1 1 6 Description of an improved Drawing Board and T Square. 



the point from going to any considerable depth, so as to permit 

 large liole.s to be made in the paper; and the drawing beii.g com- 

 plete, wliatever holes are made in the zinc may be bmnished 

 down bv rubbing the thumb nail over them, which will sufficiently 

 close them. 



When oxidation of the metal takes place, it should be suffered 

 to remain, because being white, it helps to render the thin paper 

 opaque, and consequently the lines drawn upon it will be the 

 more plainly seen. 



When the sides of the frame of a drawing-board are straight 

 lines and nicely perpenclicular to each other, parallel and perpen- 

 dicular lines may be very correctly drawn with a good T square ; 

 but as wood is continually warping with every change of whether, 

 accuracy cannot be expected from such imperfect means. 



To prevent any error arising from this circumstance, I screw 

 a solid rim of brass upon the upper surface of my frame, per- 

 mitting it to project a small distance beyond the outer ecli;es of 

 the wooden frame, so that the stock of T square may slide against 

 either edge. This brass rim beiug dressed very true and at right 

 angles, will remain so for any length of time, as nothing l>ut ex- 

 traordinary violence can injure it. 



I can assure the Society, that the present invention has sprung 

 from necessity, originating in the very minute size of the en- 

 gravings of the present day, added to which, a degree of accuracy 

 is required, that can only be attained by a corresponding improve- 

 ment in the apparatus ; and for effecting such purposes, the 

 zinc plate may be applied to a common drawing-board, and the 

 brass rim to the frame ; therefore those persons who possess a set 

 of drav/ing-boards may have them improved at a small expense. 



The same objection that ajiplies to the frame of a drawing- 

 board, applies equally to a T square made with a uooden blade 

 and fixed immovably to its stock; for if ever any injury happens 

 to the fiducial or drawmg edges, by a blow causing an indention, 

 or by the wood warping through a change of weather, it must 

 remain, because the blade being glued fast to the stock, does 

 not admit of being corrected without considerable difficulty. 



The square which I have the honour to present, is so con- 

 structed as to permit the blade to be withdrawn from the stock, 

 for the purpose of <orrection, should any accident occur to it; 

 and the same means which permit the blade to be removed for 

 the purpose of correction and adjustment, enable the draughts- 

 man to use it as a bevil at any angle, where it may be fixed by a 

 clamp and thumb screw. 



Should objection be formed to the weight of this invention, 

 I beg leave to state that the same may be made in ebony or box 



wood, 



