PLATE DVI.
Fig. 1. Fig. 2. Fig. 3. Fig. 4. Fig. 5. Fig. 6.
F. Mitchell sculp t. are two pins d and e, for suspending occasionally two eke weights for continuing the scale. These are kept hanging on adjoining hooks, ready to be lifted on by a little tackle, which is also hooked immediately above the pins d and e.
The scales of weights are laid down on the arm as follows. Let the eke-weights appropriated to the pins d and e be called D and E, and call the counterpoise C. Although the stirrup with its chains and stage weigh some hundreds, yet the length and size of the arm OP gives it a preponderancy of 300 pounds. Here, then, the scale of weights must commence. The counterpoise weighs about 125 pounds. Therefore,
1. When the load hangs by the pin b, 14 inches from the centre, the distance from one hundred to another on the scale is about 11 inches, and the first scale (on the side of the arm) reaches from 300 to 1200. In order to repeat or continue this, the eke-weight E is hung on the pin e, and the counterpoise C is brought back to the mark 300; and the two together balance 1100 pounds hanging at b. Therefore a second scale is begun on the side of the arm, and continued as far out as the first, and therefore its extremity marks 2000; that is, the counterpoise C at 2000 and the eke-weight E at e balance 2000 hanging at b.
2. To continue the scale beyond 2000, the load must be hung on the inner pin c. The eke-weight E is taken off, and the eke-weight D is hung on its pin d. The general counterpoise being now brought close to the sheers, it, together with the weight D at d, balance 2000 pounds hanging at c. A scale is therefore begun on one of the inclined planes a-top, and continued out to 4000, which falls very near to the pin d, each hundred pounds occupying about five inches on the arm. To complete the scale, hang on the eke-weight E on its pin c, and bring back the counterpoise to the sheers, and the three together balance 3800 hanging at c. Therefore when the counterpoise is now slid out to 4000, it must complete the balance with 5800 hanging at c.
It required a little consideration to find out what proportion of the three weights C, D, and E, would make the repetitions of the scale extend as far as possible, having very little of it expressed twice, or upon two scales, as is the case here. We feel that the space corresponding to a single pound is a very sensible quantity on both scales, being one-ninth of an inch on the first two scales, and one-twentieth on the last two.
This very ponderous machine, with its many weights, cannot be easily managed without some assistance from mechanics. It is extremely proper to have it susceptible of motion out and in, that it may be protected from the weather, which would soon destroy it by rust. The contrivance here is very effectual, and abundantly simple.
When the steel-yard is not in use, it is supported at one end by the iron rod F, into which the upper end of the sheers is hooked. The upper end of this rod has a strong hook E, and a little below at a it is pierced with a hole, in which is a very strong bolt or pin of tempered steel, having a roller on each end close to the rod on each side. These rollers rest on two joists, one of which is represented by MN, which traverse the building, with just room enough between them to allow the rod F to hang freely down. The other end O of the steel-yard rests in the bight of a large flat hook at the end of a chain W, which hangs down between the joists, and is supported on them by a frame with rollers H. This is connected with the rollers at G, which carry the sheers by means of two iron-rods, of which one only can be seen. These connect the two sets of rollers in such a manner that they must always move together, and keep their distance invariable. This motion is produced by means of an endless rope HI ZLKVH passing over the pulleys I and K, which turn between the joists, and hanging down in a bight between them. It is evident that by pulling on the part LZ we pull the frame of rollers in the direction GH, and thus bring the whole into the house in the position marked by the dotted figure. It is also plain, that by pulling on the part LK we force the roller frame and the whole apparatus out again.
It remains to show how the load is raised from the ground and weighed. When the steelyard is run out for use, the upper hook E just enters into the ring D, which hangs from the end of the great oaken lever BCA about 22 feet long, turning on gudgeons at C about 5 feet from this end. From the other end A descends a long iron-rod SR, which has one side formed into a toothed rack that is acted on by a frame of wheel-work turned by an endless screw and winch Q. Therefore when the hook E is well engaged in the ring D, a man turns the winch, and thus brings down the end A of the great lever, and raises the load two or three inches from the ground. Everything is now at liberty, and the weigher now manages his weights on the arm of the steelyard till he has made an equilibrium.
We need not describe the operation of letting down the load, disengaging the steelyard from the great lever, and bringing it again under cover. The whole of this service is performed by two men, and may be done in succession by one, and is over in five or six minutes.
The most compendious and economical machine of this kind that we have seen is one, first used (we have heard) for weighing the riders of race-horses, and afterwards applied to the more reputable service of weighing loaded carriages.
Fig. 5, is a plan of the machine. KLMN is the Fig. 5, plan of a rectangular box, which has a platform lid or cover, of size sufficient for placing the wheels of a cart or wagon. The box is about a foot deep, and is sunk into the ground till the platform cover is even with the surface. In the middle of the box is an iron lever supported on the fulcrum pin /k, formed like the nail of a balance, which rests with its edge on arches of hardened steel firmly fastened to the bottom of the box. This lever goes through one side of the box, and is furnished at its extremity with a hard steel pin /m, also formed to an edge below. In the very middle of the box it is crossed by a third nail of hardened steel g h, also formed to an edge, but on the upper side. These three edges are in one horizontal plane, as in a well made balance.
