Chap. XX. Of the Drill-Boxes. 317 



will it thruft the Tongue from its Middle, if its 

 Strength be fuperior to the Force that refills it, as 

 it ought to be when a Notch is pafled and before the 

 next: This Motion of the Tongue is called its Play. 



In order to meafure the Diftance (or Quantity) of 

 this Motion, we muft confider, that the Tongue, 

 moving on its Axis above, defcribes with its lower 

 End the Arch of a Circle, the Chord of which Arch 

 is the Meafure required. 



To meafure this by the Angle the Tongue makes 

 at its Centre, would be no Rule for making Boxes ; 

 becaufe fome Tongues are longer, fome fhorter, in 

 proportion to the different Diameters of the Spindles 

 they move againft ; and yet the Play of the fhorteft 

 muft be as much as that of the longeft, that is, it 

 muft defcribe as great an Arch at the Place of Pref- 

 igure (defcribed in Fig. 3. Plate 2.) ; and therefore the 

 fhorteft Tongue would make the greateft Angle. 



A fhort and eafy Way, then, for a Mechanic to 

 meafure, is thus : Screw in the Setting-fcrew until 

 the Tongue come within a quarter of an Inch of 

 touching the Spindle ; then take out the Spindle, 

 and from the Centre of the Hole draw a Line on the 

 Side of the Mortife, perpendicular to the Tongue, 

 and at the Tongue's Edge make a Mark with the 

 Compaffes, or a Pen •, then force back the Tongue 

 againft the Setting-fcrew as far as it wiil go (that is, 

 until the Spring touch the whole Back of the 

 Tongue) ; produce the faid Line to the fame Edge 

 of the Tongue, or fet the End of the Rule thereon, 

 and draw another Line, by the Rule, from the Mark 

 to the Edge of the Tongue, when fartheft back, and 

 there make the fecond Mark. The Ruler ufed this 

 Way will fhew both the Perpendicular, and the Mea- 

 fure. 



But yet a quicker Way is, to fet the Tongue, by 

 the Setting-fcrew up to the Edge of the Hole ; and, 

 when it is forced back, meafure from the Tongue 



3 tQ 



