APPLIED MECHANICS. 



[USES OF MECHANICAL DRAW I NO. 



prorcroent in our art*, than all the marvellous advance 

 if iho lost thirty yean. While we fully admit the high 

 importance of other branches of mental and moral train- 

 ing, we would aUo strongly urge the advantages of iii- 

 sight into the mechanical arts, because through their 



extended cultivation we expect to secure a great in- 

 crease of our material prosperity and comfort*, a great 

 diminution of labour, improvement of health and 

 strength, and wide diffusion of intelligence among all 



CHAPTER II. 

 MECHANICAL DRAWING. 



Conttntl. U81 OF DRAWING PLANE SURFACES DRAWING CUBES DRAWING CYLINDERS PROJECTION SECTIONS 



PERSPECTIVE PROJECTION or CURVES UNIWLDED SURFACES INTERSECTIONS HATCHING CENTRAL LINES 



SHADOWS AND SHADES INSTRUMENTS SCALE OF DRAWINGS. 



USB OF DRAWING. In the actual practice of 

 mechanical art, drawings are invaluable ; they show the 

 true forms, dimensions, and arrangements of machinery, 

 to those accustomed to their use, with greater clearness 

 than models or even the full-sized works themselves. 

 The draughtsman devises the arrangement of his machi- 

 nery ; sketches it on paper ; calculates the strength and 

 proportion of the parts ; draws them out full-size or to 

 some suitable scale ; studies their combinations on paper ; 

 improves this part, strengthens that part ; modifies the 

 forms so as to save complication, material, or cost of 

 workmanship ; traces the action of the whole ; provides 

 safeguards against accident or undue wear and strain ; 

 and having at length fully embodied his ideas on his 

 plans, sections, and elevations, places them before the 

 workman for execution. The workman has, in general, 

 no need to study any of those details which fall within 

 the province of the draughtsman ; he has only to put his 

 rule to the drawing, and measuring every dimension 

 there indicated, shape the material with which he deals 

 in exact accordance with it. Without drawings it would 

 be impossible to make any advance in mechanical art 

 The paper, pencil, ink, and colours, are cheap materials ; 

 and the time and labour occupied in making a drawing 

 are as nothing compared with the work in fashioning the 

 solid material. To scheme on paper is, therefore, most 

 advantageous in every point of view, as by that process 

 only can be secured that economy of material and labour, 

 harmony of action and justness of proportion, which con- 

 stitute the beauty of a mechanical device. To scheme in 

 the solid materials, is a most expensive as well as unsatis- 

 factory process, and one that seldom leads to successful 

 results. We know of no circumstance that can warrant 

 its adoption, except in cases where there is no practical 

 experience to guide the mechanic as to the power re- 

 quired, or the mode of operation suited to the work ho 

 has in hand. Even in such cases it is generally possible, 

 by the exercise of a little judgment and ingenuity, to 

 arrange some simple and inexpensive experiment, which 

 may give to a practical man, a tolerable notion of 

 the kind and scale of machinery that will be re- 

 quired. 



A system of mechanical notation was proposed some 

 years ago by the ingenious Mr. Babbago. In devising 

 his calculating machinery, which consisted of a great 

 number of parts, many of them merely repetitions, he 

 found it difficult to imagine all their simultaneous move- 

 ments without an excessive and painful exercise of that 

 in. iit.il power which has to deal with such matters, lie 

 tbeiefore attempted, and with considerable success, to 

 trace by written symbols the flow of motion through a 

 train of machinery. We are not aware that his system 

 of notation ha* been adopted by practical men. Indeed, 

 most of the complex machinery with which the engineer 

 has to deal, is of a kind similar in many respects to works 

 previously executed, embodying improvements that may 

 have been fmm time to time effected in them ; his mind 

 is, therefore, in a manner prepared for the conception of 

 the various motions and connections by his practice in 

 watching those in machines in action, and he has com- 

 paratively uttlo difficulty in fully imagining the intended 



action of the work on which he may be engaged. It will, 

 therefore, be our first duty to offer a few practical sug- 

 gestions on drawing, as applied to machines. 



PLANE SURFACES. When we have to draw a 

 triangle, square, circle, or any other superficial figure, 

 upon paper, we find no difficulty in giving a full aud 

 accurate delineation of it in respect of form and dimen- 

 sions. The paper on which we draw is itself a plane sur- 

 face ; and so long as we are not limited in length and 

 breadth, we can delineate any plane form whatever upon 

 it. In the same manner, we can describe any of the flat 

 faces of any solid object presented to us, such as a circle, 

 pyramid, or prism ; and give an accurate representa- 

 tion of any two of its dimensions, whether we call these 

 by the names of length and breadth, or height and width. 

 But when we have to delineate a solid body which has 

 three dimensions, length, breadth, and thickness, we 

 must either cut our paper to pieces, shape it so as to 

 correspond with the different faces of the body, and put 

 them together in similar order, so as to form a model of 

 it ; or we must have recourse to some device that shall 

 enable us to comprehend for ourselves, and to communi- 

 cate to others, an accurate notion of the solid body we 

 propose to delineate on a flat surface. When the painter 

 draws a portrait, or executes a landscape, he has to 

 imagine, that between his eye and the object which he 

 draws, there is interposed a surface or sheet of some kind 

 on which he sees an image of the object drawn as in his 

 picture. He delineates it on the canvas, therefore, ex- 

 actly as it would appear upon this interposed screen. 

 In a camera-obscura, the rays of light proceeding from 

 every point of objects presented to it, are concentrated 

 or gathered together by a lens, into a particular place or 

 focus behind it. A piece of frosted glass in this focus 

 offers a surface for these rays to illuminate, with their 

 respective lights, shades, and colours ; and an obsi 

 looking on the frosted glass, sees a perfect miniature 

 picture of the objects. A plate of prepared metal or 

 glass placed in the focus, receives exactly the same pic- 

 ture ; and certain substances spread over its surface, 

 rendered sensitive to chemical properties of rays of light, 

 which the eye cannot appreciate, undergo changes in 

 their constitution, which can be rendered permanently 

 visible in photography. 



DRAWING CUBES. Pictures are thus produced 

 by hand and by photography, which present, upon flat 

 surfaces, delineations of solid bodies as they appear to 

 the eye. But, if we were to apply a compass or a rule 

 to the measurement of the bodies represented in these 

 pictures, and also measure the objects themselves, wu 

 should find the angles, dimensions, and proportions of 

 the picture, totally different from those of the real bodies. 

 Fig. s. 



Thus, a cube, such as a die, which has six equal square 

 faces, marked by dots, as in Fig. 3, when represented on 

 a picture, might present such an appearance as in Fig. 4, 



