190.

Rumford's Differential Thermometer.

RUMFord's DiffereNTIAL THERMOMETER is repre sented in Fig. 135.

It consists of two bulbs of thin glass. A and B, connected by a fine tube bent twice at right angles, as shown in the

Fig. 135.

figure. The whole apparatus is attached to a suitable frame, which supports a scale parallel to the horizontal branch of the connecting tube. The 0 of the scale is at its middle point, and the graduation is continued from it in both directions. The bulbs and a large part of the connecting tube are filled with air; there is, however, in the tube a small drop of fluid which separates the air in the two extremities.

The instrument is so constructed that the index, n, is at the O of the scale when the temperature of the two bulbs is

(190) Describe RUMFORD'S form. Explain the scale. Explain its action. How is the scale graduated?

the same. When one of the bulbs is heated more than the other, the air in it expands and drives the index towards the other, until the tensions of the air in the two bulbs exactly balance each other.

The scale is divided by experiment by the aid of a standard mercurial thermometer.

Leslie's Differential Thermometer.

191. LESLIE'S DIFFERENTIAL THERMOMETER is shown in Fig. 136. It differs from RUMFORD'S, in having the bulbs smaller, and in containing a longer column of liquid in the tube. The scales are placed by the sides of the vertical portions of the tube, having their 0 points at the middle. There is, then, a double scale. The method of graduating and using this thermometer is the same as that described in the last article.

Fig. 136.

Pyrometer.

192. A PYROMETER is an instrument for measuring higher temperatures than can be observed by means of the mercurial thermometer.

The most important pyrometers are those of WEDGEWOOD and BROGNIART. The former is founded on the diminution of the volume of clay at high temperatures, and the latter on the principle of the expansion of metals. The indications of these instruments are very unreliable, and it yet remains

(191.) Describe LESLIE'S Differential Thermometer. (192.) What is a Pyrometer? What are the most important ones? What is the principle of each? Are they reliable?

to discover some accurate method of measuring temperatures higher than 600° F.

III. RADIATION OF HEAT.

Propagation of Heat.

193. The ethereal medium that transmits heat extends through space, and is almost perfectly elastic. It penetrates all bodies and occupies the intervals between their molecules. The heat vibrations of bodies are thus imparted to the surrounding ether, and by it are propagated outward in spherical waves similar to sound-waves in air. Heat propagated in this way is called radiant heat. A line perpendicular to a wave front is called a ray of heat.

A ray of heat indicates a direction in which heat is propagated and along which it produces its effect. In a homogeneous medium heat-rays are straight lines radiating in every direction from a heated body. Rays of heat, like rays of sound, may be refracted and reflected. Radiant heat does not impart warmth to the medium that transmits it, but when intercepted by a body the motion of the particles of ether is imparted to the molecules of the body, and the phenomena of heat are developed.

Laws of Radiant Heat.

194. The radiation of heat takes place according to the following laws:

1. Heat is radiated equally in all directions.

This law may be verified by placing thermometers at equal distances and in different directions from a heated body.

2. Rays of heat are straight lines.

This law may be verified by interposing a screen anywhere in a right line joining the heated body and the thermometer, when the thermometer will cease to rise.

(193.) How is heat transmitted through space? What is radiant heat? What are rays of heat? (194.) What is the first law of radiant heat? How verified? What is the second law? How verified?

If a ray pass from one medium to another, it is bent from its course; this bending is called refraction.

The laws of refraction for heat are the same as for sound.

1°. The plane of the incident and refracted rays is normal to the deviating surface at the point of incidence; and

2. The sine of the angle of incidence bears a constant ratio to the sine of the angle of refraction.

3. The intensity of radiant heat varies directly as the temperature of the radiating body, and inversely as the square of the distance to which it is transmitted.

The first part of this law is verified by exposing one of the bulbs of a differential thermometer to a blackened cubical box, filled with hot water, the other bulb being protected by a screen. If the water is in the first instance of a given temperature, and then falls to a half, or a third of that temperature, the differential thermometer will manifest a half, or a third of its original indication, and so on for any temperature.

The second part of the law may also be verified by means of the differential thermometer. In this case the heated body is kept always at the same temperature, and one bulb of the differential thermometer is placed at different distances from it. It will be found that at a double distance the indication is only a fourth of the original indication. at a triple distance only a ninth, and so on.

Exchange of Heat between bodies.

195. The process of radiation of heat between bodies is mutual and continuous. According to the laws given in the preceding article, those bodies which are most heated give off most heat; hence, the hottest bodies of a group give off more heat than they receive, and the coldest ones receive more than they give off. The consequence is that there is a continual tendency towards equalization of tem

What is the third law? How is the first part of the law verified? The second part? Explain the laws of refraction of heat rays. (195.) Explain the action of radiation to produce uniformity of temperature.

perature. If all the bodies are of the same temperature, each will give off as much as it receives, and no further change of temperature can occur. The process of radiation, however, goes on as before.

All the bodies in a room, for example, tend to come to a uniform temperature. We say, tend to come to a uniform temperature, bccause this condition is never fully realized. Bodies nearest the walls are continually exchanging heat with the walls, and as these are in communication, either with the outer air, or with other rooms, their temperature will be influenced thereby, and will in turn exert an influence upon the remaining bodies in the room.

V.-REFLECTION, ABSORPTION, EMISSION, AND CONDUCTIBILITY.

196.

Reflection of Heat.

When a ray of heat falls upon the surface of a body, it is divided into two parts, one of which enters the body and is absorbed, whilst the other is deflected or bent from its course. This bending is called reflection.

The point at which the bending takes place, is called the point of incidence. The ray before incidence is called the incident ray; after incidence it is called the reflected ray. If a perpendicular be drawn to the surface at the point of incidence, it is said to be normal to the surface at that point. The angle between the incident ray and the normal is the angle of incidence; the angle between the normal and the reflected ray is the angle of reflection. The plane of the incident ray and the normal is the plane of incidence; the plane of the reflected ray and the normal is the plane of reflection. These planes coincide.

Illustrate by the example of articles in a room. (196.) What is reflection of heat? What is the point of incidence? The incident ray? The reflected ray? The plane of incidence? The plane of reflection? The angles of incidence and reflection?