PRACTICAL MECHANICS FOR BOYS TEXT

Sound is also made more intense if a sonorous body is near its source. This is taken advantage of in musical instruments, where a sounding-board is used, as in the case of the piano, and in the violin, which has a thin shell as a body for holding the strings. Another curious thing is shown in the speaking tube, where the sound waves are confined, so that they are carried along in one line, and as they are not interfered with will transmit the vibrations to great distances. VELOCITY OF SOUND.--The temperature of the air has also an effect on the rate of transmission, but for general purposes a temperature of 62 degrees has been taken as the standard. The movement is shown to be about 50 miles in 4 minutes, or at the rate of 1,120 feet per second. In water, however, the speed is four times greater; and in iron nearly fifteen times greater. Soft earth is a poor conductor, while rock and solid earth convey very readily. Placing the ear on a railway track will give the vibrations of a moving train miles before it can be heard through the air. SOUND REFLECTIONS.--Sound waves move outwardly from the object in the form of wave-like rings, but those concentric rings, as they are called, may be interrupted at various points by obstacles. When that is the case the sound is buffeted back, producing what is called echoes. RESONANCE.--Materials have a quality that produces a very useful result, called _resonance_, and it is one of the things that gives added effect to a speaker's voice in a hall, where there is a constant succession of echoes. A wall distant from the speaker about 55 feet, produces an almost instantaneous reflection of the sound, and at double that measurement the effect is still stronger. When the distance is too short for the reflecting sound to be heard, we have _resonance_. It enriches the sound of the voice, and gives a finer quality to musical instruments. ECHOES.--When sounds are heard after the originals are emitted they tend to confusion, and the quality of resonance is lost. There are places where echoes are repeated many times. In the chateau of Simonetta, Italy, a sound will be repeated thirty times. SPEAKING TRUMPET.--This instrument is an example of the use of reflection. It is merely a bell-shaped, or flaring body, the large end of which is directed to the audience. The voice talking into the small end is directed forwardly, and is reflected from the sides, and its resonance also enables the vibrations to carry farther than without the use of the solid part of the instrument. The ear trumpet is an illustration of a sound-collecting device, the waves being brought together by reflection. THE STETHOSCOPE.--This is an instrument used by physicians, and it is so delicate that the movements of the organs of the body can be heard with great distinctness. It merely collects the vibrations, and transmits them to the ears by the small tubes which are connected with the collecting bell. THE VITASCOPE.--Numerous instruments have been devised to determine the rate of vibration of different materials and structures, the most important being the _vitascope_, which has a revolvable cylinder, blackened with soot, and this being rotated at a certain speed, the stylus, which is attached to the vibrating body, in contact with the cylinder, will show the number per second, as well as the particular character of each oscillation. THE PHONAUTOGRAPH.--This instrument is used to register the vibration of wind instruments, as well as the human voice, and the particular forms of the vibrations are traced on a cylinder, the tracing stylus being attached to a thin vibrating membrane which is affected by the voice or instrument. THE PHONOGRAPH.--This instrument is the outgrowth of the stylus forms of the apparatus described, but in this case the stylus, or needle, is fixed to a metallic diaphragm, and its point makes an impression on suitable material placed on the outside of a revolvable cylinder or disc. Light.-Light is the agent which excites the sensation of vision in the eye. Various theories have been advanced by scientists to account for the phenomenon, and the two most noted views are the _corpuscular_, promulgated by Sir Isaac Newton, and the _undulatory_, enunciated by Huygens and Euler. The _corpuscular_ theory conceives that light is a substance of exceedingly light particles which are shot forth with immense velocity. The _undulatory_ theory, now generally accepted, maintains that light is carried by vibrations in ether. Ether is a subtle elastic medium which fills all space. _Luminous_ bodies are those like the sun, which emit light. Rays may _diverge_, that is, spread out; _converge_, or point toward each other; or they may be _parallel_ with each other. VELOCITY OF LIGHT.--Light moves at the rate of about 186,000 miles a second. As the sun is about 94,000,000 miles from the earth, it takes 8-1/2 minutes for the light of the sun to reach us. REFLECTION.