FIG. 86.—White light is a mixture of lights of rainbow colors.FIG. 86.—White light is a mixture of lights of rainbow colors.
127. How to produce Rainbow Colors.The Spectrum.If a beam of sunlight is admitted into a dark room through a narrow opening in the shade, and is allowed to fall upon a prism, as shown in Figure 86, a beautiful band of colors will appear on the opposite wall of the room. The ray of light which entered the room as ordinary sunlight has not only been refracted and bent from its straight path, but it has been spread out into a band of colors similar to those of the rainbow.
Whenever light passes through a prism or lens, it is dispersed or separated into all the colors which it contains, and a band of colors produced in this way is called a spectrum. If we examine such a spectrum we find the following colors in order, each color imperceptibly fading into the next: violet, indigo, blue, green, yellow, orange, red.
128. Sunlight or White Light.White light or sunlight can be dispersed or separated into the primary colors or rainbow hues, as shown in the preceding Section. What seems even more wonderful is that these spectral colors can be recombined so as to make white light.
If a prismB(Fig. 87) exactly similar toAin every way is placed behindAin a reversed position, it will undo the dispersion ofA, bending upward the seven different beams in such a way that they emerge together and produce a white spot on the screen. Thus we see, from two simple experiments, that all the colors of the rainbow may be obtained from white light, and that these colors may be in turn recombined to produce white light.
FIG. 87.—Rainbow colors recombined to form white light.FIG. 87.—Rainbow colors recombined to form white light.
White light is not a simple light, but is composed of all the colors which appear in the rainbow.
129. Color.If a piece of red glass is held in the path of the colored beam of light formed as in Section 127, all the colors on the wall will disappear except the red, and instead of a beautiful spectrum of all colors there will be seen the red color alone. The red glass does not allow the passage through it of any light except red light; all other colors are absorbed by the red glass and do not reach the eye. Onlythe red ray passes through the red glass, reaches the eye, and produces a sensation of color.
If a piece of blue glass is substituted for the red glass, the blue band remains on the wall, while all the other colors disappear. If both blue and red pieces of glass are held in the path of the beam, so that the light must pass through first one and then the other, the entire spectrum disappears and no color remains. The blue glass absorbs the various rays with the exception of the blue ones, and the red glass will not allow these blue rays to pass through it; hence no light is allowed passage to the eye.
An emerald looks green because it freely transmits green, but absorbs the other colors of which ordinary daylight is composed. A diamond appears white because it allows the passage through it of all the various rays; this is likewise true of water and window panes.
Stained-glass windows owe their charm and beauty to the presence in the glass of various dyes and pigments which absorb in different amounts some colors from white light and transmit others. These pigments or dyes are added to the glass while it is in the molten state, and the beauty of a stained-glass window depends largely upon the richness and the delicacy of the pigments used.
130. Reflected Light.Opaque Objects.In Section 106 we learned that most objects are visible to us because of the light diffusely reflected from them. A white object, such as a sheet of paper, a whitewashed fence, or a table cloth, absorbs little of the light which falls upon it, but reflects nearly all, thus producing the sensation of white. A red carpet absorbs the light rays incident upon it except the red rays, and these it reflects to the eye.
Any substance or object which reflects none of the rays which fall upon it, but absorbs all, appears black; no raysreach the eye, and there is an absence of any color sensation. Coal and tar and soot are good illustrations of objects which absorb all the light which falls upon them.
131. How and Why Colors Change.Matching Colors.Most women prefer to shop in the morning and early afternoon when the sunlight illuminates shops and factories, and when gas and electricity do not throw their spell over colors. Practically all people know that ribbons and ties, trimmings and dresses, frequently look different at night from what they do in the daytime. It is not safe to match colors by artificial light; cloth which looks red by night may be almost purple by day. Indeed, the color of an object depends upon the color of the light which falls upon it. Strange sights are seen on the Fourth of July when variously colored fireworks are blazing. The child with a white blouse appears first red, then blue, then green, according as his powders burn red, blue, or green. The face of the child changes from its normal healthy hue to a brilliant red and then to ghastly shades.
