tract 34.

IN GUNNERY.

319

tape drawn out in a vibration, it is evident, is the chord ofthe arc described, and counted in inches and tenths, to theradius measured from the middle of the axis to the bottom ofthe tongue.

16. This method however was still found to be attendedwith some trouble, and many inconveniences, as well asdoubts and uncertainty sometimes. For which reasons weafterwards changed this way of measuring the chord ofvibration, for another, which answered much better inevery respect. This consisted in a block of wood, havingits upper surface ef formed into- a circular arc, whosecentre was in the middle of the axis, and consequentlyits radius equal to the length from the axis to the uppersurface of the block. In the middle of this arch was madea shallow groove of 3 or 4 inches broad, running along themiddle, through the whole length of the arch. This groovewas filled with a composition of soft-soap and wax, of aboutthe consistence of honey, or a little firmer, the upper sidebeing smoothed off, even with the general surface of thebroad arch. A sharp spear or stylette then proceeded fromthe bottom of the pendulum or gun-frame, and so low asjust to enter and scratch along the surface of the compo-sition in the groove, without having any sensible effect inretarding the motion of the body. The trace remaining,the extent of it could easily be measured. This measure-ment was effected in the following: manner :—A line ofchords was laid down upon the upper surface of the woodenarch, on each side of the groove, and the divisions markedwith lines on a ground of white paint: the edge of a straightruler being then laid across by the corresponding divisions,just to touch the farthest extent of the trace in the com-position, gave the length of the chord as marked on thearch. To make the computations by the rule for the velo-city easier, the divisions on the chords were made exactthousandth parts of the radius, which saved the troubleof dividing by the radius at every operation. The mannerin which this line of chords was constructed on the face