--One of the most important things connected with light is that of reflection. It is that quality which is utilized in telescopes, microscopes, mirrors, heliograph signaling and other like apparatus and uses. The underlying principle is, that a ray is reflected, or thrown back from a mirror at the same angle as that which produces the light. When the rays of the sun, which are, of course, parallel, strike a concave mirror, the reflecting rays are converged; and when the rays strike a convex mirror they diverge. In this way the principle is employed in reflecting telescopes. REFRACTION.--This is the peculiar action of light in passing through substances. If a ray passes through water at an angle to the surface the ray will bend downwardly in passing through, and then again pass on in a straight line. This will be noticed if a pencil is stood in a glass of water at an angle, when it will appear bent. Refraction is that which enables light to be divided up, or analyzed. In this way white light from the sun is shown to be composed of seven principal colors. COLORS.--If the light is passed through a prism, which is a triangularly shaped piece of glass, the rays on emerging will diverge from each other, and when they fall on a wall or screen the colors red, orange, yellow, green, blue, indigo and violet are shown. The reason for this is that the ray in passing through the prism has the different colors in it refract at different angles, the violet bending more than the red. THE SPECTROSCOPE.--The ability to make what is thus called a _spectrum_, brought forth one of the most wonderful instruments ever devised by man. If any metal, or material, is fused, or put in such a condition that a ray of light can be obtained from it, and this light is passed through a prism, it will be found that each substance has its own peculiar divisions and arrangements of colors. In this way substances are determined by what is called _spectrum analysis_, and it is by means of this instrument that the composition of the sun, and the planets and fixed stars are determined. THE RAINBOW.--The rainbow is one of the effects of refraction, as the light, striking the little globular particles of water suspended in the air, produces a breaking up of the white light into its component colors, and the sky serves as a background for viewing the analysis thus made. HEAT.--It is now conclusively proven, that heat, like light, magnetism and electricity, is merely a mode of motion. The _mechanical_ theory of heat may be shown by rubbing together several bodies. Heat expands all substances, except ice, and in expanding develops an enormous force. EXPANSION.--In like manner liquids expand with heat. The power of mercury in expanding may be understood when it is stated that a pressure of 10,000 pounds would be required to prevent the expansion of mercury, when heated simply 10 degrees. Gases also expand. While water, and the different solids, all have their particular units of expansion, it is not so with gases, as all have the same coefficient. CHAPTER VIII HOW DRAUGHTING BECOMES A VALUABLE AID The ability to read drawings is a necessary part of the boy's education. To know how to use the tools, is still more important. In conveying an idea about a piece of mechanism, a sketch is given. Now, the sketch may be readable in itself, requiring no explanation, or it may be of such a nature that it will necessitate some written description. [Illustration: _Fig. 95. Plain Circle_] LINES IN DRAWING.--In drawing, lines have a definite meaning. A plain circular line, like Fig. 95, when drawn in that way, conveys three meanings: It may represent a rim, or a bent piece of wire; it may illustrate a disk; or, it may convey the idea of a ball. Suppose we develop them to express the three forms accurately. Fig. 96, by merely adding an interior line, shows that it is a rim. There can be no further doubt about that expression. Fig. 97 shows a single line, but it will now be noticed that the line is thickened at the lower right-hand side, and from this you can readily infer that it is a disk. SHADING.--Fig. 98, by having a few shaded lines on the right and lower side, makes it have the appearance of a globe or a convex surface. [Illustration: _Fig. 96. Ring_ _Fig. 97. Raised Surface_ _Fig. 98. Sphere_] Shading or thickening the lines also gives another expression to the same circular line. In Fig. 99, if the upper and left-hand side of the circle is heavily shaded, it shows that the area within the circle is depressed, instead of being raised. DIRECTION OF SHADE.--On the other hand, if the shading lines, as in Fig. 100, are at the upper left-hand side, then the mind at once grasps the idea of a concave surface. The first thing, therefore, to keep in mind, is this fact: That in all mechanical drawing, the light is supposed to shine down from the upper left-hand corner and that, as a result, the lower vertical line, as well as the extreme right-hand vertical line, casts the shadows, and should, therefore, be made heavier than the upper horizontal, and the left-hand vertical lines. [Illustration: _Fig. 99. Depressed Surface_ _Fig. 100. Concave_] There are exceptions to this rule, which will be readily understood by following out the illustrations in the order given below. PERSPECTIVES.