Suppose, for example, that a white hat is held at the red end of the spectrum or in any red light. The characteristics of white objects is their ability to reflectallthe various rays that fall upon them. Here, however, the only light which falls upon the white hat is red light, hence the only light which the hat has to reflect is red light and the hat consequently appears red. Similarly, if a white hat is placed in a blue light, it will reflect all the light which falls upon it, namely, blue light, and will appear blue. If a red hat is held in a red light, it is seen in its proper color. If a red hat is held in a blue light, it appears black; it cannot reflect any of the blue light because that is all absorbed and there is no red light to reflect.
A child wearing a green frock on Independence Day seems at night to be wearing a black frock, if standing near powders burning with red,blue, or violet light.
132. Pure, Simple Colors—Things as they Seem.To the eye white light appears a simple, single color. It reveals its compound nature to us only when passed through a prism, when it shows itself to be compounded of an infinite number of colors which Sir Isaac Newton grouped in seven divisions: violet, indigo, blue, green, yellow, orange, and red.
We naturally ask ourselves whether these colors which compose white light are themselves in turn compound? To answer that question, let us very carefully insert a second prism in the path of the rays which issue from the first prism, carefully barring out the remaining six kinds of rays. If the red light is compound, it will be broken up into its constituent parts and will form a typical spectrum of its own, just as white light did after its passage through a prism. But the red rays pass through the second prism, are refracted, and bent from this course, and no new colors appear, no new spectrum is formed. Evidently a ray of spectrum red is a simple color, not a compound color.
If a similar experiment is made with the remaining spectrum rays, the result is always the same: the individual spectrum colors remain simple, pure colors.The individual spectrum colors are groups of simple, pure colors.
FIG. 88.—Violet and green give blue. Green, blue, and red give white.FIG. 88.—Violet and green give blue. Green, blue, and red give white.
133. Colors not as they Seem—Compound Colors.If one half of a cardboard disk (Fig. 88) is painted green, and the other half violet, and the disk is slipped upon a toy top, and spun rapidly, the rotating disk will appear blue; if red and green areused in the same way instead of green and violet, the rotating disk will appear yellow. A combination of red and yellow will give orange. The colors formed in this way do not appear to the eye different from the spectrum colors, but they are actually very different. The spectrum colors, as we saw in the preceding Section, are pure, simple colors, while the colors formed from the rotating disk are in reality compounded of several totally different rays, although in appearance the resulting colors are pure and simple.
If it were not that colors can be compounded, we should be limited in hue and shade to the seven spectral colors; the wealth and beauty of color in nature, art, and commerce would be unknown; the flowers with their thousands of hues would have a poverty of color undreamed of; art would lose its magenta, its lilac, its olive, its lavender, and would have to work its wonders with the spectral colors alone. By compounding various colors in different proportions, new colors can be formed to give freshness and variety. If one third of the rotating disk is painted blue, and the remainder white, the result is lavender; if fifteen parts of white, four parts of red, and one part of blue are arranged on the disk, the result is lilac. Olive is obtained from a combination of two parts green, one part red, and one part black; and the soft rich shades of brown are all due to different mixtures of black, red, orange, or yellow.
134. The Essential Colors.Strange and unexpected facts await us at every turn in science! If the rotating cardboard disk (Fig. 88) is painted one third red, one third green, and one third blue, the resulting color is white. While the mixture of the spectral colors produces white, it is not necessary to have all of the spectral colors in order to obtain white; because a mixture of the following colors alone, red, green, and blue, will give white. Moreover, by the mixture ofthese three colors in proper proportions, any color of the spectrum, such as yellow or indigo or orange, may be obtained. The three spectral colors, red, green, and blue, are called primary or essential hues, because all known tints of color may be produced by the careful blending of blue, green, and red in the proper proportions; for example, purple is obtained by the blending of red and blue, and orange by the blending of red and yellow.
135. Color Blindness.The nerve fibers of the eye which carry the sensation of color to the brain are particularly sensitive to the primary colors—red, green, blue. Indeed, all color sensations are produced by the stimulation of three sets of nerves which are sensitive to the primary colors. If one sees purple, it is because the optic nerves sensitive to red and blue (purple equals red plus blue) have carried their separate messages to the brain, and the blending of the two distinct messages in the brain has given the sensation of purple. If a red rose is seen, it is because the optic nerves sensitive to red have been stimulated and have carried the message to the brain.
