eighteen holes, and that the plate в is pierced with only one hole, and we shall consider the case where the latter coincides with one of the upper holes. When the wind from the bellows strikes the sides of this upper hole, the moveable disk begins to revolve, and the space between two consecutive holes covers the lower hole. But as the disk continues to revolve in consequence of its acquired velocity, two holes again face each other; whence a new impulse is given, and so on. Thus, during a complete revolution of the disk, the lower hole is 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 rapid. If we now suppose that the fixed plate в has eighteen holes, when the disk revolves cach 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 T 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

account.

Fig. 132, No. 2.

parts of the siren more distinctly, and it is added here on this ce is the cylindrical box, communicating, at the bottom, with a tube through which any fluid, liquid or gaseous, is made to flow for the production of sound; tttt the metallic frame-work in which the apparatus is contained; x the rod or vertical axis which revolves on itself; qg the moveable circular disk, placed as near as possible to the fixed circular plate without touching it; and 7 the toothed wheels which indicate the number of vibrations.

The siren, with equal velocity, gives the same sound in water as in air; the same sound also takes place in all gases, a fact which shows that a determinate sound depends only on the number of vibrations and not on the nature of the sonorous body,

Blowing Machine.-In acoustics, a blowing machine is a bellows, with a reservoir of air, which keeps up a continued

blast for wind instruments, such as the siren and the organ. Under the feet of a wooden table, fig. 134, is placed a bellows. c, which is put in motion by a pedal or foot-board, P. A reservoir, D, made of flexible leather or skin, is employed to collect the air thrown into it by the bellows. If this reservoir be compressed by weights placed above it, or by means of a rod, T, inoved by the hand, the air is forced by a pipe, E, into Fig. 134.

a box fixed on the table. This box is pierced with holes, to 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 researches 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. Savart 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 sound, arising from the displacement of the air. When the motion was accelerated, the sound became continuous, extremely full and deafening. Savart, by the aid of this apparatus, found that when it produced from fourteen to sixteen simple vibrations per second, the ear still recognised a sound well determined, but extremely low.

Whatever may be the method employed to count the num ber of vibrations, the results obtained are sufficiently concor dant to admit of our considering them as the expression of the truth by a very close approximation. In the middle portion of the scale of sounds, one has been particularly selected as a starting point. This selection is entirely arbitrary; but it has been fixed by custom; and the la of the diapason represents in musical language a sound of very determinate height; viz. that which corresponds to 880 simple vibrations; that is, 880 excursions of the particles of the sonorous body.

S. 1

To loose one's self, or to be loosed. λυ-ε-σθαι

Loosing one's self.

λυ-ομενος

2

λυ-ν*

3 λυ-εται

D. 1 | λυ-ομεθον

λυ-η*

λυ-ηται

λυ-ου λυ-εσθω

λυ-ωμεθον

2

λυ-εσθον*

3

λυ-εσθον*

Ρ. 1

λυ-ομεθα

λυ-ησθον* λυ-ησθον*

λυ ωμεθα λυ-ησθε λυ-ωνται

I shall loose myself.

5. 1 | λυ-σ-ομαι, the

Person

endings like the Present.

Loose thyself.

To have loosed one's self. λυσασθαι

Having loosed.

λυσαμενος

myself.

EXERCISES.-GREEK-ENGLISH.

Λυοιμην; λυσοίμην, λυομαι; λυωμαι; ελυόμην, ελεσάμην ; λελυμαι; ελελυμην; λελυσομαι; λυσομαι, ελιπομην, λυονται; ελυοντο; ελυσαντο; λελυνται ; ελέλυντο; λελυμένος ως λυσαισθε; λελυται; λελυμένοι ωσι; λελυμεθα; λιποιμην ; λιπεσθαι; λιπομενος ; λυσασθαι; λυέσθαι ; λυομενος; λυσασθε, λελυμένος είης; λιπωμαι.

ENGLISH-GREEK.

λιπου, εσθω like the Present.

λιπεσθαι

λιπομενος

loose themselves; to loose one's self; to have loosed one's

self loosing one's self, loose yourselves; I have loosed my themselves; thou mayest have loosed thyself; they shall self; they have loosed themselves; they might have loosed have loosed themselves; they remained behind; he may have remained behind, do ye remain behind, let him loose himself, to have loosed one's self.

