SPRING-BALANCE, TnE, for determining the weight of bodies, consists of a spring in the form of a eyiindrical coil, through WIlidh passes freely a graduated bar, having a hook attached to its under end, and a plate to its upper. The spring is inclosed iu an oblong or cylindrical box, quite closed except at the bottom, where there is a hole just large enough to allow the free passage up and down of the graduated bar. When the instrument is to be used, it is suspended by a ring fastened to the upper part of the box; the weight to be estimated is then hung on the hook, and pulls down the rod. the button or plate at the top of which compresses the helical spring within against the bottom of, the box; and the graduation corresponding to this amount of compression of the spring is read off at that part of the rod which just shows itself outside. In another form of the spring-balance, known as Salter's balance, a brass index-plate is attached to the side of the box, and a vertical slit through both plate and box is made from top to bottom; the weight is in this case read off on the plate by a pointer fastened to the spring, and protruding through the slit. In a third form, known as litrtin's "index weighing machine," the interior rod, instead of being graduated, is furnished with a rack on one side; this rack moves a toothed wheel fastened on the side of the instrument; and this wheel, again, has at one extremity of its axis a long index, which, on the wheel being put in motion, traverses a circular dial-plate, on which the graduations of weight are marked. The advantage of this last construction consists in the arrangement of the
size of the toothed wheel to that of the dial plate, so that, since the toothed wheel and index make a complete revolution simultaneously, a small motion of the former may produce a large motion of the latter, and the weight of the body be much more accu rately read off than can be done directly on the graduated rod. The spring-balance has one advantage over the ordinary balance, that it does not estimate unknown weight by that which is known, and is therefore applicable to the determination of "absolute" weight in all latitudes. at the equator as well as at the poles; but it has the great disad vantage of being considerably affected by change of temperature, the force of the spring to resist compression being diminished as the temperature increases at the rate of for each degree of Fahrenheit, and consequently the apparent weights of bodies must be corrected in this proportion. Various other forms of springs, semi-circular, ellipti cal, etc., are employed, instead of the helical spiral, in several French balances, but in other respects the instruments correspond. The spring-balance is also called a " dyna momete•," from its being employed to indicate the intensity of the forces exerted by ani mils or machines; for this purpose, it is attached between the force and its object, the force being applied to its object solely through the medium of the dynamometer.