ISO 



DRAWING INSTRUMENTS. 



w hii-h the lines should converge will often fall at a dis- 

 tance of !'_' and l,"> iirt from tin- j)irture, so as to ren- 

 der it impracticable to use rulers; and tlu-rr i*. except 

 this instrument and another recently invented,* no other 

 metluxl. It consists of' three rulers A, H, mid I), which 

 an- united by a common centre screw ; and have a 

 thumb screw d, which fixes them fast, at any jingle 

 where they may be placet!. E, F are two fixed weights, 

 against the edges of which the rulers AH are applied, 

 when the instrument is used ; or pins may be fixed in- 

 to the table, to answer the same end. By sliding the 

 instrument against these stationary points, as shewn in 

 the Figure, the ruler D will draw lines as shewn dot- 

 ted, which are all convergent to a common centre, the 

 distance of which will depend upon the angle of the 

 rulers A, B, and the situation of the points E, F. The 

 edge of the ruler D must in all cases be made to bisect 

 the angle of the other two. The manner of setting the 

 instrument is this : Having given the two extreme lines 

 r, s (dotted,) which converge to the intended point, we 

 suppose it is required to draw a number of others to 

 the same point. The pins or weights E, F must be set up- 

 on these linef, but situated equally distant from the cen- 

 tre point. To find their situation, place a pair of com- 

 putes with one point between the two lines r, s, so si- 

 tuated, that when a circle is described by the other 

 point, the two lines will be tangents to it. From this 

 point, as n, with a greater opening of the compasses, 

 mark off two points, as at K, F, upon the lines, and these 

 will be equidistant from the centre ; and here place the 

 weights, or fix the pins. Now apply the instrument to 

 them, with the clamp screw d loose, and slide the rulers 

 against the pins, till the ruler D comes to one of the 

 lines r or t ; and here incline the rulers on their centre 

 joints, till the edge of D exactly corresponds with the 

 line, when the other rulers are kept in contact with 

 the fixed points. Now remove the instniment to the 

 other of the two lines, and adjust it in the same manner 

 that the edge of D may correspond with it. The clamp 

 screw being fastened, fixes the rulers as they are ad- 

 )ti>ud ; and then, on sliding the two A, B, against the 

 pins, the edge of the third, D, will in all positions tend 

 to the same centre point as r, *, &c. The angle C, in 

 which the rulers A, B meet, will in this motion describe 

 a segment of a circle, as shewn by the dotted lines ; and 

 the centre to which the lines tend will be found in the 

 opposite circumference of tliat circle, by bisecting the 

 distance EF upon the dotted arch, and from this point 

 drawing a line through the centre of the circle till it cuts 

 the opposite circumference, and to this point the lints 

 will converge. 



If the instrument is required to have a greater range 

 than between r and s, other pins must be fixed for it to 

 act against, taking care that they are at the same dis- 

 tance from E or F as these are from each other. Their 

 proper situation will IK- determined by the rulers A, B 

 themselves, thus sliding the rulers against the pins till 

 the angle C comes to one of the points E or F. The 

 new point must be fixed i:i contact with the edge of 

 the ruler, which is unsupported, and at the same dis- 

 tance as is between E and F. 



The construction of the instrument will be apparent 

 from in inspection of the Figures. The two rulers A, B 

 have circular parts behind the centre, which apply one 

 upon the other ; and a projecting part b from the ruler 

 D lies over both, the centre passing through all three. 

 An arched groove is cut through both the circular 



parts, to admit tlie screw il, which also puses through nw|ng 



/, and fasten* them all three together, by screwing 



a nut, which is fitted into the arched groove of the -_" U " *[_- 



lower one. The ruler I) is math- of wood or ivory, as 



(hewn separately in l-'ig. 6', mid screwed to the under 



side of/, so that it comes into tile smiie plane with 



AB. The instniment will draw parallel lines li,.,i 



AB are set in a straight line ; and if the circular part 



i^ graduated, it will make a good protractor. 



l-'i.if. . is an extra ruler, to be applied in lieu of D. I' 1 ^ ' 

 When the instrument is required to draw lines to a'.' xx '! TI " 

 centre on the opposite side it is merely rc\cr-cd to tile *"' **" 

 other, having the hole in L, which is for the centre, in 

 the line of tile opposite edge, to that which is shewn 

 in use ; one being to draw lines tending towards the left 

 hand side, and the other towards the right hand. 'l}\]< 

 instrument has been lately rewarded by the Society of 

 Art*, who have also rewarded Mr Peter Nicholson, for 

 the invention of an instrument for the same pur) 

 which he calls a centrolinead It is on a new and 

 very ingenious principle. See the Trantaclions of the 

 Society of Art t, vol. xxxii. 



Ellipses are curves so frequently required by all ar- lartru- 

 tists who draw in perspective, that instruments for" 11 

 drawing them to any size or proportion, are almost in- J~ 

 dispensible to produce correct representations of circu- 

 lar objects. The trammel, or elliptic compasses, Figs. Ir*" 1 " 1 ^ 

 1 and 1 1 , is the only method which is in general use. ' 

 It consists of a brass cross, AA, BB, having two dove- 

 tailed grooves crossing each other at right angles. In 

 these grooves, sliders are fitted to move freely, and 

 without looseness. The sliders are perforated with 

 holes for the admission of small pins, which project 

 downwards from the sockets CD of a small beam com- 

 pass, at the extremity of which a tube E is fixed, and 

 in this a drawing pen is received. The cross is sup- 

 ported upon points, which act as legs, and at the same 

 time point out the direction of the two diameters of 

 the ellipses, which the pen will trace when the beam 

 is turned round. In doing this, the sliders being uni- 

 ted together by the beam, compel each other to ad- 

 vance and retreat in their respective grooves; and thus, 

 by a constant alteration of the centre, produces an el- 

 lipse, instead of the circle, which would be drawn if 

 the centre was immoveable. 



The rule for setting the trammel is to make ED 

 equal to half the conjugate diameter of the intended 

 ellipse, and EC equal to half the transverse diameter; 

 therefore the distance DC will be equal to the diffe- 

 rence of the two semi-diameters. Fig. 11, will ex- 

 plain this more clearly. This instrument is very de- 

 fective, because it will not draw narrow ellipses, nor 

 small ones, without interruption to the curve, by 

 the arms of the cross, as shewn in Fig. 1 1. For very 

 small om*, it cannot be used at all; neither will it draw 

 ellipses which approach very near to circles, because 

 the sliders cannot be brought sufficiently near together. 

 Another inconvenience is, that when the cross is once 

 placed, it must remain, and cannot be adjusted till it 

 come to the exact position required upon the paper. 

 For these reasons the instrument is not applicable to 

 perspective drawings, as the greater portion of those el- 

 lipses which occur could not be drawn by it. 



Fig. 9. is an elliptograph invented by Mr Farey, F ''<7'" 

 which is so general in all its applications, as to draw JT 1 ,"^"^ 

 any ellipses whatever within the si/e of the instrument, 

 and is readily adjusted. It consists of two circles A, B 



See Brtwstcr'i Treatise m A r c ftilut<>}'hical Initrumcntt, p. 189. 



