General Science by Bertha May Clark (best historical fiction books of all time TXT) π
[Illustration: FIG. 9.--Determining one of the fixed points of a thermometer.]
The Centigrade thermometer, in use in foreign countries and in all scientific work, is similar to the Fahrenheit except that the fixed points are marked 100Β° and 0Β°, and the interval between the points is divided into 100 equal parts instead of into 180.
The boiling point of
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But open fireplaces, while pleasant to look at, are not efficient for either heating or cooking. The possibilities for the latter are especially limited, and the invention of stoves was a great advance in efficiency, economy, and comfort. A stove is a receptacle for fire, provided with a definite inlet for air and a definite outlet for smoke, and able to radiate into the room most of the heat produced from the fire which burns within. The inlet, or draft, admits enough air to cause the fire to burn brightly or slowly as the case may be. If we wish a hot fire, the draft is opened wide and enough air enters to produce a strong glow. If we wish a low fire, the inlet is only partially opened, and just enough air enters to keep the fuel smoldering.
FIG. 13.βA furnace. Pipes conduct hot air to the rooms.
When the fire is started, the damper should be opened wide in order to allow the escape of smoke; but after the fire is well started there is less smoke, and the damper may be partly closed. If the damper is kept open, coal is rapidly consumed, and the additional heat passes out through the chimney, and is lost to use.
9. Furnaces. Hot Air. The labor involved in the care of numerous stoves is considerable, and hence the advent of a central heating stove, or furnace, was a great saving in strength and fuel. A furnace is a stove arranged as in Figure 13. The stove S, like all other stoves, has an inlet for air and an outlet C for smoke; but in addition, it has built around it a chamber in which air circulates and is warmed. The air warmed by the stove is forced upward by cold air which enters from outside. For example, cold air constantly entering at E drives the air heated by S through pipes and ducts to the rooms to be heated.
The metal pipes which convey the heated air from the furnace to the ducts are sometimes covered with felt, asbestos, or other non-conducting material in order that heat may not be lost during transmission. The ducts which receive the heated air from the pipes are built in the non-conducting walls of the house, and hence lose practically no heat. The air which reaches halls and rooms is therefore warm, in spite of its long journey from the cellar.
Not only houses are warmed by a central heating stove, but whole communities sometimes depend upon a central heating plant. In the latter case, pipes closely wrapped with a non-conducting material carry steam long distances underground to heat remote buildings. Overbrook and Radnor, Pa., are towns in which such a system is used.
FIG. 14.βHot-water heating.
10. Hot-water Heating. The heated air which rises from furnaces is seldom hot enough to warm large buildings well; hence furnace heating is being largely supplanted by hot-water heating.
The principle of hot-water heating is shown by the following simple experiment. Two flasks and two tubes are arranged as in Figure 15, the upper flask containing a colored liquid and the lower flask clear water. If heat is applied to B, one can see at the end of a few seconds the downward circulation of the colored liquid and the upward circulation of the clear water. If we represent a boiler by B, a radiator by the coiled tube, and a safety tank by C, we shall have a very fair illustration of the principle of a hot-water heating system. The hot water in the radiators cools and, in cooling, gives up its heat to the rooms and thus warms them.
In hot-water heating systems, fresh air is not brought to the rooms, for the radiators are closed pipes containing hot water. It is largely for this reason that thoughtful people are careful to raise windows at intervals. Some systems of hot-water heating secure ventilation by confining the radiators to the basement, to which cold air from outside is constantly admitted in such a way that it circulates over the radiators and becomes strongly heated. This warm fresh air then passes through ordinary flues to the rooms above.
In Figure 16, a radiator is shown in a boxlike structure in the cellar. Fresh air from outside enters a flue at the right, passes the radiator, where it is warmed, and then makes its way to the room through a flue at the left. The warm air which thus enters the room is thoroughly fresh. The actual labor involved in furnace heating and in hot-water heating is practically the same, since coal must be fed to the fire, and ashes must be removed; but the hot-water system has the advantage of economy and cleanliness.
FIG. 16.βFresh air from outside circulates over the radiators and then rises into the rooms to be heated.
11. Fresh Air. Fresh air is essential to normal healthy living, and 2000 cubic feet of air per hour is desirable for each individual. If a gentle breeze is blowing, a barely perceptible opening of a window will give the needed amount, even if there are no additional drafts of fresh air into the room through cracks. Most houses are so loosely constructed that fresh air enters imperceptibly in many ways, and whether we will or no, we receive some fresh air. The supply is, however, never sufficient in itself and should not be depended upon alone. At night, or at any other time when gas lights are required, the need for ventilation increases, because every gas light in a room uses up the same amount of air as four people.
