i ;> 



OR A W I X C, INS T RUM E NTS. 



iide; and they also pass through grooves in the 



tru ' ends of the Iwrs of the frmiie. By this means, when 



-J!^""iL J *'"' instrument is placed on the paper, it can be moved 



to adjusl il to the exact position, where the cllip-c i~ to 



n. IK- drawn, without disturbing the ruler 1'. To remove 



the transverse diameter of the ellipse, (the conjugate 



remaining die same,) the whole frame is moved by the 



'ing in the grooves of P; but to mo\e it in 



an opposite direction, \ iz. to adjust die position of the 



ellipse cndw ise, the nuts must be loosened, and then the 



ferews will slide in the grooves at the ends <>f the bar* 



FC!. When the nuts are screwed fast, diis motion is 



prevented, though die screws will still slide in die 



grooves of P. 



operation. "The properties of this instrument will be rendered 

 most evident, by some examples of the manner of 

 using it. All lhat is required, as data for describing 

 any ellipsis, or any number within or near each other, 

 is, to sketch them in pencil on the paper, and mark, 

 by the compasses, the four points upon each curve 

 where its two diameters inter.-ect it. Place the in-tru- 

 menl upon the paper in such a position, that, by the 

 estimation of die eye, die centre of the four ruler.-, seems 

 to coincide with the centre of die intended ellipsis, 

 the two upper rulers being parallel to the longest dia- 

 meter of die curve. Here fix the instrument by pres- 

 sing the two pins of the ruler P into die paper, and 

 hold it fast, by placing the thumb and fore finger of 

 the left hand upon the nuts XO, leaving the other 

 hand at liberty to tuni die circles about by applying 

 the finger and thumb to any opposite two of die small 

 handles^ 



Now, by turning the pinion L, remove the drawing 

 pen to one of the marks made for die cxtenl of the 

 shortest diameter of die ellipsis ; then turn die circles 

 one half round by die handles, and examine it' the point 

 of the pen conies exactly to the opposite mark, for the 

 odier end of the shortest diameter, if it does not adjust 

 the error one half by moving die pen with the pinion 

 L, and die other half by moving the whole frame on 

 the paper, then by returning die circles back again, the 

 accuracy of die adjustment will be ascertained ; for if 

 k meets die former mark, it proves that the circles are 

 in the right centre, and that the compasses are set to 

 the projK-r diameter for the conjugate axis. Now turn 

 the i>en towards the length of the ellipsis ; and, with- 

 out altering die compasses or pinion L, slide die circles 

 one upon another by die pinion K, till the point of the 

 IK-II arrives at the mark made for die length of the el- 

 lipsis ; turn die circles half round to die opposite end, 

 and if they match the mark made there, the adjust- 

 ment is correct ; if not, one half of the error must be 

 corrected, by moving the circles by their pinion K, 

 and die other by moving the whole frame sidewise on 

 the paper. To do dii, the nuts N, O must be made loose, 

 and then the frame will be at liberty to move. The 

 adjustments being made in diis manner, the pen may 

 be suffered to rest upon the paper, and trace round die 

 1 '. curve. The Society of Arts rewarded the inventor of 

 P"" 1 this instrument with their gold medal, and have publish- 

 ed a description of it in their 31st volume of Transac- 

 tions ; and also a plate of specimens of the curve drawn 

 upon die copperplate by an instrument of the same kind. 

 This instrument, however, has an additional apparatus 

 for dividing the ellipses, when drawn, into any required 

 n\im!>er of divisions, which shall be the representation of 

 a divided circle, either in true perspective, which artists 

 call the stenographic projection, where the divisions on 

 that side which is nearest to the eye will be larger than 

 those on the most distant, or it will divide in die or- 



thographic projection, where the rye being supposed 

 A an infinite di*t-iiicc. the divi-ions on the distant ami ' 

 adjacent side* of the ellipse are equal, and only tlmt ^ 

 difference made in tin- divisions which is produced !MHU " ^^*" 

 c.icli being viewed with a different degree of obliquity. 

 Numerous sjH-ciiiM-ns of tin- application of this ellipti'i- 

 graph may be seen in the Plates of our Work, parti- 

 cularly in BLOCK MM IIINKHV, and COIMM; MAUII^EH. 



Fig. II. is an instrument, called the geometrical pen. > 

 which will describe many different curves, thouh most r '- 

 of them are species of i'pV\c!..'.; . A, )',. ( an- three p, ATE 

 leg*, which form the frame, ar.il support a central axis, r. \<\ u ,i. 

 a ; upon this as a centre, a tube l> is fitted to revolve ' 'D- ' > 

 wheu the finger is applied to the milled circle r ; die 

 tube carries an arm il, which supports die wheels , and 

 J\ and therefore die centres of these describe circle* 

 round the principal axes: g is a cog wheel, firmly fixed 

 upon the lower end of die axis a, and is therefore sta- 

 tionary ; but by the revolution of other wheels round 

 i', they have a rotation given to them upon their own 

 axes. The arbor n of the wheel /] has an arm h at- 

 tached to it, and dlis at the extremity carries the tub* 

 which receives the drawing point k to trace ujxm tin- 

 paper. The pencil lias therefore a motion round its 

 arbor n, and at the same time this centre is revolving, 

 with a slower motion, round the central axis a. By 

 proportioning the velocities of the movements, a num- 

 ber of curious curves will be found, some similar to 

 that which the moon describes by the compound mo- 

 tion of revolving round the earth at the same time that 

 she is moving in her orbit round the sun. It m.ikc* 

 looped figures, resembling slurs or flowers, and of any 

 required number of loops or leaves ; but as it is im- 

 possible to describe these without giving plate!; of 

 them as examples, we shall not attempt it, but refer 

 our readers to <m exiimin: tion of the machine, or to 

 Adam's Graphical Essays, in which a great number of 

 these figures are given. He ascribes the invention of 

 the instrument to John Baptist Suardi, who enumerates 

 1273 curves which may be drawn by it. To accom- 

 plish these, many different si/cd wheels arc used, and 

 the revolving arms must be placed at various distances. 

 For diis purpose, the wheel g is tilted to the axis, so 

 that it can readily be removed, and another substituted 

 in its place; the wheel e is fitted upon a pin, which 

 slides in a groove cut through the arm d, as sliewn in 

 Fig. 12. ami can be ftXed at any part of it by a clamp 

 nut, so as to suit die radius of the other wheels. In 

 like manner, the tube which receives the axis n is fit- 

 led on a box which slides upon die arm tl, to adjust 

 the distance of die axis n from the central axis ; the 

 wheel/, which is fixed upon it, can be readily changed 

 to apply one of a different si/e ; and, lasdy, the arm 

 h of the pencil is filled in a mortise through the lower 

 end of the axis n, and fastened by a screw, by which 

 means the pencil can be removed to any required dis- 

 tance fronrthe oat : ;.nd it is by die different propor- 

 tions of these wheels, and the lengths of the arms, 

 that all the above-mentioned varieties of curves can be 

 produced. 



.Instruments for drawing in perspective are very nu- Perspective 

 merous; but the CAMERA LUCIDA, invented by Dr Wo- liutru- 

 ...rvedly t;.kr- the preference : (See that ar- u - t " l - 

 tide. ) A new and ingenious instrument, invented liy 

 Mr Turrel, has been lately rewarded by the Soricty of 

 Arts, mid promises to be x cry useful to ai lists. \\ e have 

 not been able to obtain a description of it, us the Si- 

 cicty has not yet published their account of it, but we 

 chall probably have this in our power under the artiuk- 

 PERSPBlTlVt. (j. K.) 





