the carbonic acid has been liquefied at a pressure of about 50 atmoair within would overcome the diminished pressure, and cause spheres. In conducting these experiments, allowance must be the piston to rise. If now we increase the pressure, either by made for the increase of temperature in the gas caused by the adding weights to the top of the piston or by pressing with the condensation. It should, therefore, be allowed to cool to its hand, we shall find an increasing resistance to our efforts. This original temperature before the measurement is taken. We may arises from the increased tension of the air, and, if a weight
has state, then, generally that the
volume of a gas varies inversely as been placed on, we shall find that after sinking a little way the the pressure. The importance of knowing this is great, for, as piston will come to rest, showing that the elastic force of the en- the pressure of the air is continually changing, a given weight of closed air is then equal to the pressure of 15lbs. per inch, and to any gas will occupy more space at one time than at another. the added weight in addition. Let another similar weight be now Hence, in all experiments with gases, the pressure as shown by a added to the piston, and it will sink still lower, though it will barometer has to be noted. As we have already stated, 30 not move as much as it did before. In this way we shall find inches is taken as the standard height; if, then, we have an
more weights we can compress the air to almost amount of any gas which occupies, say, 230 cubic inches when VOL. IV.
that by adding
the ḥarometer stands at 28-90, we must find how much space it in the construction of a fountain. A strong metal vessel is would occupy when the pressure is increased to 30 inches, and constructed, and a tube, dipping nearly to the bottom, is fitted this we can easily do by the following equation :
tightly to its mouth. A stopcock is inserted in this tube, and As 30 : 28-90 :: 230 : 221-56 cubic inches.
a screw is also cut in the upper end of it. The vessel is then
filled to about three-fourths of its height with water, and, by This, then, is the space which the gas would occupy when the means of a condensing syringe screwed on to the pipe, the air barometer stands at 30.
within is powerfully compressed, the fresh air bubbling up When we speak of a pressure of so many atmospheres, it through the water. The tap is then closed and the syringo remust be remembered that by an atmosphere is meant a pressure moved, and when it is desired to start the fountain we have only of 30 inches of mercury; and as a cubic inch of mercury weighs to screw a jet on the tube, and on turning the tap the tension of 0:491lb., the pressure on a square inch is 30 x 0-4911b. the air will be such as to force the water through the jet with 14.73lbs. We can thus easily solve questions like the following: sufficient velocity to raise it to a considerable height.
What is the pressure on a portion of the surface of a boiler The air, it must be remembered, does not create any force; it measuring 3 inches each way, when the steam has a tension of merely stores up the force exerted by the hand in working the 4, atmospheres ?
syringe. It is, in fact, a reservoir of power, and in many inThe pressure on each square inch is equal to the weight of stances it becomes of great service by its action in this way. 41 x 30, or 135 inches of mercury, and the surface has an area In this case the air was condensed, and its elastic force of 9 square inches; the total pressure, therefore, amounts to thereby greatly increased; at ordinary pressures, however, it has 135 X 9 X 0:4911b. = 596.565lbs.
quite enough elastic force to act in the same way if we allow In many operations it is important to have some means of the jet to play into a vacuum. There are two modes of showing measuring the prossure exerted by a gas or vapour, and the this experiment. instruments employed for this eni, are called manometers. In If a small vessel, similar to that described above, be placed nearly all of these the pressure of the atmosphere is taken as under a tall receiver, and the air rapidly removed, the effect will the unit. The pressure-gauge of a steam-engine is merely one be seen. The simplest plan of making the vessel is to take form of this instrument.
a small flask with a tightly-fitting cork, through which is passed The most common manometer is that which acts by means of a glass tube, drawn to a jet at the upper end, and reaching compressed air. It consists of a small vessel of mercury with a nearly to the bottom of the flask. To show the experiment tube closed at the upper end dipping down into it. This vessel well, the receiver must be very rapidly exhausted, otherwise the is so placed that the surface of the mercury is exposed to the air slowly expands, and merely causes the water to run slowly pressure which we want to measure, and as this increases the out of the jet. The plan usually adopted is to exhaust a second mercury is forced up into the tube, compressing the air above it, large receiver on another pump-plate, and so arrange the two and indicating the pressure by a scale marked
at the side of the that a connection may be made between them by opening a tap. tube. The graduations on the scale are not equidistant, that the air is thus almost instantaneously rarefied to a considerable marked 2 being at the middle, that marked 3 one-third of the extent, and the experiment answers. The second plate in this height from the top, and so on. Sometimes, instead of a vessel description of pump is known as the transfer-plate, and is freof mercury there is merely a U shaped tube, with the bend filled quently found very convenient. with mercury and the open end connected with the boiler. It The other mode of exhibiting the experiment is rather simpler. acts, however, in the same way.
