100 LECTURE X. 



drawn in clIfFerent directions, so that each Avord, when cut out, might indicate 

 the page to which it belongs. 



An ingenious instrument has lately been constructed, by means of which 

 copies may be multiplied with great facility ; it is called the polygraph, and 

 consists of two or more pens, so connected by frames and springs, as to move 

 always in parallel directions, each having an inkstand and a sheet of paper for 

 itself. In this manner five copies may be made at once with tolerable facility, 

 and themethod may perhaps hereafter be extended to a much greater number. 



A mode of Avriting, perfectly different from any of those which have been 

 mentioned, is performed by means of the telegrapli, which is justly consider- 

 ed as the invention of the ingenious Dr. Hooke. The ancients had attempted 

 something similar, by the exhibition of torches on elevated situations; but Dr. 

 Hooke observes, that the addition of the telescope is absolutely necessary for 

 the practical success of the process ; and the directions which he gives for its 

 performance differ very little from the plan which has since been generally- 

 adopted, first in France, and afterwards, with some variations, in this coun« 

 try. Dr. Hooke proposed the employment of alphabetical and other arbitrary- 

 characters ; at present it is usual to have six boards, each turning on its axis 

 ■so as to appear or disappear at pleasure: these admit of sixty four combina- 

 tions, 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 ta 

 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 



