WEIGHING-MACHINE. 



although it is very near to it. Tlie platform is compofed of 

 a ilroiig frame of wood (as (hewn in Jig. 2.), and the upper 

 iide is covered with wood planking. It is likewife defended 

 from wear, by iron-bars and large-headed nails, which are 

 fattened on the upper fide. Near each of the four angles 

 of the platform a piece of iron is fixed, as fliewn by b; and 

 it is on thefe four points that the platform is borne. When 

 the platform is put in its place, thefe pieces of iron apply to 

 the pins bb yfig' i.), which are fixed in four ftrong iron 

 levers, marked A A, B B. Each of thefe levers is fup- 

 ported at the extreme end c, on a fulcrum or centre-pin 

 refting on a metal fupport, as fhewn in Jig. 4, which is 

 borne by a piece of timber 0, worked into the walls of the 

 pit at the angle. At the oppofite ends the levers A A are 

 brought together, and B B the fame, fo that all four meet 

 in two points a a, and by means of links, fhewn in Jig. 5, 

 are all connefted with a long lever CC. This refts on a fup- 

 port or fulcrum D, borne by a pillar eredled from the bot- 

 tom of the pit. The end of the lever at I is received be- 

 tween two uprights to guide it, but do not any way confine 

 its motion. In the common machine, it is from the extreme 

 end of the lever C D that the iron rod before mentioned is 

 carried up to the fcale-beam, or fteel-yard, as before de- 

 fcribed ; but the patent machine in the figure is differently 

 conftrufted in that part. 



The fixed centres c, of the levers A A and B B, are at 

 the ends of thofe levers, and the points b b, on which the 

 platform bears, are very near to the centre c ; but the dif- 

 tance of the points a from c is nine times as great as from 

 c to b, confequcntly a force of one pound applied to lift up 

 the levers at the point a would balance nine pounds laid upon 

 the platform. In like manner the diftance from the point a 

 to the fulcrum D is only one-feventh part of the diftance 

 from the fulcrum to the end of the lever C ; hence, one pound 

 applied to lift up the end of the lever C would raife feven 

 pounds applied at the point a, and feven pounds applied at a 

 would balance fixty-three pounds placed on thg platform. 

 To weigh with a machine of this kind, if we ufe a fcale and 

 balance connefted with the end of the long lever, we mult 

 ufe weights which are only one fixty-third part of the marks 

 which they bear. 



Mr. Salmon's machine operates in a much more perfeft 

 manner, by the help of a felf-adjulling balance-wheel, which 

 will weigh every different body, without employing any loofe 

 weights ; and it (hews the weight by means of an index and 

 dial, like that of a clock. To efFeft this, another lever or 

 itecl-yard FG is applied, whereof Fis the fulcrum, andy the 

 point from which a link C dcfcends to the end of the long 

 If ver C D. At the extreme end G, a flrap is attached to 

 afcend to the balance-wheel i /. 



Now, as the diftance Fyis only one-tenth of the diftance 

 F G, one pound applied to lift up the end G would raife 

 ten pounds at f, or i o x 63 ^ 630 pounds placed on the 

 platform. 



To draw the end G of the lever upwards, a thin leather 

 ftrap g is attached to it, and the upper end of this is coiled 

 round a fmall roller h, which is fixed upon an horizontal 

 axis, as is fhewn on a larger fcale in Jig. 7, where the axis 

 is marked b b. This is reduced to fmall pivots at the extre- 

 mities, which are borne by friftion-wheels a a, to render 

 its motion as free as pofTible. On the fame axis b is fixed a 

 ■wheeli/, (feealfo_/j. 6.), and againll the arms of this wheel a 

 fpiral ledge is fixed, with a fufficient projeftion to admit a 

 fine filk line to wind upon the fpiral, when the wheel is 

 turned round. A weight k, which is fufpended to the line, 

 forms the counterbalance to the load placed upon the weigh- 

 bridge [(ecjig. 3. ), and the weight of the load is determined 



6 



by the diilance which the roller and wheel i are turned round. 

