VCG

VCG (fig. 5.) is the stand or pillar of this hydrostatic balance, which is to be fixed in a table. From the top A hangs, by two silk strings, the horizontal bar BB, from which it is suspended by a ring $i$, the fine beam of a balance $b$; which is prevented from descending too low on either side by the gently springing piece $t\; x\; y\; z$, fixed on the support M. The harness is annulated at $o$, to show distinctly the perpendicular position of the examen, by the small pointed index fixed above it.

The strings by which the balance is suspended, passing over two pulleys, one on each side the piece at A, go down to the bottom on the other side, and are hung over the hook at $v$; which hook, by means of a screw P, is moveable about one inch and a quarter, backward and forward, and therefore the balance may be raised or depressed so much. But if a greater elevation or depression be required, the sliding piece S, which carries the screw P, is readily moved to any part of the square brass rod VK, and fixed by means of a screw.

The motion of the balance being thus adjusted,

the rest of the apparatus is as follows. HH is a small board, fixed upon the piece D, under the scales $d$ and $e$, and is moveable up and down in a low slit in the pillar above C, and fastened at any part by a screw behind. From the point in the middle of the bottom of each scale hangs, by a fine hook, a brass wire $a\; d$ and $a\; c$. These pass through two holes $m\; m$ in the table. To the wire $a\; d$ is suspended a curious cylindric wire $r\; s$, perforated at each end for that purpose: this wire $r\; s$ is covered with paper, graduated by equal divisions, and is about five inches long.

In the corner of the board at E, is fixed a brass tube, on which a round wire $h\; l$ is so adapted as to move neither too tight nor too free, by its flat head L. Upon the lower part of this moves another tube Q, which has sufficient friction to make it remain in any position required: to this is fixed an index T, moving horizontally when the wire $h\; l$ is turned about, and therefore may be easily set to the graduated wire $r\; s$. To the lower end of the wire $r\; s$ hangs a weight L; and to that a wire $p\; n$, with a small brass ball $g$ about one-fourth of an inch diameter. On the other side, to the wire $a\; c$, hangs a large glass bubble R, by a horse hair.

Let us first suppose the weight L taken away, and the wire $p\; n$ suspended from S: and, on the other side, let the bubble R be taken away, and the weight F, suspended at $e$, in its room. This weight F we suppose to be sufficient to keep the several parts hanging to the other scale in equilibrium; at the same time that the middle point of the wire $p\; n$ is at the surface of the water in the vessel N. The wire $p\; n$ is to be of such a size, that the length of one inch shall weigh four grains.

Now it is evident, since brass is eight times heavier than water, that for every inch the wire sinks in the water it will become half a grain lighter, and half a grain heavier for every inch it rises out of the water: consequently, by sinking two inches below the middle point, or rising two inches above it, the wire will become one grain lighter or heavier. Therefore, if, when the middle point is at the surface of the water in equilibrium, the index T be set to the middle point $a$ of the graduated wire $r\; s$, and the distance on each side $a\; r$ and $a\; s$ contains 100 equal parts: then, if in weighing bodies the weight is required to the hundredth part of a grain, it may be easily had by proceeding in the following manner.

Let the body to be weighed be placed in the scale $d$. Put the weight X in the scale $e$; and let this be so determined, that one grain more shall be too much, and one grain less too little. Then the balance being moved gently up or down, by the screw P, till the equilibrium be nicely shown at $o$; if the index T be at the middle point $a$ of the wire $r\; s$, it shows that the weights put into the scale $e$ are just equal to the weight of the body. By this method we find the absolute weight of the body; the relative weight is found by weighing it hydrostatically in water, as follows.

Instead of putting the body into the scale $e$, as before, let it hang with the weight F, at the hook $e$, by a horse hair, as at R, supposing the vessel O of water were away. The equilibrium being then made, the index T standing between $a$ and $r$, at the 36 division,

Balance
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Balayze.

tion, shows the weight of the body put in to be 1095.36 grains. As it thus hangs, let it be immersed in the water of the vessel O, and it will become much lighter: the scale e will descend till the beam of the balance rest on the support z. Then suppose 100 grains put into the scale d restore the equilibrium precisely, so that the index T stand at the 36 division above a; it is evident that the weight of an equal bulk of water would, in this case, be exactly 100 grains.

After a like manner this balance may be applied to find the specific gravity of liquids, as is easy to conceive from what has been said.