3 1 6 Of the Drill-Boxes. Chap. XX. 



The common Thicknefs is about that of a Shil- 

 ling (a). 



The Degrees of Stiffnefs are meafured in this 

 manner ; viz. Fix Two Boards together, leaving a 

 Chink betwixt them, in one Place of an Inch long; 

 lay the Spring (when feafoned acrofs this Chink) with 

 its Middle exactly over it •, then put a String over 

 the Spring, which may pafs with both Ends thro* 

 the Chink, and tie fo much Weight to the Ends of 

 the String under the Boards, that will pull down the 

 Middle of the Spring, till it touch the Chink, and is 

 ftrait with both its Ends ; This will fhew the Degree 

 of Stiffnefs. But note, That the Spring muft be 

 crooked, and bear only upon its Ends, with the hol- 

 low Side upwards. 



If ten or a dozen Pounds Weight pull it down to 

 the Board, it is a good Degree of Stiffnefs, for a 

 large Box : We are not confined to be very nice or . 

 exact in the Degree of Stiffnefs; for by our Fingers 

 prefiing it, we that are pracYifed in it, know well 

 enough, whether a Spring be of a fufficient Degree 

 of Stiffnefs, without weighing it ; but for fuch who 

 are unacquainted with them, it is bed not to truft 

 to Guefs, but Weights; and to adjuft the Stiffnefs to 

 that of a Spring, that has been known to perform 

 well. 



The Spring muft bear againft the Back of the 

 Tongue at each End, and lie hollow in the Middle: 

 But the Degree of Hollownefs of the Spring is very 

 material ; for thereon depends the Diftance of the 

 Tongue's Motion towards the Spindle by Force of 

 the Spring, and back again quite to the Setting-fcrew, 

 by the Seed that is preffed againft it by Force of 

 the Notches, when they are moved by the Wheels ; 

 becaufe the more the Spring is curved, the farther 



(a) Not quite fo thick as a milled Shilling, but rather of an 

 c\d broad ftamped Shilling, which is a little thinner. 



Will 



