ON DRAWING, WRITING, AND MEASURING. 77 



employment of alphabetical and other arbitrary characters ; at present it 

 is usual to have six boards,* each turning 011 its axis so as to appear or 

 disappear at pleasure : these admit of sixty-four combinations, which are 

 sufficient, besides indicating the letters of the alphabet, for every other 

 purpose that can be required. (Plate VI. Fig. 80, 81.) 



Pens for drawing lines and figures differ sometimes from those which 

 are used for writing ; they are made of two plates of steel inclined to each 

 other, and adjusted by a screw ; or sometimes of a plate of tin folded up, 

 so as to include a receptacle for the ink ; or of a glass tube drawn to a 

 very fine point, and still remaining perforated. In all these pens, as well 

 as in common pens, the ink is retained by its cohesion, and by the 

 capillary attraction of the pen ; and it attaches itself to the paper by the 

 operation of similar powers. 



It is by no means easy to comply strictly with that postulate of geometry 

 which requires us to draw a straight line from one point to another. The 

 edge of a ruler is made straight by the instrument called a plane, which is 

 worked with a considerable velocity, and therefore naturally tends to 

 move in a right line, besides that it is guided by the flatness of its lower sur- 

 face. We judge of the straightness of a line, by means of the well known 

 property of light, which moves only in right lines, so that if we look along 

 the edge of a ruler, we easily discover its irregularities ; and this may be 

 done with still greater accuracy, if we look through a small hole made 

 with a pin in a card. Rulers of silver, brass, or ivory, have a material 

 advantage over those of wood, as they are not liable to be spoilt by warp- 

 ing. A pen filled with ink cannot be applied close to the edge of a ruler 

 without inconvenience ; it is therefore best, for diagrams which require 

 great accuracy, to draw the lines first with a steel point, or a very hard 

 black lead pencil, and to finish them with ink if necessary. The paper 

 should also be fixed on a drawing board ; and plates of lead or copper may 

 be employed, instead of paper, for very delicate purposes. The carpenter's 

 chalk line is a useful instrument for supplying the place of a very long 

 ruler ; it becomes straight when it is stretched, because a right line is the 

 shortest distance between any two points. 



For drawing a circle of a given radius we use compasses, with one point 

 generally of metal, the other of various descriptions, t Compasses are 

 sometimes made with a spring instead of a joint, and opened or shut by a 

 screw : sometimes a graduated arc is fixed in one leg, and passes through 



* This species of telegraph was invented in 1695, by Lord G. Murray ; it was 

 adopted by the Admiralty until the end of the late war, when it was discontinued, 

 and the semaphore, consisting of two arms projecting from an upright 

 post, and working about pivots, was substituted in its place. In this 

 instrument each arm has seven different positions, which afford by 

 their combinations forty-nine different arrangements. Consult Edge- 

 worth, Trans. Roy. Irish Ac. vi. 95, 319. Nicholson's Journal, ii. 319. 

 Chappe, Breguet and Betancourt, Bulletin de la Soc. Ph. n. 16. 

 Mem. de 1'Institut III. H. 22. Ph. Mag. i. 312. Nocturnal Tele- 

 graph, Rep. of Arts. x. 28. Boaz's Patent Tel. ibid. xvi. 223. Ph. 

 Mag. xii. 84. Ency. Brit. Art. Tel. Pasley, Description of the Uni- 

 versal Telegraph, 1823. Chappe, Histoire dela Telegraphe, 2 vols. Paris, 1824. 



f Duval's New Compasses, Mem. Paris, 1717, H. 83. Leup. Th. Art. t. 20, 

 a. b. 



