THE MECHANIC'S AND STUDENT'S GUIDE. CHAPTER I. OF THE ECCENTRIC CURVES. PLATE I.* (1.) ECCENTRIC curves are those curves in whose surface there is not a point which is of equal distance from the centre. The main object of these curves is to produce an alternate backward and forward sliding motion, either vertical or horizontal, just as it is required for machinery, as, for instance, for slide valves, pumps, presses, etc. It may be remarked that elipses belong as well to these curves, even the common eccentric of an engine, if in the latter the centre is not taken into consideration. The first problem we have before us is "a double eccentrical and symmetrical curve" (Fig. 1), which, by being connected by part of the circles B DE H, and A G F D, produce a stationary position on the point D, whilst the centre, C, turns round. It will be observed * The reader is supposed to be acquainted with the elements of Mechanical Drawing, so as to be able to lay down and copy drawings, He will find in the last chapter a selection of problems in geometry, the most directly applicable to the purposes of the mechanic. A that the circle A G F D1, is divided in four parts, and that, when E has turned to D, point F will be at I. Point F will now begin to rise, till it has gone through the quadrant I D1. Here the stationary position, or the point of rest, will commence, as D has now been moved to D1, where it will remain till the next quartercircle, D1 K L, has been run through. By this time A has arrived at D1, and this point, D1, will gradually return now to D, while the curve has gone through the next quadrant, leaving still one-eighth of a circle for D to get to its original position. (2.) To construct this curve, the inner circle, B D E H, and the outer circle, A G F D1, has to be determined on by the length of travel from D to D1. This having been done, divide the circle into four parts, also divide the quadrants into any number of equal parts, say six, and draw radial lines to the centre C. As we have taken six divisions, we have also to take six divisions between the outer and the inner circle, from which semicircles are struck and points of intersection found, as shown in drawing, which points have only to be connected, and the eccentrical and symmetrical curves in each quadrant are found. (3.) Fig. 2. To Construct a Double Eccentrical and Symmetrical Curve.—In the present example the point D is moved with a variable speed from D to D', the speed of its rising increasing in regular proportion from the beginning of the curve to the vertex, E, and decreasing in the same proportion, for the point D' to return to its original position, D. (4) After having determined on the size of curve by circle D' F E G, and the throw or stroke of D to D', describe the half-circle, D D', divide the circumference into any number of equal parts, say 10, and draw perpendiculars to the centre line, D' C E. The points |