70 Detail of thi CinJIru^kn sf 



are leaft elevated, and about two lines upon thofe which ftand Iiighcft. Whence the 

 -diiFerencc between the extreme heights does not exceed four lines. 



It is obvious, that fo fmall a difference is not fufficient to ftrike all the characters front 

 M to the letter /', when the wheel which governs the fcrew is put in motion by a conftant 

 weight, of which the impulfe, like that of the hammer, is increafed only by the acceleration 

 of its fall. It is evident that this requifite variation offeree might be had by changing the 

 weight; but it is equally clear, that the numbsrlefs and inceffant changes which the 

 engraving of an entire work would demand, would be incompatible with that degree 

 of fpeed which forms one of our firft requlfites. I was, therefore, obliged to render the 

 force of the weight, which turns the fcrew, variable, by caufing it to act upon levers of greater 

 or lefs lengths, according to the different quantities of impulfe required by the feveral 

 punches. For this purpofe, I adopted the following conftruftion. I connected by a fteel 

 chain to the wheel, which moves the fcrew, another wheel, having its axis horizontal, fo that 

 the two wheels refpectivcly command each other. They are of equal diameter, and the 

 chain is no longer than to make an entire turn round each wheel. This fecond wheel or 

 leading pulley, is intended to afford the requifite variations of force, which it does by means 

 of a fnail fixed upon its axis. The fnail is acted upon by a cord pafling over its fpiral 

 circumference, or groove, and bearing a weight which is only to be changed when a new 

 fet of punches for charadlers of a different fize are put into the great wheel. The 

 fpiral is fo formed, that when the weight defcends only through a fmall fpace, the part of the 

 cord, which is unwound, a£ls at a very ftiort diftance from the center of the pullyj but 

 when the fall is greater, the part of the fnail upon which it a6ts is fo far enlarged as to 

 afford a much longer lever, and, confequently, to give a proportionally greater effedl to 

 the ftroke. This conftru£tion, therefore, by giving the advantage of a longer lever to a 

 greater fall of the fcrew, affords all the power which the nature of the work and the 

 different fpaces of the letters demand. 



The fupport on which the plate is fixed, muft, as has before been remarked, move fo as to 

 form ftrait lines. This motion, which ferves to fpace the different charaders with precifion, 

 is obtained by means of a fcrew, the axis of which remains fixed, and carries a female fcrew or 

 nut. The nut itfolf is attached to the fupport of the metallic plate, which receives the 

 letters and carries it in the right lined dirediion without any deviation, becaufe it is confined 

 in a groove formed between two pieces of metal. The fcrew is moved by a lever whicri 

 can turn it in one direcStion only, becaufe it acts by a click upon a ratchet-wheel, which is 

 fixed to the head of the fcrew. The adtion of this lever always begins from a fixed flop ; 

 but the fpace through which it moves is variable, according to the refpetSlive breadths of the 

 letters. This new confideration induced me to fix upon the rule or plate of the regifter, a 

 number of pins, corrcfponding with the different divifions which anfwer to each punch : 

 thefe pins determine the diftance to which the lever can move. It, therefore, becomes a con- 

 dition that its pofition in the machine fhould be oppofite the fixed index which determines 

 the charader at any time beneath the prefling-fcrew. The lever and its pin are, therefore?, 



the 