 This diftance is fhewn by an index c fixed on the extreme 

 end of the axis b, and pointing to different divifions engraved 

 round a dial, as fhewn in Jig- 6. 



This fingle weight i can counterbalance all the different 

 weights which may be placed on the platform, becaufe the 

 line by which the weight /• is fufpended, when it winds 

 upon the circumference of the fpiral, continually applies 

 itfelf at a different diftance from the centre of the axis, fo 

 as to operate with greater force. Hence, when any weight 

 is placed upon the platform E, it preffes down the levers 

 A A and B B ; thefe deprefs the long lever C D, and this 

 again aftuates the lever F G, and draws down the ftrap g, 

 which unwinds from the roller on the axis b b, fo as to turn 

 it round, together with the wheel and fpiral. The weight 

 i winds upon the fpiral, but the fufpending line foon arrives 

 at that part of the fpiral where its radius is fufSciently 

 increafed, to enable the weight k to counterbalance the load 

 upon the platform ; the balance-wheel being then come to 

 an equilibrium, will move no farther, and the index points 

 out upon the dial the weight of the load upon the platform. 

 The fpiral originates in the central part, at a circle which 

 is of the fame diameter as the roller, upon which the ftrap ^, 

 _fg. 3. winds; and the weight i muft be equal to the fix-hun- 

 dred-and-thirtieth part of the weight of the platform and 

 levers A A, B B, when there is no weight upon it. When 

 the weight i hangs from this commencement of the fpiral, the 

 index c Hands at zero, as (hewn in Jig. 6. The fpiral is fo 

 made, that to turn it round fixty degrees vi'ill require one 

 ton weight to be laid on the platform, and every additional 

 ton will turn the wheel and index round another fixty 

 degrees, fo that the machine will bear fix tons before the 

 index makes a complete revolution. Each fpnce of fixty 

 degrees is divided into twenty parts, which reprefent hun- 

 dred weights ; and each one is fubdivided into halves or 

 quarters, which divifions are very apparent on a large dial. 



In conftrufting a machine of this kind, evepy attention 

 muft be paid to accuracy in the centres of motion of the 

 different levers ; all thefe points (liould be made of fteel, 

 and hardened. The form of the centres fhould be that of a 

 fliarp edge, like a blunt knife, with the edge refting on a 

 furface of hard fteel, made rather concave, ({ee Jig. 4.) 

 Centres, or bearing points of this kind, made (harp, vvilJ 

 move with very little friftion ; and if the fteel is good, and 

 perfeftly hard, the edges will not become blunt in many 

 years' ufe. In all cafes, the bearing-pins with the fharp 

 edges muft be fixed in the levers ; becaufe if the levers were 

 made with plain furfaces, and to have (harp pins to bear 

 upon them, there would be no certainty as to the effeftive 

 lengths of the different levers, and they would vary in their 

 power whenever the fharp edge changed its place upon the 

 fupporting furface. 



The four principal levers, A A, B B, are made double, 

 or with open loops at the ends, as (hewn in Jigs. I and 4 ; 

 two fteel pins are put throus;h the double part, one of 

 them marked c, being made with a (harp edge at the lower 

 fide, but the other, b, is fliarpened on the upper fide. The 

 former bears upon a fixed fupport faced witli hard fteel, and 

 the other receives the metal ftems, b,Jig. 2. which are fixed to 

 the underfide of the platform. The two levers, A A, are 

 joined together at the point a ; and the two levers, B B, are 

 alfo joined in a fimilar manner. Each pair of levers are 

 conr.efted by a crofs-bar, as (liewn in _fg. i , fo as to make 

 two triangles. 



The ends p or s. Jig. 5. of the compound levers A A or 

 B B, where they join together, have a fcrew fitted through 

 them, as fhewn in_fig. 5 ; the ends of thefe fcrews are made 



of 



