212 



BALANCE. 



Balance, moves two pinions concentric with itself, and within 

 *~" v ' the box, which act on props P, P that support the 

 pans, r allow their motion as business may require. 

 A strong brass pillar F occupies the centre of the in- 

 terior ot the box, supporting at top a square plat- 

 form ; on the front and opposite side of this are erec- 

 ted two arches m, m 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 , 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 beam 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. iThese 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 a*gate 

 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 being 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 

 Plate LI. a piece which is pushed inwards by a screw, while a 

 J 'S - 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 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 

 in. indefinite, and varied at pleasure by the adjusting 



weight : they arc, however, of great use in estimating Balance- 

 small quantities. For hydrostatical purposes there v '' 

 are only required to be added a common tumbler Plate LI. 

 of water, and a brass wire hooked at both ends. A *'' * 

 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: 



10 100 1000 

 20 200 2000 

 40 400 4000 

 80 800 8000 

 Greater weights than these might endanger the parts 

 of action. 



The following instructions how to adjust this kind Method of 

 of balance may not be unacceptable. 1st, To bring adjusting 

 the three points of action into a right line. Without the balance, 

 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, yon 

 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 oil the scales by 

 unhooking them, and hang them into contrary ends 

 of the beam, which if now poised, the thing is done j 

 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 Trcugh- 

 beam of the common form, such as is used for as- ton's Assay 

 saying of metals, or nice chemical purposes, in which usance, 

 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 i3 fixed an obtuse an- 

 gle of hardened steel, which, supporting the very a- 

 cutc angle of the axis, forms the principal part of ac- 