FIG. 17.βThe air which goes to the schoolrooms is warmed by passage over the radiators.
In the preceding Section, we learned that many houses heated by hot water are supplied with fresh-air pipes which admit fresh air into separate rooms or into suites of rooms. In some cases the amount which enters is so great that the air in a room is changed three or four times an hour. The constant inflow of cold air and exit of warm air necessitates larger radiators and more hot water and hence more coal to heat the larger quantity of water, but the additional expense is more than compensated by the gain in health.
12. Winds and Currents. The gentlest summer breezes and the fiercest blasts of winter are produced by the unequal heating of air. We have seen that the air nearest to a stove or hot object becomes hotter than the adjacent air, that it tends to expand and is replaced and pushed upward and outward by colder, heavier air falling downward. We have learned also that the moving liquid or gas carries with it heat which it gradually gives out to surrounding bodies.
When a liquid or a gas moves away from a hot object, carrying heat with it, the process is called convection.
Convection is responsible for winds and ocean currents, for land and sea breezes, and other daily phenomena.
The Gulf Stream illustrates the transference of heat by convection. A large body of water is strongly heated at the equator, and then moves away, carrying heat with it to distant regions, such as England and Norway.
Owing to the shape of the earth and its position with respect to the sun, different portions of the earth are unequally heated. In those portions where the earth is greatly heated, the air likewise will be heated; there will be a tendency for the air to rise, and for the cold air from surrounding regions to rush in to fill its place. In this way winds are produced. There are many circumstances which modify winds and currents, and it is not always easy to explain their direction and velocity, but one very definite cause is the unequal heating of the surface of the earth.
13. Conduction. A poker used in stirring a fire becomes hot and heats the hand grasping the poker, although only the opposite end of the poker has actually been in the fire. Heat from the fire passed into the poker, traveled along it, and warmed it. When heat flows in this way from a warm part of a body to a colder part, the process is called conduction. A flatiron is heated by conduction, the heat from the warm stove passing into the cold flatiron and gradually heating it.
In convection, air and water circulate freely, carrying heat with them; in conduction, heat flows from a warm region toward a cold region, but there is no apparent motion of any kind.
Heat travels more readily through some substances than through others. All metals conduct heat well; irons placed on the fire become heated throughout and cannot be grasped with the bare hand; iron utensils are frequently made with wooden handles, because wood is a poor conductor and does not allow heat from the iron to pass through it to the hand. For the same reason a burning match may be held without discomfort until the flame almost reaches the hand.
Stoves and radiators are made of metal, because metals conduct heat readily, and as fast as heat is generated within the stove by the burning of fuel, or introduced into the radiator by the hot water, the heat is conducted through the metal and escapes into the room.
Hot-water pipes and steam pipes are usually wrapped with a non-conducting substance, or insulator, such as asbestos, in order that the heat may not escape, but shall be retained within the pipes until it reaches the radiators within the rooms.
The invention of the "Fireless Cooker" depended in part upon the principle of non-conduction. Two vessels, one inside the other, are separated by sawdust, asbestos, or other poor conducting material (Fig. 18). Foods are heated in the usual way to the boiling point or to a high temperature, and are then placed in the inner vessel. The heat of the food cannot escape through the non-conducting material which surrounds it, and hence remains in the food and slowly cooks it.
FIG. 18.βA fireless cooker.
A very interesting experiment for the testing of the efficacy of non-conductors may be easily performed. Place hot water in a metal vessel, and note by means of a thermometer the rapidity with which the water cools; then place water of the same temperature in a second metal vessel similar to the first, but surrounded by asbestos or other non-conducting material, and note the slowness with which the temperature falls.
Chemical Change, an Effect of Heat. This effect of heat has a vital influence on our lives, because the changes which take place when food is cooked are due to it. The doughy mass which goes into the oven, comes out a light spongy loaf; the small indigestible rice grain comes out the swollen, fluffy, digestible grain. Were it not for the chemical changes brought about by heat, many of our present foods would be useless to man. Hundreds of common materials like glass, rubber, iron, aluminum, etc., are manufactured by processes which involve chemical action caused by heat.
CHAPTER II TEMPERATURE AND HEAT14. Temperature not a Measure of the Amount of Heat Present. If two similar basins containing unequal quantities of water are placed in the sunshine on a summer day, the smaller quantity of water will become quite warm in a short period of time, while the
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