Εικόνες σελίδας
PDF
Ηλεκτρ. έκδοση

slow motion be given at first to the toothed wheel, the success the toothed wheel, e is the place where the card is fixed which sive strokes of the teeth upon the card are distinctly heard; catches the teeth of the wheel. but if the velocity be gradually increased, a continued sound The Siren.---The siren is a small apparatus employed, like is obtained, which gradually rises higher and higher. When, the preceding, for the purpose of measuring the exact num. by this means, the sound is produced whose number of vibra- ber of the vibrations of a sonorous body in a given time. M. tions are required, the same velocity is kept up during a deter. Cagniard de Latour, the inventor, gave this name to the minate number of seconds; and by reading off on the counter instrument, because it can be made to yield sounds under the number of turns of the toothed wheel B, we have only to water. It is made wholly of brass, and is represented in

[merged small][graphic]

multiply this number by that of the teeth it contains, in order fig. 131, mounted on the box of a blowing machine or bellows, to obtain the whole number of vibrations. Then dividing this hereafter described, which is employed to send a continued product by the corresponding number of seconds, the quotient current of air into the siren. Fig. 132, No. 1, and fig. 133, will be the number of vibrations per second. The following show the interior details of the siren. The lower part of this representation of the same instrument in outline may suggest instrument consists of a cylindrical box, o, surmounted by a Fig. 130, No. 2,

fixed plate, B. On this plate rests a vertical rod T, to which

is fastened a disk A, that turns freely with the rod; several d'

holes are made at equal distances, in a circular forin, in the 3

plate B; and in the disk A an equal number, of the same size and at the same distance from the centre as those of the plate, are perforated. These holes are not perpendicular to the planes of the plate and the disk; but they are all inclined to ihem at the same angle, those in the plate being inclined in one direction, and those in the disk in the contrary direction, in such a manner that when the holes in the plate and the disk face each other they are arranged as seen at mn, fig. 133. From this arrangement it follows, that when a rapid current of

air comes from the bellows into the cylindrical box and into improvement or simplicity in the construction, fig. 130, No. 2. the hole m, it obliquely strikes the sides of the hole n, and a a is the oaken bench, 6'is the large wheel furnished with a imparts to the disk A, a motion of rotation in the directions winch c, d is an axle on which is placed the toothed wheel d'n A. and a pulley of small radius, x is the band which passes over the In order to simplify the explanation of the play of the siren, large wheel and the pulley, and communicates the motion to we shall first suppose that the moveable disk å is pierced with

[ocr errors]
[merged small][merged small][merged small][graphic][graphic][merged small]
[ocr errors]

eighteen holes, and that the plate B is pierced with only one blast for wind instruments, such as the siren and the organ.
hole, and we shall consider the case where the latter coincides Under the feet of a wooden table, fig. 134, is placed a bellows,
with one of the upper holes. When the wind from the bellowsc, which is put in motion by a pedal or foot-board, P. A
strikes the sides of this upper hole, the moveable disk begins reservoir, D, made of flexible leather or skin, is employed to
to revolve, and the space between two consecutive holes collect the air thrown into it by the bellows. If this reservoir
covers the lower hole. “But as the disk continues to revolve be compressed by weights placed above it, or by means of a
in consequence of its acquired velocity, two holes again face rod, t, inoved by the hand, the air is forced by a pipe, e, into
each other; whence a new impulse is given, and so on. Thus,
during a complete revolution of the disk, the lower hole is

Fig. 134,
eighteen times open and eighteen times shut; whence arises
a series of blasts and stops which puts the air into vibration,
and produces a sound when the successive impulses are very

T rapid. If we now suppose that the fixed plate B has eighteen holes, when the disk revolves each hole will produce at once the same effect as a single hole; the sound will, therefore, be eighteen times more intense, but the number of vibrations will not be increased.

