In the centre of the semicircular plate C, (Fig. 2.) a light steel index I' is fixed upon its axis by a screw, and has a double point, viz. at in and n, for indicating the di visions on both scales. A small piece of leather, or cloth, is glued upon the lower side of the circular part C, to render uniform, and diminish the friction on the plate. The point in of the index, and the scale or myviogrammcs is employed in all experiments, in which the spring is to be pulled in the direction of its greater axis AA, and the point n aryl the scale of kilogrammes is used, when it is to be employed for experiments in which the two sides of the spring are to be compressed.
The mechanism now described is covered with a small plate of brass CD, Fig. 5, to prevent it from being in jured. Upon this plate is a divided arc, corresponding with the first arc of the machine, and by the play of a small index b (Fig. 4.) under the plate, the movements of the spring may be ascertained. Through the aper ture K, a small turnscrew is introduced, for the purpose of relieving or tightening the index, when necessary.
A pallet of brass L, Fig. 3. has a screw, with a cap like that on the needle in the mariner's compass, in which the lower pivot of the lever that pushes rc-.ind the index is made to play. This pallet, acting like a spring, yields to any sudden concussion, and prevents the mechanism from being deranged. AI (Fig. 5.) is a socket rivetted on the plate CD, in which the upper pivot of the lever turns. N, N, N, are small cylindrical pillars, to which the plate CD is fixed by three screws.
When it is intended to try the strength of the human body, it is necessary to have an iron rack, Fig. 6. on the lower part of which the feet of the person must rest ; and also a double handle of wood, with an iron hook, Fig. 7. which lie must at the same time hold with both his hands.
When the strength of animals is to be determined, it is necessary to have a double iron hook, shewn in Fig. 8. one end of which is to be hooked to the end of the spring, and the other to a rope fixed to a stake, as at c, c, Fig. 9.
The three methods of using the dynamometer are re presented in Fig. 9, 10, and 11.
Fig. 9. represents the method of applying the dyna mometer to ascertain the strength of animals. The ani mal is yoked to the chains PQ, and the force which it exerts against the fixed obstacle R is shewn by the in dex of the interposed dynamometer T. The pin d, Fig. 4. pushes forward the index, which always remains at the place to which it is brought. M. Regnier made experi ments with four excellent horses, and obtained the fol lowing results : Fig. 10. shews the method of determining the strength of the hands, or the muscular force of the arms. The person lays hold of the two sides of the spring nearest to the centre, so that his arms may be a little stretched and inclined downwards, at an angle almost of 45 degrees. This position is considered by Regnier as the most na tural, and as that in which a man can exert his whole force. The index will then point ont on the scale of ki logrammes the force which has been exerted. The strength of each hand may be tried separately, and the sum of the two will always be found equal to that of both when exerted together.
Fig. I 1. represents the method of measuring the strength of the reins of the human body. The person places his feet on the bottom part of the rack, shewn separately in Fig. 6, after one of the ends of the spring has been placed in one of the hooks of the rack, and the hook of the handle, Fig. 7, put into the other end of the spring. The strength which he exerts in this position is then indicated upon the scale of my•iogrammes.
The preceding dynamometer is obviously an instru ment of great use; and as it does not weigh more than two pounds and a half, there are few practical purposes to which it may not be conveniently applied. For cases,
however, where the load or the resistance is stationary, as in those represented in Fig. 9, 10, and 11, we think that a much simpler and more accurate dynamometer might be constructed in the following manner.
Let ABCD (Fig. 12.) be a vessel containing water, and EF a long cylinder, made of any substance heavier than water, suspended in the fluid by a rope Gill at tached to the hook G, and passing over a pulley II. When the upper surface E of the cylinder is on a level with the surface of the water, the weight of the cylinder, or the force which it exerts upon the rope Gill, will be equal to the absolute weight of the cylinder in air, dimi nished by the weight of a quantity of water of the same magni•ude as the cylinder. When a horse or a man pulls at the rope III, so as to raise the cylinder above the fluid surface, the weight of the cylinder will gradually in crease; and if the magnitude and specific gravity of the cylinder are duly adjusted to the description of force which is applied, there will be a particular position of the cylinder, at which its weight will exactly balance the force which is applied at I. The forces which are then in equilibria or the force required to be measured, will be equal to the absolute weight of the cylinder diminish ed by the weight of a quantity of water equal to the mag nitude of the immersed part of the cylinder. In this way the force of men and animals, and of particular parts of the human body, may be ascertained with the utmost accuracy and facility; and by fixing a scale upon the cy linder, the measure of the force in cwts. or pounds may be seen by simple inspection. The length of the scale will obviously be increased by lengthening the cylinder and diminishing its diameter ; or a dial plate with an in dex may be readily applied, so as to obtain the most mi nute subdivisions.
In measuring the strength of the reins, we have only to apply a hook, similar to that shewn in Fig. 7. to the book G; the person who exerts the force standing upon a board placed at a convenient height from the ground; or the same thing may be effected by sinking the vessel AB either wholly or partly into the ground.
If we suppose the solid cylinder EF to weigh two cwt. and to be made of a solid whose specific gravity is twice that of water, then its weight, when wholly hn merscd in the fluid, will be only one cwt. and by means of it all forces between one and two cwt. may be accu rately measured. As the specific gravity of the cylin der is increased, the range of the scale will diminish in the same proportion. A greater range of scale, hov, ever, may be obtained by making the cylinder hollow ;— and by increasing its magnitude, any force front sinallest to the greatest may be ascertained. It the cy linder, for example, weighs six cwt. in air, and is made so hollow as to weigh only one pound in water, we have a length of scale extending from one pound to six cwt.
The slightest consideration of this instrument must satisfy every person, that in measuring forces where the instrument itself is stationary, it is greatly superior to Regnier's, and has the advantage of a natural and im moveable scale ; whereas the scale of the spring dyna mometer must be determined experimentally ; and with every precaution that can be taken, the spring must to a certain extent lose its elasticity, and derange the scale. See Desagulier's Course of Experimental Philosophy, vol. i. p. 291; Journal De L'Ecole Polytechnique ; loch's Philosophical Magazine, vol. i. p. 599. (119