--The utility of the heavy lines will be more apparent when drawing square, rectangular, or triangular objects. Let us take Fig. 101, which appears to be the perspective of a cube. Notice that all lines are of the same thickness. When the sketch was first brought to me I thought it was a cube; but the explanation which followed, showed that the man who made the sketch had an entirely different meaning. He had intended to convey to my mind the idea of three pieces, A, B, C, of metal, of equal size, joined together so as to form a triangularly shaped pocket as shown in Fig. 101. The addition of the inner lines, like D, quickly dispelled the suggestion of the cube. [Illustration: _Fig. 101. Fig. 102. Fig. 103. Fig. 104. Forms of Cubical Outlines_] "But," he remarked, "I want to use the thinnest metal, like sheets of tin; and you show them thick by adding the inner lines." Such being the case, if we did not want to show thickness as its structural form, we had to do it by making the lines themselves and the shading give that structural idea. This was done by using the single lines, as in Fig. 103, and by a slight shading of the pieces A, B, C. [Illustration: _Fig. 105. Fig. 106. Shading Edges_] THE MOST PRONOUNCED LINES.--If it had been a cube, or a solid block, the corners nearest the eye would have been most pronounced, as in Fig. 104, and the side next to the observer would have been darkest. This question of light and shadow is what expresses the surface formation of every drawing. Simple strokes form outlines of the object, but their thickness, and the shading, show the character enclosed by the LINES. DIRECTION OF LIGHT.--Now, as stated, the casting of the shadow downward from the upper left-hand corner makes the last line over which it passes the thickest, and in Figs. 105 and 106 they are not the extreme lines at the bottom and at the right side, because of the close parallel lines. In Figs. 109 and 110 the blades superposed on the other are very thin, and the result is the lines at the right side and bottom are made much heavier. [Illustration: _Fig. 107. Fig. 108. Illustrating Heavy Lines_] This is more fully shown in Figs. 107 and 108. Notice the marked difference between the two figures, both of which show the same set of pulleys, and the last figure, by merely having the lower and the right-hand lines of each pulley heavy, changes the character of the representation, and tells much more clearly what the draughtsman sought to convey. SCALE DRAWINGS.--All drawings are made to a scale where the article is large and cannot be indicated the exact size, using parts of an inch to represent inches; and parts of a foot to represent feet. In order to reduce a drawing where a foot is the unit, it is always best to use one-and-a-half inches, or twelve-eighths of an inch, as the basis. In this way each eighth of an inch represents an inch. If the drawing should be made larger, then use three inches, and in that way each inch would be one-quarter of an inch. [Illustration: _Fig. 109. Fig. 110. Lines on Plain Surfaces_] The drawing should then have marked, in some conspicuous place, the scale, like the following: "Scale, 1-1/2" = 1'"; or, "Scale 3" = 1'." DEGREE, AND WHAT IT MEANS.--A degree is not a measurement. The word is used to designate an interval, a position, or an angle. Every circle has 360 degrees, and when a certain degree is mentioned, it means a certain angle from what is called a _base line_. [Illustration: _Fig. 111. Illustrating Degrees_] Look at Fig. 111. This has a vertical line A, and a horizontal line B. The circle is thus divided into four parts, and where these lines A, B, cross the circle are the cardinal points. Each of the four parts is called a quadrant, and each quadrant has 90 degrees. Any line, like C, which is halfway between A and B, is 45 degrees. Halfway between A and C, or between B and C, like the line D, is 22-1/2 degrees. MEMORIZING ANGLES.--It is well to try and remember these lines by fixing the angles in the memory. A good plan is to divide any of the quadrants into thirds, as shown by the points E, F, and then remember that E is 30 degrees from the horizontal line B, and that F is 60 degrees. Or, you might say that F is 30 degrees from the vertical line A, and E 60 degrees from A. Either would be correct. [Illustration: _Fig. 112. Section Lining_] SECTION LINING.