384 



BALANCE. 



Kgypt and Arabia. It is of the guitar kiiul, but lias 

 only two strings. 



BALANCE; an instrument employed for determining 

 the quantity of any substance equal to a given 

 weight. Balances are of various forms; tliat most 

 commonly used, is represented in plate IX. fig. 2, 

 being usually denominated, the beam and scales. 

 The horizontal beam AB rests, and is capable of 

 turning, on the centre of motion C. The scales D 

 and E are suspended by chains from the extremities 

 A and B of the beam, called the centres of suspen- 

 sion. Midway between the centres of suspension, 

 and directly above the centre of motion, there rises 

 from the upper surface of the beam, a perpendicular 

 slender stem, called the tongue, which, when the 

 beam is level, points to the top of the handle F, by 

 which the whole is suspended. A good balance is 

 necessary, not less for the ordinary commerce of soci- 

 ety, thnn for the purposes of science ; and there are 

 few indeed to whom it would be a matter of indif- 

 ference to know the principles of construction which 

 contribute to the excellence of this simple and useful 

 instrument. We shall, therefore, as briefly as possible, 

 state the properties of a good balance, and also the 

 means by which these properties are to be secured. 

 The characteristics of a good balance are three : 1st. 

 That tin- beam should rest in a horizontal position 

 when the scales are either empty or loaded with 

 equal weights. 2d. A very small addition of weight 

 put into either scale, should cause the beam to devi- 

 ate from the level, which property is denominated 

 the sensibility of the balance. 3d. When the beam 

 is deflected from the horizontal position by ine- 

 quality of the weights in the scales, it should liave 

 a tendency speedily to restore itself, and come to 

 rest in the level, which property is called the stability 

 of the balance. The remarks which follow will 

 guide in the construction of a balance, which shall 

 possess the foregoing properties : The arms of the 

 beam should be exactly similar, equal in weight and 

 length, and as long as possible. The centres of 

 gravity and suspension ought to be in one straight 

 line, and the centre of motion should be immediately 

 alx)ve the centre of gravity. The centre of motion 

 and the centres of suspension should cause as little 

 friction as possible, and their axes ought to be at 

 right angles to the line which measures the length 

 of the beam. The centre of motion ought to be a 

 knife-edge; and if the balance requires to be very 

 delicate, the centres of suspension ought to be knife- 

 edges also : and if the centres of suspension be not 

 knife-edges, the rings with which they are formed 

 should be hard, polished, and of an oval form. There 

 are means of testing whether or not these conditions 

 have been observed in the construction of a balance. 

 For if the balance have no tendency to one position 

 more than another, when the scales are either loaded, 

 empty, or off altogether, it is a proof that the centres of 

 gravity and motion coincide, and the remedy is to low- 

 er the centre of gravity. If the beam is disturbed by 

 a small addition of weight to either scale, the arm at 

 the loaded end descending, and having no tendency 

 to resume the horizontal position ; then we may infer 

 that the centre of gravity is above the centre of mo- 

 tion ; and it is to be observed, that the quicker the 

 descent of the loaded arm of the beam is, the farther 

 must the centre of gravity be lowered before the beam 

 will acquire the requisite stability. If it require a 

 considerable addition of weight in either scale to de- 

 flect the beam from the level, we may infer either 

 that there is too much friction at the centre of motion, 

 or that the centre of gravity is too low. If two 

 weights are found to be in equipoise, one being in each 

 scale, when a transfer of them is made, that which 

 was in the one scale being put into the other, then 



if there lie no longer an equilibrium, we may in- 

 fer that the arms of tlie beam are of unequal lengths. 

 Various contrivances have been employed, with a 

 view to correct the defects of the common balance. 

 The whole apparatus is not inifrequeiitly enclosed in 

 a glass case, which prevents the heat from expanding 

 the arms unequally, or currents of air from disturb- 

 ing the equilibrium. A small weight lias been made. 

 to slide up or down on the tongue, by which means 

 the centre of gravity may lie raised or depressed at 

 pleasure ; and to regulate the equality of the length 

 of the arms, a regulating screw is employed, by 

 means of which the centre of suspension of either arm 

 may be moved nearer to or farther from the centre of 

 motion. Balances used for delicate purposes, such 

 as for assaying, have the centre of motion suspend- 

 ed ; but that centre is fixed on a pedestal, which firmly 

 supports the whole. Such is the case in the avay 

 balance, plate IX. fig. 1. The hydrostatic balance 

 (plate IX. fig. 4) is another modification of the com- 

 mon beam and scale. The pedestal D rises from the 

 table AB, to a convenient height, and supports the 

 tablet E, into which the stalk F is fixed. From this 

 stalk, the beam of the instrument is suspended, in a 

 manner which the engraving will easily show. To 

 the bottom of one ot the scales, there is hooked a 

 small wire, C, which suspends the substance whose. 

 specific gravity is to be taken, and which in the figure; 

 is represented as immersed in the water contained in 

 the glass vessel R. The minutia? of construction 

 and use of this balance, can only be understood by 

 following the method usually employed for the deter- 

 mination of specific gravities, which will be found ex- 

 plained under the article Gravity, specific. The ba- 

 lances, which we have considered above, all require an 

 assortment of weights ; and it now remains for us to 

 describe the more important of those balances which 

 require only one weight, but are, nevertheless, capa- 

 ble of determining a great many. Of this description 

 is the statera, or Roman steel-yard, represented in 

 plate IX. fig. 3, where AB is a steel bar, moving 

 on the fulcrum C, after the same manner as the com- 

 mon balance beam. If the arm CA be one inch 

 long, and the arm CB any convenient length, being 

 divided into inches, and also smaller than the short 

 arm CA, so that when the weight which is seen at 

 E is taken off, and the empty scale hung from A, 

 the bar AB will be level, and at rest. If now a 

 weight of two ounces were put into the scale D, the 

 equilibrium will be destroyed, unless a weight of two 

 ounces be hung on the long arm, at the distance of 

 one inch from the centre of motion, or one ounce at the 

 distance of two inches, if a six ounce weight be put 

 into the scale D, it will be balanced by one ounce at 

 the distance of six inches from the fulcrum C, on the 

 long arm, &c., which follows from the principles of 

 the Lever. And thus, if the arm CB were sixteen 

 inches long, and CA one inch, then would one ounce 

 weight at B balance one pound avoirdupois in the 

 scale D. A modification 

 of this instrument has 

 lately been employed 

 with much advantage in 

 taking specific gravi- 

 ties. The bent-lever, 

 or quadrant balance, is 

 represented in the ac- 

 companying cut, and de- 

 pends on principles near- 

 ly the same with those: 

 of the steel-yard. The 

 article to lie weighed is 

 put into the scale, and 

 the index will rise on the 

 graduated quadrant; un- 