A snowy field stimulates equally all three sets of optic nerves—the red, the green, and the blue. Lavender, which is one part blue and three parts white, would stimulate all three sets of nerves, but with a maximum of stimulation for the blue. Equal stimulation of the three sets would give the impression of white.
A color-blind person has some defect in one or more of the three sets of nerves which carry the color message to the brain. Suppose the nerve fibers responsible for carrying the red are totally defective. If such a person views a yellow flower, he will see it as a green flower. Yellow contains both red and green, and hence both the red and green nerve fibers should be stimulated, but the red nerve fibers are defective and do notrespond, the green nerve fibers alone being stimulated, and the brain therefore interprets green.
A well-known author gives an amusing incident of a dinner party, at which the host offered stewed tomato for apple sauce. What color nerves were defective in the case of the host?
In some employments color blindness in an employee would be fatal to many lives. Engineers and pilots govern the direction and speed of trains and boats largely by the colored signals which flash out in the night's darkness or move in the day's bright light, and any mistake in the reading of color signals would imperil the lives of travelers. For this reason a rigid test in color is given to all persons seeking such employment, and the ability to match ribbons and yarns of all ordinary hues is an unvarying requirement for efficiency.
"The night has a thousand eyes,And the day but one;Yet the light of the bright world diesWith the dying sun."
"The night has a thousand eyes,And the day but one;Yet the light of the bright world diesWith the dying sun."
136.Most bodies which glow and give out light are hot; the stove which glows with a warm red is hot and fiery; smoldering wood is black and lifeless; glowing coals are far hotter than black ones. The stained-glass window softens and mellows the bright light of the sun, but it also shuts out some of the warmth of the sun's rays; the shady side of the street spares our eyes the intense glare of the sun, but may chill us by the absence of heat. Our illumination, whether it be oil lamp or gas jet or electric light, carries with it heat; indeed, so much heat that we refrain from making a light on a warm summer's night because of the heat which it unavoidably furnishes.
137. Red a Warm Color.We have seen that heat and light usually go hand in hand. In summer we lower the shades and close the blinds in order to keep the house cool, because the exclusion of light means the exclusion of some heat; in winter we open the blinds and raise the shades in order that the sun may stream into the room and flood it with light and warmth. The heat of the sun and the light of the sun seem boon companions.
We can show that when light passes through a prism and is refracted, forming a spectrum, as in Section 127, it isaccompanied by heat. If we hold a sensitive thermometer in the violet end of the spectrum so that the violet rays fall upon the bulb, the reading of the mercury will be practically the same as when the thermometer is held in any dark part of the room; if, however, the thermometer is slowly moved toward the red end of the spectrum, a change occurs and the mercury rises slowly but steadily, showing that heat rays are present at the red end of the spectrum. This agrees with the popular notion, formed independently of science, which calls the reds the warm colors. Every one of us associates red with warmth; in the summer red is rarely worn, it looks hot; but in winter red is one of the most pleasing colors because of the sense of warmth and cheer it brings.
All light rays are accompanied by a small amount of heat, but the red rays carry the most.
What seems perhaps the most unexpected thing, is that the temperature, as indicated by a sensitive thermometer, continues to rise if the thermometer is moved just beyond the red light of the spectrum. There actually seems to be more heat beyond the red than in the red, but if the thermometer is moved too far away, the temperature again falls. Later we shall see what this means.
138. The Energy of the Sun.It is difficult to tell how much of the energy of the sun is light and how much is heat, but it is easy to determine the combined effect of heat and light.
FIG. 89.—The energy of the sun can be measured in heat units.FIG. 89.—The energy of the sun can be measured in heat units.
Suppose we allow the sun's rays to fall perpendicularly upon a metal cylinder coated with lampblack and filled witha known quantity of water (Fig. 89); at the expiration of a few hours the temperature of the water will be considerably higher. Lampblack is a good absorber of heat, and it is used as a coating in order that all the light rays which fall upon the cylinder may be absorbed and none lost by reflection.
Light and heat rays fall upon the lampblack, pass through the cylinder, and heat the water. We know that the red light rays have the largest share toward heating the water, because if the cylinder is surrounded by blue glass which absorbs the red rays and prevents their passage into the water, the temperature of the water begins to fall. That the other light rays have a small share would have been clear from the preceding Section.