Conjugate, according to the active and middle paradigms, these verbs:-raidevo, I instruct, educate; ßaoiλevw. I reign: the chief parts are-παιδεύω, παιδεύσω, πεπαιδευκα, πεπαι

LESSONS IN CHEMISTRY.-No. XXIV. BEFORE entering any further upon the investigation of chemical bodies, it will be necessary to pause awhile and describe certain manipulative operations having reference to the application of heat. An examination of the contributions to the editorial letter-file points out the necessity of this. One correspondent desires to know whether a gas flame may not be substituted for a spirit-lamp flame; another wishes to be instructed as to the best means of conducting distillation. I shall proceed, therefore, in this lesson, to impart a notion of the economy of heat chiefly in reference to gas. Hitherto I have directed the employment of charcoal and of spirit as sources of heat. This course was pursued in reference to the necessities of those who could not command the agency of coal gas, which whenever at hand, affords a cheap, commo

gated the chemical nature of carbon. Suffice it to say, that the combustion of coal gas alone yields smoke, whereas the combustion of coal gas and atmospheric air, or coal gas and oxygen, the latter being the effective constituent in atmospheric air, yields no smoke.

But most people know, I assume, that a mixture of coal gas and atmospheric air constitutes the terribly explosive fire-damp of the miner; how, then, are we to burn this mixture without danger? Not only does this problem admit of being solved, but the most explosive gaseous mixture in nature can be tranquilly burned by the method, slightly modified, that we shall adopt for producing the mixed gas flame, namely, outside a piece of wire gauze, the effect of which material in checking the progress of flame can be admirably recognised in the following experiment, for conducting which, either a spiritlamp flame or a gas jet flame admits of being employed. Procure 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.

If, in the preceding experiment, the wire gauze be held at a sufficient distance from the jet, and the resulting flame examined, it will be seen to yield a very diminished amount of smoke; indeed, if the wire gauze be held exactly at the correct distance from the issue jet, a flame absolutely devoid of smoke may result. See now how these principles are applied to the construction of a mixed gas burner.

Commence by taking an iron or brass or copper tube, having an internal diameter of about two inches, and a length of about four. Tightly whip, by means of wire, a sheet of wire gauze over one end of the tube, as represented at a, and scollop the other end as represented at b fig. 13. These directions being followed, the student will have made a contrivance for using the mixed gas flame.

Fig. 13.

By proceeding in this manner, the flame will be seen 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 spiritlamp be employed, the extinguished gaseous contents which permeate the gauze will be invisible; if, however, a gas flame be the subject of experiment, the volatile emanations will be seen to be charged with smoke. Before proceeding to expatiate upon these phenomena, I ought to remark that a common candle will serve the purpose of a gas flame, though on a smaller, and therefore a less evident and less satisfactory scale. Two important facts will have been learned by the performance of the preceding experiments. The first is, that coal gas contains the matter of smoke, although it may not smoke on burning, and a spirit-lamp flame does not. The second fact is, that flame cannot, or rather cannot readily, pass through small apertures.

Advancing now a step further, the operator may demonstrate by means of a spirit-lamp, a gas jet, or a candle flame, that although flame does not pass through wire gauze, yet the gaseous material, the food of flame (thus to express one's self), does. Consequently it admits of being ignited on the other or upper side of the wire gauze and conversely, supposing a gas jet employed (a spirit-lamp or candle will no longer aid our illustration)—supposing, I say, a gas jet employed, and caused to pass through the wire gauze, that portion which traverses the wire gauze may be burned on the upper surface of the latter, without the transmission of flame through the wire gauze down to the orifice of the jet, fig. 12.

Fiz. 12.

The use of the scollops at the lower end of the tube b, is for the double purpose of admitting free passage to the atmospheric air and to the end of a gas-pipe c, and the rationale of the instrument will soon be rendered evident. Very slight consideration of what has been said will show that the conditions of the arrangement are such as to cause the admixture of gas with atmospheric air; which mixture ascending, must pass through the wire gauze layer at a, and escape. Being there ignited, we get a flame without smoke, because the material burned' is no longer gas, but a mixture of gas and atmospheric

air.

I have described this instrument in its simplest form, which, if not better than, is at least as good as any other. Nothing so simple can be procured in the shops of philosophic instrument makers; these gentlemen devoting much time and material to the manufacture of an apparatus very pretty to look at certainly, but not better in practice than that just described. It may here be remarked, that the two great points to be attended to in the use of this instrument, are (1) to apportion the amount of gas to that of air, and (2) to promote accurate admixture between the two. The former condition is secured by the very obvious means of a stop-cock, the second by well distributing the issue of gas: to which latter end one of a few very simple expedients will suffice.

Supposing the laboratory tube employed for conveying the gas to be made of india-rubber or gutta-percha, to one end of it should be attached about a foot length of pewter or lead gastubing, a material which admits of being readily bent or otherwise 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 sufficiently perfect will usually ensue. If not, little holes may be bored through the sides of the tube, until the exact condition of perfect admixture is achieved. This may be known to have occured when a piece of glass held in the flame is no