--In representing many parts of a machine, or article, it is necessary to show the parts cut off, which must be illustrated by what is called "section lining." Adjacent parts should have the section lines running at right angles to each other, and always at 45 degrees. Look at the outside and then the inside views of Fig. 112, and you will see how the contiguous parts have the angles at right angles, and clearly illustrate how every part of the wrench is made. Skill in depicting an article, for the purpose of constructing it from the drawing, will make the actual work on the bench and lathe an easy one. [Illustration: _Fig. 113. Drawing an Ellipse_] MAKING ELLIPSES AND IRREGULAR CURVES.--This is the hardest thing to do with drawing tools. A properly constructed elliptical figure is difficult, principally, because two different sized curves are required, and the pen runs from one curve into the other. If the two curves meet at the wrong place, you may be sure you will have a distorted ellipse. Follow the directions given in connection with Fig. 113, and it will give you a good idea of merging the two lines. First. Draw a horizontal line, A, which is in the direction of the major axis of the ellipse--that is, the longest distance across. The narrow part of the ellipse is called the minor axis. Second. Draw a perpendicular line, B, which we will call the center of the ellipse, where it crosses the line A. This point must not be confounded with the _focus_. In a circle the focus is the exact center of the ring, but there is no such thing in an ellipse. Instead, there are two focal points, called the _foci_, as you will see presently. Third. Step off two points or marking places, as we shall term them, equidistant from the line B, and marked C, C. These marks will then represent the diameter of the ellipse across its major axis. Fourth. We must now get the diameter of the minor axis, along the line B. This distance will depend on the perspective you have of the figure. If you look at a disk at an angle of about 30 degrees it will be half of the distance across the major axis. So you may understand this examine Fig. 114. The first sketch shows the eye looking directly at the disk 1. In the second sketch the disk is at 30 degrees, and now the lines 2 2, from the eye, indicate that it is just half the width that it was when the lines 3 3 were projected. The marks D D, therefore, indicate the distance across the minor axis in Fig. 113. [Illustration: _Fig. 114. Perspection in Angles_] Fifth. We must now find the focal points of the ellipse. If the line A on each side of the cross line B is divided into four parts, the outer marks E may be used for the foci, and will be the places where the point of the compass, or bow pen, is to be placed. Sixth. Describe a circle F, so it passes through the mark C, and move the point of the compass to the center of the ellipse, at the star, and describe a circle line G, from the mark C to the line B. This will give a centering point H. Then draw a line I from H to E, and extend it through the circle F. Seventh. If the point of the compass is now put at H, and the pencil or pen on the circle line F, the curve J can be drawn, so the latter curve and the curve F will thus merge perfectly at the line I. THE FOCAL POINTS.--The focal points can be selected at any arbitrary point, between C and the line B, and the point H may be moved closer to or farther away from the line A, and you will succeed in making the ellipse correct, if you observe one thing, namely: The line I, which must always run from H to E, and intersects the circle F, is the starting or the ending point for the small curve F or the large circle J. [Illustration: _Fig. 115. Fig. 116. Fig. 117. Perspectives of Cubes_] ISOMETRIC AND PERSPECTIVE.--A figure may be drawn so as to show an isometric or a perspective view. Thus, a cube can be drawn so as to make an isometric figure, as in Fig. 115, where the three sides are equal to each other. Isometric means a method of drawing any object in such a manner that the height, length and breadth may be shown in the proportion they really bear to each other. Fig. 115 has the sides not only equal to each other, in appearance to the eye, but they have the same outlines and angles. Contrast this figure with Figs. 116 and 117. In Fig. 116 two of the sides are equal in angles and outline; and in Fig. 117 each side has a different outline, and different angles. Nevertheless, all the cubes are, in reality, of the same dimension. THE PROTRACTOR.--This is a most useful tool for the draughtsman. It enables the user to readily find any angle. Fig. 118 shows an approved form of the tool for this purpose. [Illustration: _Fig. 118. Protractor. Section Lining Metals_] SUGGESTIONS IN DRAWING.--As in the use of all other tools, so with the drawing instrument, it must be kept in proper order. If the points are too fine they will cut the paper; if too blunt the lines will be ragged. In whetting the points hold the pen at an angle of 12 degrees. Don't make too long an angle or slope, and every time you sharpen hold it at the same angle, so that it is ground back, and not at the point only. [Illustration: _Fig. 119. Using the Protractor._]