In order to find the number of vibrations corresponding to the sound which the apparatus gives during its motion of rotation, we must know how many revolutions the disk makes in a second. This is effected in the following manner :-on the rod r is placed an endless screw, which transmits its motion to a wheel of 100 teeth. This wheel, which advances one tooth at every revolution of the disk, carries on it a pin, P; and this pin causes a second wheel to advance one tooth at every revolution, as seen to the left in fig. 132, No. 1. The axes of these wheels carry two indexes or hands, which move round the dials shown in fig. 131. One of these indexes shows the number of the revolutions of the disk, and the other the hundreds of these revolutions. Two knobs, c and D, are used to engage or disengage at pleasure the small wheel and the endless screw. As the sound rises in proportion as the velocity of the disk increases, a determinate sound may be obtained by increasing the force of the wind from the bellows. By keeping up the same current of air during a certain time, say two minutes, we can then read on the dials the number of revolutions which have been made by the disk. Next, by multiplying this number by 18, and dividing the product by the number of seconds, the quotient will give the number of vibrations per second. Fig. 132, No. 2, shows some of the

a box fixed on the table. This box is pierced with holes, to
Fig. 132, No. 2

which are fitted small metal valves, which can be opened at
pleasure by pressing on stops or keys placed in front of the
box. On these holes are placed the box of the siren or the
pipes of the organ, F.

Limits of Perceptible Sound.-Before the experimental re-
searches of M. Savart, philosophers believed that the ear
ceased to perceive sound when the number of simple vibra-
tions per second was below thirty-two for low sounds, and

above 18,000 for high sounds. But this experimenter has
shown that these limits were too contracted, and that the
faculty of perceiving, more or less easily, low sounds and high
sounds depends more on intensity than height; so that when
the extreme sounds are not heard, it is because that these
sounds are not produced with sufficient intensity to make an
impression on the organ of hearing. By increasing the diame-
ter of his toothed wheel, and consequently the amplitude
and intensity of the vibrations, M. Savar has extended the
limit of high sounds to 48,000 simple vibrations per second.
For low sounds he substituted for his toothed wheel a bar of
iron of about 26 inches in length, revolving between two thin
slips of wood distant from the bar only about one-thirteenth

part of an inch. At every passage it produced only a dry
parts of the siren more distinctly, and it is added here on this sound, arising from the displacement of the air. When
account. cc is the cylindrical box, communicating, at the the motion was accelerated, the sound became continuous,
bottom, with a tube through which any fuid, liquid or extremely full and deafening. Savart, by the aid of this
gaseous, is made to flow for the production of sound; tttt apparatus, found that when it produced from fourteen to six-
the metallic frame-work in which the apparatus is contained; teen simple vibrations per second, the ear still recognised a
I the rod or vertical axis which revolves on itself; 99 the sound well determined, but extremely low.
moveable circular disk, placed as near as possible to the fixed Whatever may be the method employed to count the num-
circular plate without touching it; and ll the toothed wheels ber of vibrations, the results obtained are sufficiently concor-
which indicate the number of vibrations.

dant to admit of our considering them as the expression of the The siren, with equal velocity, gives the same sound in truth by a very close approximation. In the middle portion of water as in air; the same sound also takes place in all gases, the scale of sounds, one has been particularly selected as a a fact which shows that a determinate sound depends only on starting point. This selection is entirely arbitrary; but it has the number of vibrations and not on the nature of the sonorous been fixed by custom; and the la of the diapason represents body,

in musical language a sound of very determinate height; viz. Blowing Machine. -In acoustics, a blowing machine is a that which corresponds to 880 simple vibrations; that is, 880 bellows, with a reservoir of air, which keeps up a continued l excursians of the particles of the sonorous body,

[graphic]
[ocr errors][ocr errors]

LESSONS IN GREEK.-No. XXV.

By JoHN R. BEARD, D.D.

PARADIGM OF THE REGULAR VERB Auw, I LOOSE.-MIDDLE VOICE.

