DESCRIPTION OF A SECRET LOCK. 277 



over the brafs which refts on an elaftic plane, which yields (oConftrudlion of 

 the unavoidable irregularities of execution in a work of a mo- ceffible onJy to* 

 derate price. the proprietor. 



It muft likewife be obferved, that the heads of the fcrews, 

 as well.as the face itfelfof the lock, ought to be cafe hardened, 

 in order to protect it from external injuries, and to prevent the v 



enlargement of the holes which receive the fcrews. The fur- 

 face ought to be highly polilhed, and the heads of the fcrews 

 fhould be blue. 



Advantages of the new Loch 



IJi, As in Reignier's lock, which likewife has fcrew-heads, 

 ferrils, and teeth without wedges, thefe pieces cannot be put 

 at diftances extremely precife, it neceffarily follows that one 

 of the pins or teeth, the fourth for example, will always meet 

 its fernl before the others meet theirs. Let us fuppofe that 

 with a turnfcrew we endeavour to turn the fcrew-head which 

 moves the bolt, whilft with another hand and turnfcrew we 

 fucceffively turn the other fcrew-heads, they will all turn 

 freely, except a fourth; which will be immoveable; It will be 

 concluded that the fourth fcrew-head is not in the pofition to 

 open the door, that is to fay, that the deep cut of the ferril is 

 not oppofite the pin of the bolt: that figure or letter of the 

 fcrew-head will be noticed among thofe which do not foFm the 

 fecret. The petition of the fcrew-head muft therefore be 

 changed, and the above procefs repeated till the pin is felt to 

 fall into the notch. The fame experiment muft then be made 

 with that fcrew-head among the remaining ferrils which alone 

 is now preffed by its pirn And after eighty attempts at moil, 

 ii' there be eight fcrew-heads, each having eleven figures, we 

 lhall know the eight figures which compofe the fecret. 



Here then is a regular and infallible procefs to difcover in 

 lefs than an hour the combinations with which the lock is 

 clofed. This defect, which is common to the generality of 

 compounded locks, does not exiftin mine. 



When my bolt is moved, the wedges which precede the 

 teeth enter at the fame time into the cuts of the ferrils, which 

 are of fuch a height that they graze the interior furface of the 

 lock : all the four ferrils then become immoveable, and it is im- 

 poftible to difcover which is the ferril that is rirft met by the 

 ££>rrefpondent tooth, becaufe tiieir fixation is produced at the 



fame 



