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Observe that

of an iron or copper ball heated to redness. below the ball the air is only heated to a very short distance. This shows what a bad conductor air is.

EXPT. 61. Place a lighted candle-end on a saucer and pour some water round it; over the candle place a lamp-glass. The flame flickers awhile and then goes out. There is an open outlet above for the heated air and products of combustion, but no fresh air can get in below and so the candle cannot burn. Repeat the experiment and introduce a piece of cardboard down the middle of the lamp-glass as in Fig. 55. The flame appears to be blown about a bit at first, but it brightens up and keeps alight. The cardboard has roughly divided up the chimney into two halves and the flame makes use of one of these to get rid of the hot gases while the other brings fresh air down to it. The existence of these two currents can be shown by holding smouldering paper near the top of the lamp-glass.

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Fig. 55.

83. Ventilation of Rooms. In our climate dwelling-rooms are usually warmer than the air outside. This is not only due to fires and lights, but also to the heat given out by our own bodies. This hot and impure air rises to the top of the room, as you may easily prove by standing on a table in a closed room where several gas-jets have been burning. In order to keep the air sufficiently pure for respiration we require some system of ventilation, i.e. some means of getting rid of hot impure air and replacing it by cool fresh air.

If you slightly open the door of a room and hold a lighted candle in the gap, you will generally find that near the floor the flame is blown inwards (Fig. 56). At the top (unless the room is very high) it is drawn outwards: while a position (about half-way up) can generally be found in which the flame burns

Two

steadily. Thus cold air tends to make its way into the room at the bottom, driving out the warm light air above. things are therefore necessary before a room can be properly ventilated: an outlet for warm air just under the ceiling, and an inlet for fresh air near the floor.

84. Ventilation of Mines. -Coal-mines have to be thoroughly ventilated in order to supply fresh air for breathing and also to prevent accumulation of the dangerous and inflammable fire-damp referred to in Art. 74. For this purpose two vertical shafts are provided at opposite ends of the mine.

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At

Fig. 56.

the bottom of one of these (called the up-cast shaft) a large fire is kept burning, and this creates a powerful upward draught. Fresh cold air enters through the other shaft (called the down-cast shaft), and has to make its way through the various workings of the mine before it reaches the up-cast shaft. The action of the system may be illustrated as follows:

EXPT. 62.-Take off the front of a flat wooden box and replace it by a sheet of glass. Cut two holes in the top of the box (at opposite ends) and put a lighted candle in the box under one of these. Over each of the holes place a lampchimney or wide glass tube, as in Fig. 57. The left-hand tube represents the up-cast shaft, the candle the fire, the right-hand tube the down-cast shaft, and the box itself the mine.

By holding smouldering paper at the top of the tubes, it can be shown that there is a steady current of air down the righthand tube, through the box, and up the left-hand tube.

85. Convection-Currents in Nature.-The student should here read Arts. 24, 25 again. Winds, which are natural convection-currents on a large scale, will be treated of in Chapter XIII, after we have considered radiation.

Ocean currents may be regarded as convection-currents produced by the unequal heating of the surface of our globe.

[graphic][subsumed][merged small]

In general they follow the direction of the prevailing winds. These in tropical regions are easterly (i.e. blow from east to west), and so in the Atlantic Ocean there is an equatorial current from east to west. This passes along the north-east shoulder of South America, into the Caribbean Sea, and flows out of the Gulf of Mexico as a mighty stream of warm waterthe Gulf Stream-which skirts the coast of the United States and then sweeps across the Atlantic to the north-west coasts of Europe. As an example of the effect of the Gulf Stream on climate, it may be stated that the harbour of Hammerfest in Norway is free from ice all the year round; whereas the mouth of the Baltic (12 degrees farther south) and the river Hudson (in the same latitude as Rome) are frozen over three months in the year. The comparative mildness of winter in the British Isles is largely due to the influence of the Gulf Stream.

CHAPTER XII

TRANSMISSION OF HEAT-RADIATION

86. We have already referred (Art. 68) to radiation as a third mode of transmission of heat. It is by radiation that the sun warms our earth. We may at once mention two respects in which radiation differs from the other modes of transmission of heat. A hot body emits radiation in all directions and in straight lines. An obstacle interposed in the direct line between you and a source of heat at once cuts off the radiation. You can protect your face from the heat of a fire by holding a book or paper between; and when the sun's rays are too powerful you seek relief by getting into the shade.

The transmission of heat by convection, on the other hand, always takes place in one direction (e.g. by upward currents); and conduction is not restricted to straight lines, for a bent wire conducts heat as well as a straight one.

Light is known to travel at the rate of about 186,400 miles per second. Now, when an eclipse of the sun takes place, it is found that the light and heat are cut off at the same time; hence both must travel with the same enormous speed. Another characteristic of heat-radiation is that the medium or substance through which it passes is not thereby sensibly warmed. It is true that many media are only partially transparent to heat; these absorb a portion of the radiation as it passes through them (just as partially transparent glass absorbs a certain proportion of light), and are thereby warmed. With this reservation we may define the process of radiation as follows :— Heat is said to be transmitted by radiation when it passes

H

from one point to another in straight lines, with great speed, and without heating the medium through which it passes.

87 Much of what is stated in this chapter will be better understood after the student has read the next section of this book (Light). The laws of reflection and refraction are the same for heat as for light. If an image of the sun be formed with a lens on paper or on the back of the hand, the heat is found to be focussed at about the same spot, a fact with which schoolboys are sufficiently familiar, and which shows that heatradiation can be refracted as light is. Again, the law of inverse squares (p. 124) applies here as in the case of light, and it is best to study the two sets of phenomena in connection with one another.

A body heated in a dark room to a comparatively low temperature, say below a dull-red heat, is not visible; it emits only heat-radiation. But when it is heated above this temperature it becomes visible; it now emits both kinds of radiation, or it would be more correct to say that the effect produced by its radiation depends upon the object on which that radiation falls. It affects the surface of our bodies as heat; it produces in our eyes the sensation of light.

The common use of the term 'radiant heat' might lead the student to suppose that there are different kinds of heat, of which 'radiant heat' is one. This is not so. Strictly speaking, radiation is not heat at all: at any rate, it does not exhibit the ordinary properties of heat. If it were, it could not pass, as it is known to do, through bodies without heating them. There is reason for believing that radiation travels in the form of waves, much as waves are propagated on the surface of water by the upward and downward motion of the particles of water. The disturbing cause producing these waves is the hot body emitting the radiation. The waves, in general, travel freely outwards in all directions; but when they strike against an obstacle they may be partly reflected from its surface, partly absorbed by it, and partly transmitted through it. Bright polished tinplate may be taken as a type of a good reflector; lamp-black as a type of a good absorber; and rock-salt as the best example of a substance which is transparent to heat-radiation.

88. Apparatus, etc.-For detecting the presence of heat

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