All the energy of the sunshine which falls upon the cylinder, both as heat and as light, is absorbed in the form of heat, and the total amount of this energy can be calculated from the increase in the temperature of the water. The energy which heated the water would have passed onward to the surface of the earth if its path had not been obstructed by the cylinder of water; and we can be sure that the energy which entered the water and changed its temperature would ordinarily have heated an equal area of the earth's surface; and from this, we can calculate the energy falling upon the entire surface of the earth during any one day.
Computations based upon this experiment show that the earth receives daily from the sun the equivalent of 341,000,000,000 horse power—an amount inconceivable to the human mind.
Professor Young gives a striking picture of what this energy of the sun could do. A solid column of ice 93,000,000 miles long and 2-1/4 miles in diameter could be melted in a single second if the sun couldconcentrate its entire power on the ice.
While the amount of energy received daily from the sun by the earth is actually enormous, it is small in comparison with the whole amount given out by the sun to the numerous heavenly bodies which make up the universe. In fact, of the entire outflow of heat and light, the earth receives only one part in two thousand million, and this is a very small portion indeed.
139. How Light and Heat Travel from the Sun to Us.Astronomers tell us that the sun—the chief source of heat and light—is 93,000,000 miles away from us; that is, so far distant that the fastest express train would require about 176 years to reach the sun. How do heat and light travel through this vast abyss of space?
FIG. 90.—Waves formed by a pebble.FIG. 90.—Waves formed by a pebble.
A quiet pool and a pebble will help to make it clear to us. If we throw a pebble into a quiet pool (Fig. 90), waves or ripples form and spread out in all directions, gradually dying out as they become more and more distant from the pebble. It is a strange fact that while we see the ripple moving farther and farther away, the particles of water are themselves not moving outward and away, but are merely bobbing up and down, or are vibrating. If you wish to be sure of this, throw the pebble near a spot where a chip lies quiet on the smooth pond. After the waves form, the chip rides up and down with the water, but does not move outward; if the wateritself were moving outward, it would carry the chip with it, but the water has no forward motion, and hence the chip assumes the only motion possessed by the water, that is, an up-and-down motion. Perhaps a more simple illustration is the appearance of a wheat field or a lawn on a windy day; the wind sweeps over the grass, producing in the grass a wave like the water waves of the ocean, but the blades of grass do not move from their accustomed place in the ground, held fast as they are by their roots.
If a pebble is thrown into a quiet pool, it creates ripples or waves which spread outward in all directions, but which soon die out, leaving the pool again placid and undisturbed. If now we could quickly withdraw the pebble from the pool, the water would again be disturbed and waves would form. If the pebble were attached to a string so that it could be dropped into the water and withdrawn at regular intervals, the waves would never have a chance to disappear, because there would always be a regularly timed definite disturbance of the water. Learned men tell us that all hot bodies and all luminous bodies are composed of tiny particles, called molecules, which move unceasingly back and forth with great speed. In Section 95 we saw that the molecules of all substances move unceasingly; their speed, however, is not so great, nor are their motions so regularly timed as are those of the heat-giving and the light-giving particles. As the particles of the hot and luminous bodies vibrate with great speed and force they violently disturb the medium around them, and produce a series of waves similar to those produced in the water by the pebble. If, however, a pebble is thrown into the water very gently, the disturbance is slight, sometimes too slight to throw the water into waves; in the same way objects whose molecules are in a state ofgentle motion do not produce light.
The particles of heat-giving and light-giving bodies are in a state of rapid vibration, and thereby disturb the surrounding medium, which transmits or conveys the disturbance to the earth or to other objects by a train of waves. When these waves reach their destination, the sensation of light or heat is produced.
We see the water waves, but we can never see with the eye the heat and light waves which roll in to us from that far-distant source, the sun. We can be sure of them only through their effect on our bodies, and by the visible work they do.
140. How Heat and Light Differ.If heat and light are alike due to the regular, rapid motion of the particles of a body, and are similarly conveyed by waves, how is it, then, that heat and light are apparently so different?
