metal, r, which may be elevated or depressed by a a rod, b, serves to lengthen or shorten the vibrating part of the reed. This arrangement enables us to diminish or increase the rapidity of vibration at pleasure.

The mouth-piece, as described, connects with the tube T, and is set in a rectangular box, KN, which is in communication with a bellows, from which the wind is supplied. For the purpose of class demonstration, the upper part of the tube KN, has glass windows on three sides to show the motion of the reed.

When a current of air is forced into the tube KN, the reed is set in rapid vibration, causing a succession of rarefactions and condensations in the air of the pipe T, and causing it to emit a sound. The air entering the tube KN, first closes the opening by pressing the

Fplain its action.

reed against it; the reed then recoils by virtue of its elasticity, permitting a portion of condensed air to enter the pipe, when the reed is again pressed against the opening, and so on as long as the current of air is kept up. It is evident, that the rapidity of vibration will be increased by increasing the tension of the air from the bellows, and also by shortening the vibrating part of the reed.

Fig. 123 shows the arrangement of the free reed. The vibrating plate, l, is placed so as to pass backwards through an opening in the side of the tube ca, alternately closing and opening a communication between the tube and the air from the bellows. The regulator, 1, is entirely similar to that shown in Figs. 121 and 122, as are the remaining parts of the arrangement. The explanation of the action of this species of reed is entirely similar to that already described.

Explain the arrangement of the Free Reed. What is its mode of action f

The Bellows.

174. Fig. 124 represents one form of the Bellows, used in causing pipes to sound. It is worked by a lever. The air enters a valve, S, through which it passes to a leathern reservoir, R. The top of the reservoir is weighted so as to force the air into a box, from which it is admitted to the pipes by means of valves, which are opened and shut at the will of the player.

Wind Instruments.

175. WIND INSTRUMENTS of music consist of pipes, either straight or curved, which are made to sound by a current of air properly directed.

In some, the current of air is directed by the mouth upon on opening made in the side, as in the flute. In others, the current of air is made to enter through a mouth-piece, as in the flageolet. In others, a reed is used, as in the clarionet. In the organ, there is a collection of tubes, similar to those shown in Figs. 116 and 118. In some instruments, as the trumpet and the horn, a conical mouth-piece is used, of the form shown in Fig. 125, within which the lips of the musician vibrate in place of the reed. The rapidity of vibration can be regulated at will.

Fig. 125.

(174.) Describe the Bellows used with wind instruments. (175) What are Wind Instruments? Explain their different varieties.

CHAPTER V.

I. GENERAL

HEAT.

PROPERTIES OF HEAT

Definition of Heat.

176. HEAT is the physical agent that produces the sensation we call warmth; the term heat is also applied to the sensation itself. Cold is a negative term used to express the absence of heat.

Theories of Heat.

177. Two theories have been advanced to explain the phenomena of heat: the emission theory and the undulatory theory.

According to the emission theory, heat is a fluid, destitute of weight and capable of passing from one body to another with great velocity. Its particles repel each other, but are attracted by the particles of all other bodies. A body becomes heated by receiving more of this fluid than it gives out; it becomes cooled by giving out more than it receives.

According to the undulatory theory, the heat of a body is caused by a rapid vibration of its molecules; this motion may be transmitted from one body to another through an elastic medium called ether, in the same way that sound is transmitted through the air. According to this view heat is a mode of motion, and those bodies are hottest whose molecules vibrate with greatest velocity, and through the greatest amplitudes.

(176.) What is heat? Cold? (177.) What two theories of heat have been advanced? Explain the emission theory. The undulatory theory.

The undulatory theory is the one generally adopted by physicists; it affords a better explanation of the phenomena and at the same time serves to show the intimate relation between heat and light. In what follows the phenomena will be explained, as far as possible, independently of both theories.

General Effects of Heat.

178. Heat, accumulating in bodies, penetrates into their substance, and acting upon their ultimate molecules, gives rise to repellent forces which counteract those of cohesion. Hence, the most noticeable phenomenon of heat is, that it causes bodies to expand. If applied in sufficient quantity, the particles of solids are so far repelled, as to move freely amongst each other, becoming liquid; or if still greater quantities of heat are applied, the body passes into a state of vapor. When heat is abstracted from a vapor, it returns to a liquid state, and if still more heat be abstracted, it becomes solid, and if the process be continued, the solid goes on contracting under the influence of the molecular forces.

Hence we say, that heat dilates bodies, and cold contracts them. Heat also converts solids into liquids, liquids into vapors, and acting upon gases and vapors, causes them to expand.

Expansion of Bodies by Heat.

179. All bodies are expanded by heat, but in very different degrees. The most dilatable bodies are gases, then vapors, then liquids, and finally solids. In fluids we regard only increase of volume, but in solids we distinguish two kinds of expansion, linear expansion, that is, expansion in length, and expansion of volume.

(178.) Describe the general effects of heat on solids. On liquids. What effect has cold on vapors? On liquids? (179.) What bodies are most dilatable? The least dilatable? What is linear expansion? Expansion of volume ?