Having thus stated very briefly the precautions that are necessary in the construction of a delicate balance, we shall proceed to describe three of the best balances that have yet been made. The two first were made by the celebrated Mr Troughton, and the last by Messrs Miller and Adic, mathematical instrument makers, Edinburgh. (o) In Figure 1. AB represents the scale-beam in its '1 most perfect state, as made by Mr Troughton. A t strong mahogany box contains the whole; it has drawers below for holding weights, &c.; and a long / one behind, wherein the beam is packed, when out of use or in carriage. There are doors in the ends through which the pans, or scales, are loaded and unloaded, and through another at the top the beam may be taken out. The front and back are.of plate glass. On the upper surface of the drawer depart ment are shewn two spirit levels, L, L by means of which, and screws below, the balance is easily ad justed to its due position. There passes through the box, a little below the levels, an iron rod, having at each end a handle, by which it may be turned round. One of these is seen in the figure at H. This rod moves two pinions concentric with itself, and within the box, which act on props P, P that support the pans, tor allow their motion as business may require. A strong brass pillar F occupies the centre of the in terior of the box, supporting at top a square plat form ; on the front and opposite side of this are erec ted two arches al, az nearly semicircular ; and, on their vertices, are fixed two horizontal planes of agate, which afford together the proper fulcrums for the beam. Within the pillar F a cylindric tube is elevated or depressed by a lever, the handle of which is seen at G between the drawers below : from the upper end of the tube, just above the platform, springs an in verted arch a, of as much greater radius than the other two which it crosses at right angles, as brings its span on the outside of them. The beard is formed of two hollow cones of brass joined together by a short cy linder in the middle, and is altogether about 18 inches long. ,These are formed of a substance not exceeding 0,02 of an inch thick, but, by means of circular rings driven hard into them at proper intervals, are rendered almost inflexible. The beam is crossed in the middle of its length by a cylinder of steel, the lower diameter of which coincides with its centre. The lower side of this cylinder being reduced to an edge, the angle of which is about 30°, hardened and well polished, forms the principal axis, and rests through a length of about 0,05 of an inch, before and behind, upon the ate planes. Exterior to the parts of action, the axis is worked into two small pivots ; and the extreme ends of the inverted arch being formed into angles, the latter when lifted up by the lever below, carries with it the beam, and thus relieves the axis when the in strument is out of use. On lowering the beam, the angles leave the axis in its proper position on the cen tres of the planes. There no contrivance to_pre vent the axis from wandering from the centres of the agates, when this is seen or suspected, the beam must have its position rectified by means of the inverted arch. A weight for adjusting the motion of the beam, respecting the time in which it performs its vi brations, is raised or lowered by a screw at top: its mean position is the centre of the beam. ' The points of suspension for the scales are both adjustable, the one on the left horizontally, for making the arms of the balance of equal length : the axis is here fixed in a piece which is pushed inwards by a screw, while a strong spring of coiled wire, in the inside of the cone, presses it in an opposite direction. The axis on the right is adjustable vertically, for the purpose of bring ing the three points of action into a right line: this is brought about by fixing i the axis in a sliding piece si milar to the other, and it is acted on by two screws which press it in opposite directions. The action at the ends, like that at the centre, is on double bearings; but, instead of by two sharp edges resting upon two planes, is here performed by two concave edges acting at right angles on two other concave edges : the for mer pair formed in the axis itself being sharp upwards, and the latter pair sharp downwards, formed in the spur shaped pieces from which the scale-strings de pend. The ends of the beam terminate in points, and play contiguous to divided ivory scales, which are fixed in the inside of the box : the value of these divisions is indefinite, and varied at pleasure by the adjusting weight : they are, however, of great use in estimating small quantities. For hydrostatical purposes there '
are only required to be added a common tumbler P of water, and a brass wire hooked at both ends. A 11 set of accurately adjusted grain weights usually ac company the instrument ; and, perhaps every thing considered, the following series may be as convenient as any: ,001 ,01 ,1 1 10 100 1000 ,002 ,02 ,2 2 20 200 2000 ,004 ,0 ,4 4 40 400 4 000 ,008 ,08 ,8 8 80 800 8000 Greater weights than these might endanger the parts of action.
The following instructions how to adjust this kind of balance may not be unacceptable. 1st, To bring the three points of action into a right line. Without II weights, poise the beam by throwing into the scales any bits of light substance, and raise or lower the weight within the beam until the vibrations are there by rendered very slow : now, put weights into the pans, equal to about half the greatest load you mean your instrument to carry, so that the beam may be poised again : if it now vibrates slow just as it did before, it proves the adjustment to be perfect ; but, in case it either oversets or vibrates too quick, you must restore it to slow motion by the adjusting weight, noting the number of turns of the screw, and parts of a turn, which were required to produce slow motion : now, turn the screw the contrary way, through double the noted quantity, and then produce the required slow motion by the proper adjustment at the right end of the beam. Repeat the operation until the adjustment is perfect. 2d, To make the arms of the beam of an equal length. With weights in the scales as in the last adjustment, poise the beam with the greatest care : now take off the scales by unhooking them, and hang them into contrary ends of the beam, which if now poised, the thing is done ; but, if not, take as much hair or wire as when put into the apparently lighter scale, will restore the ba lance : take away half of it ( which is accurately found by doubling and cutting), and poise the beam by the proper adjustment at the left end. Repeat this operation also for greater certainty. The above are the two grand adjustments of the balance : secondary ones, (as poising the beam itself, is done by simply screwing in or out a small screw at the right end ; and equalising the pans and hook for weighing in water, which is done by scraping off a little of their substance,) can only require to be enumerated. When the instrument is adjusted in all its parts, the adjust ing weight may be moved up or down at pleasure, according as exactness or dispatch may be required.
In Figure 4. of Plate LI. is represented a small • beam of the common form, such as is used for as saying of metals, or nice chemical purposes, in which 1 a load greater than 4 ounces is not required. It re quires to be inclosed in a glass case, and is supported upon a pillar. A tube inside the pillar is acted on by a lever below, and connected with a square plat form and two semicircular arches at top : on the summit of each of these arches is fixed an obtuse an gle of hardened steel, which, supporting the very a cute angle of the axis, forms the principal part of ac.
win. The circular ends of the beam are excavated in the middle of their thickness, on the lower side, a good deal above the centre : the axis is fastened in the two sides of the circular ends, with its sharp edge upwards a litti. concave, having passed through a piece of hardened and polished steel of the shape of an 8 ; the inner edges of the two eyes being worked to sharpness : by the upper eye it hangs upon the axis within the part excavated, forming the point of suspension, and with the lower the scale-strings are connected by a ring. A piece of wire extending above the centre is worked into a screw, and upon this an adjustable ball of brass is placed for the pur pose of procuring quick or slow vibrations. Oppo site to this, extending downwards, is a long index pointing to a graduated scale of ivory for ascertaining the position of the beam. The lever below is fixed to the pillar, and by its action lowers the beam until the pans rest upon the bottom of the glass case, or raises it when in use. At the top of all there is a light index of brass-wire, which, by being turned round with the finger, serves to adjust the index below to the zero of the ivory scale : this, insignificant as it ap pears to be, is extremely convenient in practice. This kind of beam is adjusted, respecting equality of length in the arms, by the hammer, and bent by the hand for bringing the three points of action into a right line. (E. T.