A jet is screwed into the aperture of the pump-plate, and the pump If, instead of the pressure on any gas being greater than that is so constructed that the plate, together with a portion of the of the air, it is less, the gas still expands in the same proportion. exhaust-pipe closed by a stopcock, may be removed without the Thus, if one-half the pressure be removed it will fill twice the admission of any air. The end of this tube is then plonged space. To prove this, a graduated tube is nearly filled with beneath the surface of water, and on opening the tap, the water mercury, and inverted into a tubular vessel filled with the same will be forced up through the jet by the pressure of the air, and liquid. It is first sunk so deep that the level of the mercury is thus produce a very pretty fountain in vacuo. the same inside as outside, and the volume of the contained air Having seen the mode of ascertaining the alteration which is is carefully noted. The tube is now raised till the air has ex- caused in the volume of any quantity of a gas by variation in panded to exactly double this volume, and the mercury in the the pressure, we have now to examine the effects produced by tube will then be found to stand at just half the height of the variations in the temperature. These variations are considerable ; barometer above that outside.
it is therefore necessary, in order to measure the exact quantity A simple experiment shows that if external pressure be of a gas, to bring it to a standard temperature, and, as already removed air will expand forcibly. Procure a very shrivelled stated, 60° has been fixed on as the most convenient. There is apple, and having placed it under the receiver of the pump, however, often difficulty and loss of time in bringing & 523 remove the air. The apple will expand, and look quite plump exactly to any temperature ; we want, therefore, when we know and fresh. If, however,
you admit the air in order to remove and the volume at any other temperature, to be able to calculate enjoy the apple, the pressure at once shrivels it up as before. what it would be at 60o. In the same way, if one of the thin india-rubber balls frequently If we dip the neck of a retort beneath the surface of water. sold in the streets be nearly emptied of air and tied at the and apply heat to the bulb, we shall find a number of bubbles of mouth, the little air left in will, when it is placed under the air passing off through the water; and when the source of beat receiver, expand with sufficient force to distend the ball. So, is removed and the air cools again, the water will rise in the too, if a hole be pricked in the large end of an egg, the bubble neck of the retort to take the place of the displaced air. So, of air at the other end will expand sufficiently to drive out all the likewise, if we nearly fill a bladder with air, and, having tied the contents.
neck tightly, place it before the fire, the air in it will expand Several experiments may also be easily performed showing 50 as completely to distend, and perhaps burst the bladder
. the large amount of elastic force which may be stored up in We see, then, that
the air alters in its volume by a change of compressed air. The air-gun is, perhaps, the simplest illustra- temperature. tion of this. In it the elastic force of the air takes the place of This property of air is sometimes employed in the construc gunpowder, and propels the bullet with great velocity. A strong tion of a thermometer. Two forms of air-thermometer are recopper ball is made to screw on just below the lock of the presented in Fig. 16. In one, a straight glass tube, B, with a gun. By means of a condensing syringe, air is powerfully com- bulb blown at one end, is placed with its open end downwards pressed into this, and when the trigger falls it presses a pin, in a vessel of coloured water, A. Heat is first applied to the bulb
, and thus opens a valve in the ball and allows a portion of the c, greater than that which
it is required to indicate; a portion of air to escape. This strikes the bullet, and imparts such velocity the air is thus driven out of the tube, B, and the water rises to it as to make it a very deadly missile. If the ball be weil to replace it. The height at which this water stands depends charged, the gun may be discharged
from twelve to twenty times upon the pressure of the air in the bulb, c, and as this varies successively without condensing the air in it afresh;
the with the temperature, the column serves as'a
thermometer. In power, however, diminishes slightly each time that it is fired, as the other form represented, the tube is turned up so that a small the air becomes less dense.
quantity of air may be included
in the bulb, c, above the water, anı! This expansive force of compressed air is sometimes employed this, as it expands or contracts by the heat, causes the water to