the toothed wheel, e is the place where the card is fixed which catches the teeth of the wheel. slow motion be given at first to the toothed wheel, the successive strokes of the teeth upon the card are distinctly heard; The Siren. The siren is a small apparatus employed, like but if the velocity be gradually increased, a continued sound is obtained, which gradually rises higher and higher. When, the preceding, for the purpose of measuring the exact numby 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 Fig. 130, No. 1. fig. 131, mounted on the box of a blowing machine or bellows, hereafter described, which is employed to send a continued current of air into the siren. Fig. 132, No. 1, and fig. 133, show the interior details of the siren. The lower part of this instrument consists of a cylindrical box, o, surmounted by a 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 holes are made at equal distances, in a circular forin, in the 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 them 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 the hole m, it obliquely strikes the sides of the hole », and imparts to the disk A, a motion of rotation in the directions Fig. 133. Fig. 13, No. 1. 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, moved 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 vibrations 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 diameter 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 number of vibrations, the results obtained are sufficiently concordant 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. LESSONS IN GREEK.-No. XXV. BY JOHN R. BEARD, D.D. PARADIGM OF THE REGULAR VERB λυω, I LOOSE.-MIDDLE VOICE. S. 1 | λυ-ομαι λυ-ωμαι To loose one's self, or to be loosed, λυ-ε-σθαι Loosing one's self. λυ-ομενος 2 Av-y* 3 λυ-εται D. 1 | λυομεθον 2 λυεσθον* 3 λυ-εσθον Ρ. 1 | λυ-ομεθα 2 λυ-εσθεί 3 λυονται I was loosing myself. λυ-ησθε λυ-ωνται λυ-εσθεί commonly εσθον EXERCISES.-GREEK-ENGLISH. Λυοιμην; λυσοιμην; λυομαι; λυωμαι; ελυόμην, ελυσαμην ; λελυμαι; ελελυμην; λελυσομαι; λυσομαι; ελιπομην; λυονται; ελυοντο; ελυσαντο; λελυνται ; eλeλvvro; λeλvμEvos ; λυσαισθε; λελυται; λελυμένοι ωσι; λελυμεθα; λιποιμην; λιπεσθαι; λιπομενος ; λυσασθαι ; λυεσθαι ; λυομενος; λυσασθε; λελυμένος ειης; λιπωμαι. ENGLISH-GREEK. | like the Present. loose themselves; to loose one's self; to have loosed one's self, they have loosed themselves; they might have loosed self; loosing one's self; loose yourselves; I have loosed mythemselves; thou mayest have loosed thyself; they shall 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:-raidevw, I instruct, educate; ẞaoiλevw. I reign: the chief parts are—παιδεύω, παιδεύσω, πεπαιδευκα, πεπαι I might loose myself; he might loose himself; they might | δευμαι; and βασιλευω, βασιλεύσω, βεβασιλευκα, βεβασιλευμαι. 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 dious, and an elegant means of conducting many chemical operations. Until within the last few years, the most usual method of employing coal gas in chemical laboratories as a source of heat, consisted in utilising the flame of a common argand burner, the construction of which is so well known that it scarcely need be detailed. At present, the argand burner is almost thrown out of use by the mixed gas flame presently to be described. The great advantage possessed by the mixed gas burner over the argand flame is the absence of all smoke. The student may here reply, that a well-regulated argand burner does not smoke, True enough; but it nevertheless deposits a thick coating of smoke or soot on the surface of any body held within it, whereas, on the contrary, a mixed gas flame, if well regulated, deposits no soot whatever. The theory of this soot deposition cannot be understood, until we have fully investi 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 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 Recond 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. Fig. 12.. for the double purpose of admitting free passage to the atmosThe use of the scollops at the lower end of the tube b, is pheric 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. if not better than, is at least as good as any other. Nothing so I have described this instrument in its simplest form, which, makers; these gentlemen devoting much time and material to simple can be procured in the shops of philosophic instrument the manufacture of an apparatus very pretty to look at cer tainly, 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 admix. ture 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 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 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 |