Light and heat differ as much as the short, choppy waves of the ocean and the slow, long swell of the ocean, but not more so. The sailor handles his boat in one way in a choppy sea and in a different way in a rolling sea, for he knows that these two kinds of waves act dissimilarly. The long, slow swell of the ocean would correspond with the longer, slower waves which travel out from the sun, and which on reaching us are interpreted as heat. The shorter, more frequent waves of the ocean would typify the short, rapid waves which leave the sun, and which on reaching us are interpreted as light.
141. We seldom consider what life would be without our wonderful methods of illumination which turn night into day, and prolong the hours of work and pleasure. Yet it was not until the nineteenth century that the marvelous change was made from the short-lived candle to the more enduring oil lamp. Before the coming of the lamp, even in large cities like Paris, the only artificial light to guide the belated traveler at night was the candle required to be kept burning in an occasional window.
With the invention of the kerosene lamp came more efficient lighting of home and street, and with the advent of gas and electricity came a light so effective that the hours of business, manufacture, and pleasure could be extended far beyond the setting of the sun.
The production of light by candle, oil, and gas will be considered in the following paragraphs, while illumination by electricity will be reserved for a later Chapter.
142. The Candle.Candles were originally made by dipping a wick into melting tallow, withdrawing it, allowing the adhered tallow to harden, and repeating the dipping until a satisfactory thickness was obtained. The more modern method consists in pouring a fatty preparation into a mold, at the center of which a wick has been placed.
The wick, when lighted, burns for a brief interval with a faint, uncertain light; almost immediately, however, the intensityof the light increases and the illumination remains good as long as the candle lasts. The heat of the burning tallow melts more of the tallow near it, and this liquid fat is quickly sucked up into the burning wick. The heat of the flame is sufficient to change most of this liquid into a gas, that is, to vaporize the liquid, and furthermore to set fire to the gas thus formed. These heated gases burn with a bright yellow flame.
143. The Oil Lamp.The simple candle of our ancestors was now replaced by the oil lamp, which gave a brighter, steadier, and more permanent illumination. The principle of the lamp is similar to that of the candle, except that the wick is saturated with kerosene or oil rather than with fat. The heat from the burning wick is sufficient to change the oil into a gas and then to set fire to the gas. By placing a chimney over the burning wick, a constant and uniform draught of air is maintained around the blazing gases, and hence a steady, unflickering light is obtained. Gases and carbon particles are set free by the burning wick. In order that the gases may burn and the solid particle glow, a plentiful supply of oxygen is necessary. If the quantity of air is insufficient, the carbon particles remain unburned and form soot. A lamp "smokes" when the air which reaches the wick is insufficient to burn the rapidly formed carbon particles; this explains the danger of turning a lamp wick too high and producing more carbon particles than can be oxidized by the air admitted through the lamp chimney.
One great disadvantage of oil lamps and oil stoves is that they cannot be carried safely from place to place. It is almost impossible to carry a lamp without spilling the oil. The flame soon spreads from the wick to the overflowing oil and in consequence the lamp blazes and an explosion may result. Candles, on the other hand, are safe from explosion; the dripping grease is unpleasant but not dangerous.
The illumination from a shaded oil lamp is soft and agreeable, but the trimming of the wicks, the refilling of bowls, and the cleaning of chimneys require time and labor. For this reason, the introduction of gas met with widespread success. The illumination from an ordinary gas jet is stronger than that from an ordinary lamp, and the stronger illumination added to the greater convenience has made gas a very popular source of light.
