Chap. XIII.

DYXANOMETEIi.

1065

within *4 of an inch of each other, each perpendicular, which is *7 of an inch when thereis no pressure, is reduced to *5 of an inch. The brass limbs on which the scales are drawnis fixed on the centre of the arc A'B' of the spring, and the opposite arc A B carries acounterpoise ob , 3*1 inches long; the extremity 6 of this counterpoise acts on a smallbranch ftH, -3 of an inch of a bent lever, tile, whose other branch lie is a needle, 2-4inches long, from the centre, II, of rotation to the index, c. Below this index is a smallcylindrical thread, *1 of an inch, which is fixed to the needle lie, and serves as a foot whenit turns on the centre II, parallel to the limb. This first needle by turning commu-nicates a rotatory movement round the centre K to another needle Kdd f , which rolls onthe smooth portion of the screw K, and leans on the limb by a foot G furnished with awasher to reduce friction. This second needle has two indices d and d', to mark thepressure and traction ; it is moved by the first needle PI c ; the divisions of the arcsdescribed by the two indices are numbered in pounds, which indicate the weight broughtby the needle to these divisions: as long as the spring is not stretched, the needle Picpreserves its primitive position, and the needle K dd' of the scales remains on the divisions

Fig. 1633.

to which it is drawn by the tension of the spring; whence it is seen that the system oftwo needles gives us the power of preserving the measure of a tension, when the forcewhich produced this tension no longer acts. The greatest arc which the needle lie candescribe is determined by the course of the counterpoise a ft, which is *39 of an inch. Thepoints b and c of the bent lever GPIe describe arcs of the same number of degrees : withregard to the positions of the two needles He, K dd' to the right line II KL of theircentres, the angle c K L, which the second needle makes with this right line, is equal to theangle cPI L of the first with the same right line, increased by the vertical angle II cK of thetwo needles, for in the triangle IIcK, whose sides He and HK are constant, the anglecKII is the supplement of the angle cKL, and of the sum of the two angles cKII andIIcK. The distance II K of the centres II and K is about *46 of an inch, and it resultsthat the total arc described by the needle K dd' is nearly one-third of the circumference.

To preserve from injury the system of these three pieces, viz. the counterpoise aft, thebent lever ftHc, and the needle with two indices K dd\ the limb is covered by a plateNNN, which rests on the three pillars, *39 of an inch high. If the axis of rotation of thebent lever were prolonged, it would meet this plate in the point IT, the centre of the arcof a circle m! m" which terminates it, and whose radius is equal to the length of the needlelie. The divisions of the arc m f m' f are figured, and the figures indicate the same tensionsas those on the scale of traction.