A Machine for engraving metallic Plata. 67 



centre of the preiUng-fcrew ; and a line of divifion then drawn upon the moveable rule, to 

 which the letter itfelf muftbe added to diftinguifli it. The index, already mentioned, being 

 placed oppofite, and upon this firft divifion, will ferve to place immediately beneath the prefl- 

 ing-fcrew, the punch, «r rather the charadter, correfponding with the divifion u])on the rule, 

 without its being afterwards neceflary to infps£l the place either of the punch or the fcrcw, 

 with regard to each other. Confequently, as foon as the divifions which correfpond with ail 

 the punches iiiferted in the wheel, are engraved upon the fliraight rule, the fixed index will 

 immediately determine the pofition into which that wheel muft be brought, in order to place 

 the punches under the prefling-fcrew, in the order which the work may require. 



This regifter (tableau), for by this name I fhall hereafter diftinguiOi the rule and its index, 

 has no other funftion in the machine, than to guide the hand of the operator, and to fhcw 

 when the punch is very near its proper pofition beneath the prefling-fcrew. When this is the 

 cafe, the required pofition is accurately obtained by means of a detent, or catch. 



The detent which I ufe for this operation, is a lever with two tails, one of which is urged 

 toward the circumference of the wheel, by a fpring. To this extremity of the lever is fixed a 

 piece of hardened fteel, which has the figure of a wedge, which, by means of a fpring, is 

 preffed towards the axis of the great wheel, but may be relieved, or drawn back, by prefTure 

 on the oppofite tail of the lever, fo as to permit the great wheel to revolve at liberty. 



In the next place it mufl: be explained how this detent takes hold of the wheel, fo as to re- 

 tain it prccifely in the fituation neceflary to caufe any one of the punches, at plcafure, to give its 

 ibipreflion to the plate. For this purpofe there are a number of notches cut in the circum- 

 ference of the wheel, for the purpofe of receiving the detent. Thefe notches may be about 

 half an inch deep, wider towards the circumference than elfewhere, and it will be of advantage 

 that this outer width {hould be as great as the circumference of the wheel can conveniently 

 allow. By this contrivance, the wedge will not fail to prefent itfelf oppofite one of the notches 

 into which it will fall, and draw the wheel exaftly to its due fituation, even though the index of 

 the regiflrer fhouldnot be brought precifely to the line of divifion appropriated to any particular 

 letter. For, if this laft degree of precifion were required in working the machine, it would be 

 very prejudicial to the requifite fpeed which, above all things, is required in its ufe. When the 

 wedge is, therefore, left at liberty, it not only enters immediately into its place, and moves the 

 wheel till its two fides apply fairly to the interior furfaces of the notch, but retains the wheel in 

 this fl:ate with the neceflary degree of fliability. 



The method of giving the proper figure to thefe notches is very eafy. For this purpofe 

 neceflary, in the firfl: place, to imprefs all the characters contained in the wheel, on a plate of 

 copper or pewter. The fupport on which the plate is fixed muft be moved in a right line, after 

 each ftroke of the punch, through fuch a fpace, that the charaflers may be arranged one after 

 the other without touching. Now, as the perfect linear arrangement (fuppofing every other pari; 

 to be true) inufl: depend on the notches, it might feem fufficient to cut thefe according to the 

 method ufcd for the wheels of clock-work : but as it is very difficult to avoid fome obliquity 

 on the face of the punch, and, perhaps, in the hole through which it paflTes, it is in almoft every 



K 2 « cafe 