144. Gas Burners and Gas Mantles.For a long time, the only gas flame used was that in which the luminosity resulted in heating particles of carbon to incandescence. Recently, however, that has been widely replaced by use of a Bunsen flame upon an incandescent mantle, such as the Welsbach. The principle of the incandescent mantle is very simple. When certain substances, such as thorium and cerium, are heated, they do not melt or vaporize, but glow with an intense bright light. Welsbach made use of this fact to secure a burner in which the illumination depends upon the glowing of an incandescent, solid mantle, rather than upon the blazing of a burning gas. He made a cylindrical mantle of thin fabric, and then soaked it in a solution of thorium and cerium until it became saturated with the chemical. The mantle thus impregnated with thorium and cerium is placed on the gas jet, but before the gas is turned on, a lighted match is held to the mantle in order to burn away the thin fabric. After the fabric has been burned away, there remains a coarse gauze mantle of the desired chemicals. If now the gas cock is opened, the escaping gas is ignited, the heat of the flame will raise the mantle to incandescence and will produce a brilliant light. A very small amount of burning gas is sufficient to raise the mantle to incandescence, and hence, by the use of a mantle, intense light is secured at little cost. The mantle saves us gas, because the cock is usually "turned onfull" whether we use a plain burner or a mantle burner. But, nevertheless, gas is saved, because when the mantle is adjusted to the gas jet, the pressure of the gas is lessened by a mechanical device and hence less gas escapes and burns. By actual experiment, it has been found that an ordinary burner consumes about five times as much gas per candle power as the best incandescent burner, and hence is about five times as expensive. One objection to the mantles is their tendency to break. But if the mantles are carefully adjusted on the burner and are not roughly jarred in use, they last many months; and since the best quality cost only twenty-five cents, the expense of renewing the mantles is slight.
145. Gas for Cooking.If a cold object is held in the bright flame of an ordinary gas jet, it becomes covered with soot, or particles of unburned carbon. Although the flame is surrounded by air, the central portion of it does not receive sufficient oxygen to burn up the numerous carbon particles constantly thrown off by the burning gas, and hence many carbon particles remain in the flame as glowing, incandescent masses. That some unburned carbon is present in a flame is shown by the fact that whenever a cold object is held in the flame, it becomes "smoked" or covered with soot. If enough air were supplied to the flame to burn up the carbon as fast as it was set free, there would be no deposition of soot on objects held over the flame or in it, because the carbon would be transformed into gaseous matter.
Unburned carbon would be objectionable in cooking stoves where utensils are constantly in contact with the flame, and for this reason cooking stoves are provided with an arrangement by means of which additional air is supplied to the burning gas in quantities adequate to insure complete combustion of the rapidly formed carbon particles. An opening is made in the tube through which gas passes to the burner,and as the gas moves past this opening, it carries with it a draft of air. These openings are visible on all gas stoves, and should be kept clean and free of clogging, in order to insure complete combustion. So long as the supply of air is sufficient, the flame burns with a dull blue color, but when the supply falls below that needed for complete burning of the carbon, the blue color disappears, and a yellow flame takes its place, and with the yellow flame the deposition of soot is inevitable.
146. By-products of Coal Gas.Many important products besides illuminating gas are obtained from the distillation of soft coal. Ammonia is made from the liquids which collect in the condensers; anilin, the source of exquisite dyes, is made from the thick, tarry distillate, and coke is the residue left in the clay retorts. The coal tar yields not only anilin, but also carbolic acid and naphthalene, both of which are commercially valuable, the former as a widely used disinfectant, and the latter as a popular moth preventive.
From a ton of good gas-producing coal can be obtained about 10,000 cubic feet of illuminating gas, and as by-products 6 pounds of ammonia, 12 gallons of coal tar, and 1300 pounds of coke.
147. Natural Gas.Animal and vegetable matter buried in the depth of the earth sometimes undergoes natural distillation, and as a result gas is formed. The gas produced in this way is called natural gas. It is a cheap source of illumination, but is found in relatively few localities and only in limited quantity.
148. Acetylene.In 1892 it was discovered that lime and coal fused together in the intense heat of the electric furnace formed a crystalline, metallic-looking substance called calcium carbide. As a result of that discovery, this substance was soon made on a large scale and sold at a moderate price.The cheapness of calcium carbide has made it possible for the isolated farmhouse to discard oil lamps and to have a private gas system. When the hard, gray crystals of calcium carbide are put in water, they give off acetylene, a colorless gas which burns with a brilliant white flame. If bits of calcium carbide are dropped into a test tube containing water, bubbles of gas will be seen to form and escape into the air, and the escaping gas may be ignited by a burning match held near the mouth of the test tube. When chemical action between the water and carbide has ceased, and gas bubbles have stopped forming, slaked lime is all that is left of the dark gray crystals which were put into the water.
When calcium carbide is used as a source of illumination, the crystals are mechanically dropped into a tank containing water, and the gas generated is automatically collected in a small sliding tank, whence it passes through pipes to the various rooms. The slaked lime, formed while the gas was generated, collects at the bottom of the tanks and is removed from time to time.