In the four corners A, A', E', E, of the box are firmly fixed four blocks of tempered steel, having their upper surfaces formed into spherical cavities, well polished and hard tempered. ABCDE represents the upper edge of an iron bar of considerable strength, which rests on the cavities of the steel blocks in A and E, by means Steel-yard, of two hard steel studs projecting from its under edge, and formed into obtuse angled points or cones. These points are in a straight line parallel to the side KN of the box. The middle part C of this crooked bar is faced with hard-tempered steel below, and is there formed into an edge parallel to AE and KN, by which it rests on the upper edge of the steel pin g h which is in the lever. In a line parallel to AE, and on the upper side of the crooked bar ACE, are fixed two studs or points of hardened steel B and D projecting upwards above half an inch. The platform-cover has four short feet like a stool, terminated by hard steel studs, which are shaped into spherical cavities and well polished. With these it rests on the four steel points B, B', D', D. The bar ACE is kneed in such a manner vertically, that the points A, B, D, E and the edge C are all in a horizontal plane. These particulars will be better understood by looking at the elevation in fig. 6. What has been said of the bar ACE, must be understood as also said of the bar A' C' E'.
Draw through the centre of the box the line abc perpendicular to the line AE, BD. It is evident that the bar ACE is equivalent to a lever abc, having the fulcrum or axis AE resting with its extremity C on the pin g h and loaded at b. It is also evident that a c is to ab as the load on this lever to the prelure which it exerts on the pin g h, and that the same proportion subsists between the whole load on the platform and the prelure which it exerts on the pin g h. It will also appear, on an attentive consideration, that this proportion is nowise deranged in whatever manner the load is placed on the platform. If very unequally, the two ends of the pin g h may be unequally pressed, and the lever wrenched and strained a little; but the total prelure is not changed.
If there be now placed a balance or steelyard at the side LK, in such a manner that one end of it may be directly above the pin /m in the end of the lever EOF, they may be connected by a wire or flender rod, and a weight on the other arm of the balance or steel-yard may be put in equilibrium with any load that can be laid on the platform. A small counterpoise being first hung on to balance the apparatus when unloaded, any additional weight will measure the load really laid on the platform. If ab be to ac as 1 to 8, and EO to EF, also as 1 to 8, and if a common balance be used above, 64 pounds on the platform will be balanced by one pound in the scale, and every pound will be balanced by \( \frac{1}{64} \)th of an ounce. This would be a very convenient partition for most purposes, as it would enable us to use a common balance and common weights to complete the machine: Or it may be made with a balance of unequal arms, or with a steelyard.
Some have thought to improve this instrument by using edges like those of the nails of a balance, instead of points. But unless made with uncommon accuracy, they will render the balance very dull. The final deviation of the two edges A and E, or of B and D, from perfect parallelism to KN, is equivalent to a broad surface equal to the whole deviation. We imagine that, with no extraordinary care, the machine may be made to weigh within \( \frac{1}{1000} \) of the truth, which is exact enough for any purpose in commerce.
It is necessary that the points be attached to the bars. Some have put the points at A and E in the blocks of steel fastened to the bottom, because the cavity there lodged water or dirt, which soon destroyed the instrument with rust. But this occasions a change of proportion in the first lever by any shifting of the crooked bars; and this will frequently happen when the wheels of a loaded cart are pulled on the platform. The cavity in the steel stud should have a little rim round it, and it should be kept full of oil. In a nice machine a quarter of an inch of quicksilver would effectually prevent all these inconveniences.
The simplest and most economical form of this machine is to have no balance or second steelyard; but to make the first steelyard EOF a lever of the first kind, viz. having the fulcrum between O and F, and allow it to project far beyond the box. The long or outward arm of this lever is then divided into a scale of weights, commencing at the side of the box. A counterpoise must be chosen, such as will, when at the beginning of the scale, balance the smallest load that will probably be examined. It will be convenient to carry on this scale by means of eke-weights hung on at the extremity of the lever, and to use but one moveable weight. By this method the divisions of the scale will always have one value. The best arrangement is as follows: Place the mark O at the beginning of the scale, and let it extend only to 100, if for pounds; or to 112, if for cwt.; or to 10, if for floes; and let the eke-weight be numbered 1, 2, 3, &c. Let the lowest weight be marked on the beam. This is always to be added to the weight shown by the operation. Let the eke-weights stand at the end of the beam, and let the general counterpoise always hang at O. When the cart is put on the platform, the end of the beam tilts up. Hang on the heaviest eke-weight that is not sufficient to press it down. Now complete the balance by sliding out the counterpoise. Suppose the constant load to be 312 lb. and that the counterpoise stands at 86, and that the eke-weight is 9; we have the load \( = 986 + 312 = 1298 \) lbs.