230 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



[July, 



If now the carriage with its stylus be moved parallel to itself over 

 the surface of a drawing board, it is clear that any given point of 

 the carriage will describe precisely the same path as the point of the 

 stylus does, and since the pencil is attached to the carriage, this will 

 also be true for it; so that if the tracing point C of the stylus be 

 made to pass transversely across a series of mouldings, and the 

 point be at the same time kept always in contact with their surface, 

 the pencil will simultaneously describe upon the paper the exact form 

 of the section of these mouldings of the same size as the original. 



But the surfaces of mouldings are inflected in various directions to the 

 right and to the left, and the tracing point of the stylus must therefore 

 be capable of following such changes of direction. This it is enabled to 

 do bv its curved form combined with its power of rotation about its 

 axis; for by turning it, it can be applied at various degrees of incli- 

 nation, either to the right or left side of a moulded surface, as the 

 form of the latter may require. And as the rotation has been shown 

 not to affect the actual position of the tracing point C with respect to 

 the carriage, so neither can it affect the truth of the section drawn by 

 the pencil. 



For example, at C the tracing point is applied to the right of the 

 stone surface, but at X on the opposite side of the rib it is applied to 

 the left of that surface, and is also turned into a different angular 

 position to enable it to enter the hollow. But in passing from X to Y 

 it must be gradually turned into the position shown by the dotted 

 lines, to enable it to clear the projection at Z. In fact, during its 

 progress across a rib of this kind, the point must be carefully watched, 

 and the stylus turned into the best angular position that the momen- 

 tary form of the moulding requires. 



i have said that the carriage of the stylus is always to move parallel 

 to itself, and will now describe the means by which this is compelled. 

 The base of the machme is a drawing board of mahogany F G (| in. 

 thick), 10^ in. by Hi in., and made to fold in the middle like a book, 

 for the convenience of carriage. When in use it is kept open by a 

 button R on the lower surface. The carriage is guided by a parallel 

 motion somewhat resembling a double parallel ruler, but the propor- 

 tion of whose jointed arms are altered to suit the different circum- 

 stances of its employment. 



A plate H is secured to the board by a button-headed screw, K, 

 having a milled headeil nut below at L. As K merely enters a 

 notch in the plate H, a single turn to right or left of this nut L is suffi- 

 cient to detach or fix the plate, together with the instrument, while 

 the slip of metal M, screwed to the board, serves to keep the plate in 

 its proper position. 



Two arms N N of equal length are jointed to the extremities of H, 

 and also to an arm P of the same length as H ; consequently P will be 

 parallel to H in all positions. Two other arms Q Q, also of equal 

 length, are jointed at one end to the arm P, and at the other to the 

 carriage of the stylus, so that the latter will be parallel to P in all 

 positions, and consequently to H. This arrangement enables the car- 

 riage to move freely and steadily from one side of the board to the 

 other, and over every part occupied by the paper, and at the same 

 time keeps it always parallel to the plate H. The paper TS, upon 

 which the mouldings are to be copied, is secured to the board by a 

 pair of spring catches V, W, of which the last is placed near the cor- 

 ner S of the paper, to prevent the latter from being caught and turned 

 up by the arm N of the instrument during its motion. 



It is absolutely necessary that the board of the instrument should 

 be held steadily, in the same position, against the mouldings during 

 the process of tracing. The two retaining pieces ac, b(l,3.xe provided 

 for this purpose ; these are attached to the lower side of the board by 

 thumb-screws, e,/. In Fig. 3 they are shown in the position requisite 

 for closing the instrument, but in Fig. 1 they are in the position for 

 use. When the thumb screws are relaxed, either of the retaining 

 pieces may be drawn out, and turned to the right or left so as to touch 

 some convenient projection of the moulded surface, and thus when 

 clamped fast, to retain the instrument in the same position so long as 

 it is pressed into contact with the mouldings. 