The cost of an acetylene generator is about $50 for a small house, and the cost of maintenance is not more than that of lamps. The generator does not require filling oftener than once a week, and the labor is less than that required for oil lamps. In a house in which there were twenty burners, the tanks were filled with water and carbide but once a fortnight. Acetylene is seldom used in large cities, but it is very widely used in small communities and is particularly convenient in more or less remote summer residences.
Electric Lights.The most recent and the most convenient lighting is that obtained by electricity. A fine, hairlike filament within a glass bulb is raised to incandescence by the heat of an electric current. This form of illumination will be considered in connection with electricity.
149. Labor-saving Devices.To primitive man belonged more especially the arduous tasks of the out-of-door life: the clearing of paths through the wilderness; the hauling of material; the breaking up of the hard soil of barren fields into soft loam ready to receive the seed; the harvesting of the ripe grain, etc.
FIG. 91.—Prying a stone out of the ground.FIG. 91.—Prying a stone out of the ground.
The more intelligent races among men soon learned to help themselves in these tasks. For example, our ancestors in the field soon learned to pry stones out of the ground (Fig. 91) rather than to undertake the almost impossible task of lifting them out of the earth in which they were embedded; to swing fallen trees away from a path by means of rope attached to one end rather than to attempt to remove them single-handed; to pitch hay rather than to lift it; to clear a field with a rake rather than with the hands; to carry heavy loads in wheelbarrows (Fig. 92) rather than on the shoulders; to roll barrels up a plank (Fig. 93) and to raise weights by ropes. In every case, whether in the lifting of stones, or the felling of trees, or the transportation of heavy weights, or the digging of the ground, man used his brain in the invention of mechanicaldevices which would relieve muscular strain and lighten physical labor.
If all mankind had depended upon physical strength only, the world to-day would be in the condition prevalent in parts of Africa, Asia, and South America, where the natives loosen the soil with their hands or with crude implements (Fig. 94), and transport huge weights on their shoulders and heads.
FIG. 92.—The wheelbarrow lightens labor.FIG. 92.—The wheelbarrow lightens labor.
Any mechanical device (Figs. 95 and 96), whereby man's work can be more conveniently done, is called a machine; the machine itself never does any work—it merely enables man to use his own efforts to better advantage.
FIG. 93.—Rolling barrels up a plank.FIG. 93.—Rolling barrels up a plank.
150. When do we Work?Whenever, as a result of effort or force, an object is moved, work is done. If you lift a knapsack from the floor to the table, you do work because you use force and move the knapsack through a distance equal to the height of the table. If the knapsack were twice as heavy, you would exert twice as much force to raise it to the same height, and hence you would do double the work. If you raised theknapsack twice the distance,—say to your shoulders instead of to the level of the table,—you would do twice the work, because while you would exert the same force you would continue it through double the distance.
FIG. 94.—Crude method of farming.FIG. 94.—Crude method of farming.
Lifting heavy weights through great distances is not the only way in which work is done. Painting, chopping wood, hammering, plowing, washing, scrubbing, sewing, are all forms of work. In painting, the moving brush spreads paint over a surface; in chopping wood, the descending ax cleaves the wood asunder; in scrubbing, the wet mop rubbed over the floor carries dirt away; in every conceivable form of work, force and motion occur.
A man does work when he walks, a woman does work when she rocks in a chair—although here the work is less than in walking. On a windy day the work done in walking is greater than normal. The wind resists our progress, and we must exert more force in order to cover the same distance. Walking through a plowed or rough field is much more tiring than to walk on a smooth road, because, while the distance covered may be the same, the effort put forth is greater, and hence more work is done. Always the greater the resistance encountered, the greater the force required, and hence the greater the work done.
The work done by a boy who raises a 5-pound knapsack to his shoulder would be 5 × 4, or 20, providing his shoulders were 4 feet from theground.
The amount of work done depends upon the force used and the distance covered (sometimes called displacement), and hence we can say that
Work = force multiplied by distance,orW = f × d.