[blocks in formation]
[blocks in formation]

EXERCISES.-GREEK-ENGLISH.

loose themselves; to loose one's self; to have loosed one's Λυοιμην; λυσoιμην; λυομαι και λυωμαι; ελυομην; ελυσαμης και self; they have loosed themselves; they might have loosed

self; loosing one's self; loose yourselves ; I have loosed my: λελυμαι; ελελυμην; λελυσομαι; λυσομαι και ελιπομην; λυονται ; | themselves ; thou mayest have loosed thyself; they shall ελυοντο; ελυσαντο; λελυνται ; El eluvto; de vuevos w; have loosed themselves ; they remained behind; he may λυσαισθε και λελυται; λελυμενοι ωσι; λελυμεθα; λιπoιμην; | have remained behind ; do ye remain behind; let him loose λιπεσθαι; λιπομενος και λυσασθαι; λυεσθαι; λυομενος; λυσασθε; | himself; to have loosed one's self. λελυμενος ειης; λιπωμαι.

Conjugate, according to the active and middle paradigms, these verbs :

-aidevw, I instruct, educate ; Baoilevw. I reign : ENGLISH-GREEK.

the chief parts are-παιδευω, παιδευσω, πεπαιδευκα, πεπαιI might loose myself; he might loose himself; they might | δευμαι; and βασιλευω, βασιλευσω, βεβασιλευκα, βεβασιλευμαι.

dious, and an elegant means of conducting many chemical LESSONS IN CHEMISTRY.-No. XXIV. operations.

Until within the last few years, the most usual method of BEFORE entering any further upon the investigation of chemi-employing coal gas in chemical laboratories as a source of heat, cal bodies, it will be necessary to pause awhile and describe consisted in utilising the flame of a common argand burner, certain manipulative operations having reference to the appli. the construction of which is so well known that it scarcely cation of heat. An examination of the contributions to the need be detailed. At present, the argand burner is almost editorial letter-file points out the necessity of this. One thrown out of use by the mixed gas flame presently to be correspondent desires to know whether a gas flame may not described. he substituted for a spirit-lamp flame ; another wishes to be The great advantage possessed by the mixed gas burner over instructed as to the best means of conducting distillation. I the argand flame is the absence of all smoke. The student shall proceed, therefore, in this lesson, to impart a notion of may here reply, that a well-regulated argand burner does not the economy of heat chiefly in reference to gas. Hitherto I smoke. True enough; but it nevertheless deposits a thick have directed the employment of charcoal and of spirit as coating of smoke or soot on the surface of any body held sources of heat. This course was pursued in reference to the within it, whereas, on the contrary, a mixed gas flame, if well necessities of those who could not command the agency of regulated, deposits no soot whatever. The theory of this soot coal gas, which whenever at hand, affords a cheap, commo- deposition cannot be understood, until we have fully investigated the chemical nature of carbon. Suffice it to say, that If, in the preceding experiment, the wire gauze be held at a the combustion of coal gas alone yields smoke, whereas the sufficient distance from the jet, and the resulting flame ex. combustion of coal gas and atmospheric air, or coal gas and amined, it will be seen to yield a very diminished amount of oxygen, the latter being the effective constituent in atmos- smoke; indeed, if the wire gauze be held exactly at the cor. pheric air, yields no smoke.

rect distance from the issue jet, a flame absolutely devoid of But most people know, I assume, that a mixture of coal gas smoke may result. See now how these principles are applied and atmospheric air constitutes the terribly explosive fire-damp to the construction of a mixed gas burner. of the miner; how, then, are we to burn this mixture without Commence by taking an iron or brass or copper tube, having danger? Not only does this problem admit of being solved, an internal diameter of about two inches, and a length of about but the most explosive gaseous mixture in nature can be four. tranquilly burned by the method, slightly modified, that we Tightly whip, by means of wire, a sheet of wire gauze over shall adopt for producing the mixed gas flame, namely, outside one end of the tube, as represented at a, and scollop the other a piece of wire gauze, the effect of which material in checking end as represented at 6 fig. 13. These directions being the progress of flame can be admirably recognised in the followed, the student will have made a contrivance for using following experiment, for conducting which, either a spirit. the mixed gas flame. lamp flame or a gas jet flame admits of being employed. Pro

Fig. 13, cure a piece of wire gauze (of copper wire by preference), and about six inches square. Hold this wire gauze horizontally over the apex of the flame, and gradually lower it to the base of the same, as represented in the annexed diagram, fig. 11.

