DRAWING INSTRUMENTS. 



131 



Drawing 

 Instru- 

 ments. 



united together with screws, with any required degree 

 of eccentricity from each other. These revolve between 

 four rulers, DE and FG, firmly screwed together to 

 PLATE form a frame: but they are in two different planes, as 

 ecxxxvm. shewn by Fig. 3 ; so that the upper circle is included 

 Tig- 3. between FG, and the lower between DE. Both circles 

 are accurately fitted, with liberty to slide freely be- 

 tween their respective rulers, but have no other motions; 

 therefore the centres of the two circles always move in 

 right lines, which are parallel to the rulers, and at 

 right angles to each other, as shewn by the dotted 

 lines ED and FG. The curve is traced by the pen of 

 a pair of compasses MH, situated as shewn in Fig. 3, 

 the leg being stuck fast into a socket H, which moves 

 on a centre or axis ; and the pen M of the compasses 

 is therefore capable of being lifted up at pleasure, tliat 

 it may not mark upon the paper ; and when in use, 

 this centre permits the pen to follow the surface of the 

 paper, and always press upon it with a proper force to 

 draw neatly. The circles of the instrument are turned 

 round by means of six small handles f fixed in the cir- 

 cumference of the upper circle, and to any opposite 

 two of these the finger and thumb of the right hand is 

 applied, whilst the frame of the instrument is held firm- 

 ly down upon the paper by the finger and thumb of the 

 left hand pressing upon the two nuts O, N ; then turn- 

 ing the circles round in their frame, the pen will draw 

 the ellipsis as in the Figure. The principle on which this 

 instrument operates, is the same as the trammel. To 

 shew this, Fig. 11. represents the same ellipsis as in 

 Its prin- Fig. 2. with a trammel properly set for describing it 

 *P ' in the manner before explained. The alteration con- 



sists in extending the diameters of the pins which act 

 in the grooves, till they become the large circles, AB, Fig. 

 2 ; and then the rulers DE and FG represent the sides 

 of the grooves in which the pins move. The point of 

 the pen of the compasses M now represents the point E, 

 Fig. 1 1. and draws the curve in the same manner : but 

 by these alterations, the instrument becomes general in 

 its application; for the point M has the power of extend- 

 ing any distance from the centre, and any required ec- 

 centricity can be given to the circles, still preserving 

 the advantage that the point M can be actually brought 

 to coincide with one of the points 1) or C, Fig. 1 1. when 

 of course it will draw a straight line, and if brought to 

 agree with both of them, it will describe only a point ; 

 therefore this instrument will describe all possible va- 

 rieties of ellipses within the limits of its radius, either 

 , . j. with respect to size or the proportion of its diameters. To 

 i'.-iu-. explain the adjustment of the instrument, we must return 

 to Fig. 2, which shews that the circles have no central 

 bar, but instead of it have two bars it, a parallel to each 

 other, and at some distance from the centre, leaving an 

 open space between them, in which the drawing pen or 

 tracing point is situated. There are also crooked arms b, b 

 proceeding from the bars a, a to the circular rim, to give 

 them sufficient strength ; and these being all the liars 

 across the circle, it leaves them sufficiently open to see 

 the curve as it is traced beneath by the drawing pens. 

 The circles are united by screws, which keep them to- 

 gether, but at the same time allow them to slide one 

 upon the other, in the manner of the Figure, by means 

 of a pinion K, the centre pin of which is fixed on one 

 of the arms of the lower circle, and acts upon a rack rf, 

 ( screwed to the upper circle, so that it separates the two 



when turned round by its head K. But the circles are 

 fitted together so tight by the screws, that they will not 

 separate from each other, except by the power of the 

 pinion, and may, in the motion of the instrument, be con- 

 sidered as firmly united together, though capable of 



having any degree of eccentricity given to them by Drawing 

 means of the pinion K. The other adjustment, viz. Instru- 

 that which removes the drawing point to any required J" ent8 -^ 

 distance from the centre, is produced by a pinion L on p LATE 

 the opposite side, which gives motion to a small car- ccxxxvm. 

 riage or frame g. This carriage is fitted into the space be- Fig- 3. 

 tween the bars a, a, and slides freely from one end of 

 the opening to the other by means of a rack h screwed 

 on one side of it to act in the teeth of the pinion L, 

 which turns on a centre pin fixed in the upper circle 

 BB. 



The frame g has the brass socket H within it fixed on 

 a centre pin extended across the frame. There is a hole 

 in the socket for the reception of the leg of the draw, 

 ing compasses H, which stand, as in Fig. 3. when in 

 use, the pen tracing the curve upon the paper, by the 

 weight of the compasses bearing upon it. 



The manner of fitting the frame g into the bars of Fig- * 

 the circle is shewn in Fig. 4 ; and also the socket H 

 moving on its centre pin, the rack d and the pinion K, 

 which are for the purpose of separating the circles, and 

 the other rack and pinion L, h for moving along the 

 frame g between the bars. The frame g is so fitted that 

 it continues at the same point with respect to the upper 

 circle B, when the two circles are separated from each 

 other by the motion of the pinion K. We have now 

 shewn that the circles can be set with any required 

 degree of eccentricity, and in this state are capable of 

 revolving in the frame; and also that the tracing point or 

 pen can be removed to any required distance from the 

 centre of the upper circle B. We shall now proceed to 

 point out the extreme cases of the instrument's action. 

 Suppose the two circles set by the pinion K, exactly 

 concentric with/each other, and the pinion L turned till 

 the end of the frame g comes in contact with the rim 

 of the circle, 'then the point of the pen M will come ex- 

 actly in the centre of both circles, which being turned 

 round in the frame by their handles, the pen will only 

 mark a small point on the paper, which will be the 

 centre of any curve the instrument may be afterwards 

 made to describe. By turning the pinion L, the point 

 of the pen may be removed to any distance from the 

 centre within the radius of the instrument; and it will 

 when turned round describe a circle which may be 

 made of any radius from the smallest point to the size 

 of the circles. This is the simplest case of the instru- 

 ment, and may be considered as an ellipsis when the 

 difference of its diameters is infinitely small. When the 

 circles are rendered eccentric, it draws an ellipsis, the 

 breadth of which will be determined by tuniing the 

 pinion L; and by the other K, the difference between its 

 breadth and length is regulated. Suppose K turned, 

 to render the circles concentric, without moving the 

 other pinion, the pen therefore remains in the centre of 

 the upper circle. In this case, the pen will describe a 

 straight line, equaljn length to twice the eccentricity of 

 the circles. This is evident, because,the circumference 

 of the upper circle BB, moving between the straight 

 edges F and G, its centre must describe a line parallel 

 to them. This case may be considered as an ellipsis with- 

 out breadth ; for if the pen is set the smallest quantity 

 out of the centre of the upper circle, it will describe a 

 very narrow ellipsis ; and by setting it at different dis- 

 tances from the centre, any required proportion of el- 

 lipses may be described. 



When the instrument is held down to draw, it is kept 

 steady by two sharp points, fixed in the ruler P, and 

 penetrating the paper. This ruler is united to the frame 

 by screws, on which the nuts N, O are screwed. The 

 screws are fitted into grooves in the ruler P, in which 