151. Machines. A glance into our machine shops, our factories, and even our homes shows how widespread is the use of complex machinery. But all machines, however complicated in appearance, are in reality but modifications and combinations of one or more of four simple machines devised long ago by our remote ancestors. These simple devices are known to-day, as (1) the lever, represented by a crowbar, a pitchfork; (2) the inclined plane, represented by the plank upon which barrels are rolled into a wagon; (3) the pulley, represented by almost any contrivance for the raising of furniture to upper stories; (4) the wheel and axle, represented by cogwheels and coffee grinders.
FIG. 95.—Primitive method of grinding corn.FIG. 95.—Primitive method of grinding corn.
Suppose a 600-pound bowlder which is embedded in the ground is needed for the tower of a building. The problem of the builder is to get the heavy bowlder out of the ground, to load it on a wagon for transportation, and finally to raise it to the tower. Obviously, he cannot do this alone; the greatest amount of force of which he is capable would not suffice to accomplish any one of these tasks. How then doeshe help himself and perform the impossible? Simply, by the use of some of the machine types mentioned above, illustrations of which are known in a general way to every schoolboy. The very knife with which a stick is whittled is a machine.
FIG. 96.—Separating rice grains by flailing.FIG. 96.—Separating rice grains by flailing.
FIG. 97.—The principle of the lever.FIG. 97.—The principle of the lever.
152. The Lever. Balance a foot rule, containing a hole at its middle pointF, as shown in Figure 97. If now a weight of 1 pound is suspended from the bar at some point, say 12, the balance is disturbed, and the bar swings about the pointFas a center. The balance can be regained by suspending an equivalent weight at the opposite end of the bar, or by applying a 2-pound weight at a point 3 inches to the left ofF. In the latter case a force of 1 pound actually balances a force of 2 pounds, but the 1-pound weight is twice as far from the point of suspension as is the 2-pound weight. The small weight makes up in distance what it lacks in magnitude.
Such an arrangement of a rod or bar is called a lever. In any form of lever there are only three things to be considered: the point where the weight rests, the point where the force acts, and the point calledthe fulcrum about which the rod rotates.
The distance from the force to the fulcrum is called the force arm. The distance from the weight to the fulcrum is called the weight arm; and it is a law of levers, as well as of all other machines, that the force multiplied by the length of the force arm must equal the weight multiplied by the length of the weight arm.
Force × force arm = weight × weight arm.
A force of 1 pound at a distance of 6, or with a force arm 6, will balance a weight of 2 pounds with a weight arm 3; that is,
1 × 6 = 2 × 3.
Similarly a force of 10 pounds may be made to sustain a weight of 100 pounds, providing the force arm is 10 times longer than the weight arm; and a force arm of 800 pounds, at a distance of 10 feet from the fulcrum, may be made to sustain a weight of 8000 pounds, providing the weight is 1 foot from the fulcrum.
153. Applications of the Lever. By means of a lever, a 600-pound bowlder can be easily pried out of the ground. Let the lever, any strong metal bar, be supported on a stone which serves as fulcrum; then if a man exerts his force at the end of the rod somewhat as in Figure 91 (p. 154), the force arm will be the distance from the stone or fulcrum to the end of the bar, and the weight arm will be the distance from the fulcrum to the bowlder itself. The man pushes down with a force of 100 pounds, but with that amount succeeds in prying up the 600-pound bowlder. If, however, you look carefully, you will see that the force arm is 6 times as long as the weight arm, so that the smaller force is compensated for by the greater distance through which it acts.
At first sight it seems as though the man's work were done for him by the machine. But this is not so. The man mustlower his end of the lever 3 feet in order to raise the bowlder 6 inches out of the ground. He does not at any time exert a large force, but he accomplishes his purpose by exerting a small force continuously through a correspondingly greater distance. He finds it easier to exert a force of 100 pounds continuously until his end has moved 3 feet rather than to exert a force of 600 pounds on the bowlder and move it 6 inches.
By the time the stone has been raised the man has done as much work as though the stone had been raised directly, but his inability to put forth sufficient muscular force to raise the bowlder directly would have rendered impossible a result which was easily accomplished when through the medium of the lever he could extend his small force through greater distance.
154. The Wheelbarrow as a Lever.The principle of the lever is always the same; but the relative position of the important points may vary. For example, the fulcrum is sometimes at one end, the force at the opposite end, and the weight to be lifted between them.