Fig, 11.

[graphic]

By proceeding in this manner, the flame will be seon unable to extend through the wire gauze, that is to say, will be unable to traverse it and reappear at its upper surface. If a spirit.

The use of the scollops at the lower end of the tube 6, is lamp be employed, the extinguished gaseous contents which for the double purpose of admitting free passage to the atmos. permeate the gauze will be invisible; if, however, a gas flame be pheric air and to the end of a gas-pipe c, and the rationale of the the subject of experiment, the volatile emanations will be seen instrument will soon be rendered evident. Very slight consito be charged with smoke. Before proceeding to expatiate deration of what has been said will show that the conditions upon these phenomena, I ought to remark that a common of the arrangement are such as to cause the admixture of gas candle will serve the purpose of a gas flame, though on a with atmospheric air; which mixture ascending, must pass emaller,

and therefore a less evident and less satisfactory through the wire gauze layer at a, and escape. Being there scale. Two important facts will have been learned by the ignited, we get a tame without smoke, because the material performance of the preceding experiments. The first is, that Burned' is no longer gas, but a mixture of gas and atmospheric coal gas contains the matter of smoke, although it may not air. smoke on burning, and a spirit-lamp flame does not. The

I have described this instrument in its simplest form, which, Recond fact is, that flame cannot, or rather cannot readily, pass if not better than, is at least as good as any other. Nothing so through small apertures. Advancing now a step further, the operator may demonstrate makers ; these gentlemen devoting much time and material to

simple can be procured in the shops of philosophic instrument by means of a spirit-lamp, a gas jet, or a candle flame, that the manufacture of an apparatus very pretty to look at cer. although Aame does not pass through wire gauze, yet the tainly, but not better in practice than that just described. It gaseous

material, the food of flame (thus to express, one's self), may here be remarked, that the two great points to be attended does. Consequently it admits of being ignited on the other or to in the use of this instrument, are (i) to apportion the upper side of the wire gauze and conversely, supposing a gas amount of gas to that of air, and (2) to promote accurate admix. jet employed (a spirit-lamp or candle will no longer aid our ture between the two. The former condition is secured by the illustration) -supposing, I say, a gas jet employed, and caused to very obvious means of a stop.cock, the second by well distripass through the wire gauze, that portion which traverses the buling the issue of gas : to which latter end one of a few very wire gauze may be burned on the upper surface of the latter, simple expedients will suffice. without the transmission of flame through the wire gauze down

Supposing the laboratory tube employed for conveying the to the orifice of the jet, fig. 12.

gas to be made of india-rubber or gutta-percha, to one end of Fig. 12.

it should be attached about a foot length of pewter or lead gas. tubing, a material which admits of being readily bent or other. wise manipulated. If such a terminal leaden pipe be made to open without further preparation directly into the cylinder, it is probable that perfect mixture of the gas and the air will not result; in which case the operator should proceed as follows.

Taking a pair of pliers, let him tightly compress the delivery end of the metallic tube as represented in a, or more evidently in the section b, fig. 14. In this manner it is evident the gas will be delivered in a sort of fish-tail jet, and an admixture Bufficiently perfect will usually ensue. If not, little holes may be bored through the sides of the tube, until the exact condi. tion of perfect admixture is achieved. This may be known to have occured when a piece of glass held in the flame is no

« ΠροηγούμενηΣυνέχεια »