This adjustment of the retaining pieces must always be made pre- 



viously to taking the profile of any mouldings ; the board must then 

 be grasped in the left hand at O, and held firmly against the mould- 

 ings. The button at D is managed by the right hand, which will be 

 found sufficient as well to guide and turn the point of the stylus as to 

 keep the pencil in contact with the paper, and to raise it off the paper 

 when necessary, which the elasticity of the parallel motion will allow 

 of easily. 



Fig. 4 is a section of the edge of the board at W, to show the form 

 of the paper springs. These, which are also seen at It k, li k, Fig. 3, 

 are screwed to the lower side of the board in sunk recesses, and are 

 released from the paper by pressing them at k. When the plate H, 

 with the parallel motion and carriage, are detached from the board, 

 they may be deposited in a recess sunk for the purpose on the lower 

 surface, as shown at i m ; the button R is then to be turned into the 

 position shown by the dotted lines, and the board may be folded to- 

 gether, and secured by a hook in front. The recess at n receives the 

 stylus, and those at s,s, s, receive the milled nuts e,f, L, respectively. 

 When a series of mouldings are to be copied which exceed the 

 limits of the paper, they must be taken piecemeal, as shown at Fig. S. 

 This figure shows how the entire rib of Fig. 1 may be represented, 

 which rib is too large to be entirely comprehended within the boun- 

 daries of the paper. In this and similar cases it is better to apply 

 the instrument against the right and left faces of the rib in succession. 

 In the first operation the mouldings from o top (Fig. 1) wiU be taken, 

 and in the next operation those from q to e (the same letters are em- 

 ployed in Figs 1 and 5). 



Care being taken always to draw an overlapping portion of the 

 mouldings in the successive drawings as in this case from q to p, there 

 will be no difficulty in joining the separate pieces into one continuous 

 line by tracing them on a larger sheet. 



A small black-lead pencil and common paper may be used for the 

 purposes of this machine, but as the point is very liable to break and 

 become troublesome, I find it much more convenient to employ the 

 metallic paper, which is prepared by Messrs. Harwood, SG.Fenchurch 

 Street, and requires only a brass or other metallic point instead of 

 black lead. Besides this advantage, the trace is indelible. The paper 

 for the instrument should be cut to the proper size, and carried in a 

 portfolio. By a somewhat different arrangement of the instrument, 

 the paper might be made to remain always in its place on the drawing 

 board in the form known by the name of a block, from which the 

 sheets are to be detached as fast as the drawings are made. This 

 would increase the size of the machine, however, by making it exactly 

 as large as the paper, and the folding hinge would be placed in a 

 direction transverse to that which it occupies in the present design. 



This instrument requires no great accuracy of workmanship. The 

 two essential points upon which the precision of its action depends are, 

 1st, that every pair of arms N N, Q Q, of which the parallel motion 

 consists shall have exactly the same distance between their joint holes. 

 This is easily secured by drilling them all separately, from a pattern arm 

 made previously. Similarly the holes in the plates H, P, and the 

 carriage, must be at the same distance respectively. 2nd, that the 

 point of the stylus shall be in the axis of rotation. This is also easily 

 effected, and is as easily verified at any moment by twirling the stylus 

 in its carriage opposite to a fixed point. 



The parallel motion may be made of thin slips of sheet iron ri vetted 

 together, but better of sheet steel. The hinges of the board must be 

 so placed that when it is closed or folded, there may be room between 

 its inside surfaces for the retaining plates e/and the button R, which 

 must be made all of sheet iron or brass of the same thickness. 



The length which I have given to the arms of the parallel motion, 

 is just sufficient to allow the pencil to travel over the board. The 

 limit of the motion to the right will be found to be that the arras Q Q 

 will come into contact, and thus prevent farther motion in that direc 

 tion ; and similarly the limit to the left will be the contact of the arms 

 N N. If the size of the instrument is increased, th« same proportions 

 must be preserved between the arms and board. 



R. Willis. 



Cambridge, June 10, 1842. 



