An instrument employed to measure the comparative strengths of men and cattle, and to ascertain the force required in drawing carriages upon land, and vessels upon canals. These effects are usually estimated by the compression or distension of a strong spring, or by a steelyard upon the principle of a bent lever balance; but in both these constructions the instrument is subject to great vibration, owing to inequalities in the resistance and in the moving force, which render the indications very uncertain. This o jection to spring dynamometers has been obviated by Mr. H. R. Palmer, as follows: - a piston, fitting loosely in a cylinder filled with oil, so as to allow the oil to escape slowly past its sides as it moves up and down, is connected with the springs by means of a fine steel wire passing through stuffing boxes at each end of the cylinder; the friction, therefore, is so extremely slight as not to be worth taking into account in any experiments, whilst it prevents that vibratory motion of the index, from jerks, and allows the resistance to be ascertained with the greatest accuracy.
The preceding engraving is a representation of the instrument, a a represents the back of the dial plate of any ordinary spring weighing machine, the front of which is shown separate in Fig. 2 (but without the graduated scale, as being unnecessary); b b are two spiral springs enclosed in cylindrical cases, (similar to the well-known domestic article called pocket steelyards,) the upper ends of which are fixed to a sliding frame c c, and at their lower ends they are hooked to a cross bar f, which bar is made fast to a piece of metal o at the back of a a; d is the cylinder of oil, which is firmly fixed at the back of a a; to this cylinder is fixed four pieces of metal e e e e, having angular grooves, in which the frame c c slides when the springs are acted upon. The piston in the cylinder is shown by dots, the rod to which (a fine steel wire) passes through stuffing boxes at each end of the cylinder, and each end of the rod is then made fast to the frame c c. Now the bar g, which proceeds from the circular box a a, and acts upon the spring, is connected at the swivel ring i with the upper end of the frame c c, as one solid piece; therefore when the ring k is made fast to a carriage, and power applied to the rope at i, the bar g is drawn out of a, while the frame c c acts simultaneously upon the steelyard springs c c, which move along with them the piston in the fixed cylinder d; the oil therein being incompressible, is compelled to pass from one side of the piston to the other, through the extremely narrow interstice between the periphery and the cylinder.
In the dynamometer invented by Mr. Milne, of Edinburgh, an iron plunger is by the force exerted in traction caused to descend in an open vessel containing mercury, by which means the latter rises to a height indicated by a glass tube open at top, and connected to the mercurial vessel at bottom by a neck, the bore of which is contracted to diminish the vibrations of the mercurial column; half the height in inches of the mercury in the tube multiplied by the area of the base of the plunger in inches will be the amount of the force of traction in pounds.