552 NATURE 
ON THE SCIENCE OF WEIGHING AND 
MEASURING, AND THE STANDARDS OF 
WEIGHT AND MEASURE * 
VII. 
WEIGHING AND MEASURING INSTRUMENTS, AND 
THEIR USE 
HE instrument universally used for weighing is the 
balance, with its various modifications. It serves to 
determine the weight of bodies by comparison with a 
body of known weight, such as a standard weight. The 
simplest form of balance is a beam made to vibrate upon 
a centre or axis of motion, with pans hanging from the 
extremities of the two arms of the balance. These two | may be, in relation to their thickness and the weight they 
pans hold the bodies compared, and their equality or 
difference of weight can thus be determined. 
Balances are of two kinds:—1. Ordinary balances 
with equal arms, which have the beam suspended by the 
middle. If an equal-armed balance is accurately ad- 
justed, so that the beam is exactly horizontal when the 
pans are empty, the beam will also be horizontal, and the 
balance will be in equilibrium when equal weights are 
placed inthe pans. 2. Balances with unequal arms, in 
which the beam vibrates upon the centre of motion placed 
more or less near one of the extremities. In both of these 
kinds of balance the beams are levers of the first order, 
the fulcrum upon which the beam vibrates being placed 
between the power and the weight, that is to say, between 
the extremities of the beam which support the bodies 
compared. On the principle of the lever, the power of 
any weight to move a balance is proportionately greater 
according as the part of the beam which supports that 
weight is more distant from the fulcrum or centre of 
motion of the balance. Hence it follows that the power 
of the weight to move a balance is in a ratio compounded 
of the weight itself and of its distance from the centre of 
motion of the balance. A multiplying or proportionate 
balance may consequently be constructed for determining 
the weight of a body placed in the pan suspended from 
the shorter arm of the bearer, and required to be equal 
to any multiple of a given unit weight placed in the pan 
suspended from the longer arm of the beam, termed 
the weight pan. For this purpose, if the beam be divided 
into, say three equal parts, and the centre of motion be 
placed at the first division, one pound placed in the 
weight pan will form an equipoise with two pounds placed 
in the other pan, and so on. This principle is greatly 
extended in larger weighing machines by lengthening the 
longer arm, through the use of compound levers, so that 
one pound can be made to form an equipoise with 100 
pounds or more. 
The ancient Roman balance is perhaps the earliest form 
of a well-constructed multiplying balance, and corre- 
sponds with our modern steelyard. It has been remarked 
by Sir Gardiner Wilkinson that no instance has been 
found of the existence of the steelyard before the Roman 
era. But the principle of its construction was in use 
amongst the ancient Egyptians, who ascertained the 
weight of articles suspended from different parts of a 
scale beam by means of a heavy determinate weight 
placed in one scale. The Roman balance consists of a 
determinate weight attached to the longer arm of the 
beam, and made to traverse along a number of divisicns 
marked upon it. The multiplied power of the traversing 
weight when resting on the several sub-divisions, as they 
increase in distance from the centre of motion, is indi- 
cated by corresponding figures upon the graduated beam. 
The following figure (iaken by permission from the 
“Tmperial Journal of Art,” vol. i. p. 85) represents an 
ancient Roman balance of an elegant form, found at Pom- 
peii, and in use A.D. 77. It is described as having the 
graduated divisions on the longer arm of the beam marked 
* Continued from p. 49% 
a 
[ Oct. 30, 1873 
with Roman numerals from X. to XXXX. (probably 
Roman pounds), and with a V. on the half of each deci- 
mal series, the smaller subdivisions being also marked. 
The inscription on the shorter arm of the beam (shown 
in a separate and enlarged figure) denotes its having been — 
proved at the Capitolin the 8th of Vespasian Emperor 
Augustus, and in the 6th Consulate of Titus Emperor 
Augustus his son. This steelyard is consequently a duly 
verified standard weighing machine. 
For the justness of an equal-armed balance, it is requisite 
(1) that the points of suspension of the pans from the beam | 
be exactly in the same line as the centre of motion ; (2) that 
these points be precisely equidistant from the centre of 
motion ; (3) that the arms be as long as conveniently 
are intended to carry, in other words, consistently with — 
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SSS SS SSS 
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POLICE = 
Fic. 15.—Ancient Roman Balance, 
the stability of the balance; (4) that there be as little 
friction as possible at the centre of motion and the points — 
of suspension ; (5) that the centre of gravity of the bear 
be placed a little below the centre of motion. 
_ The fulcrum upon which the beam of a balance rests 
is formed with a steel knife edge, and the two pans at its 
extremities are hung upon similar knife edges. In 
ordinary trade balances, these knife edges, are placed in 
contact with steel bearings having a spherical curve. But 
in the practical construction of balances of a high degree 
of sensibility, such as are required for scientific purposes 
or for the comparison of standards in which very minute — 
differences of weight are to be determined, there are 
many circumstances to which attention is requisite, that 
may properly be neglected in balances used for com- 
mercial purposes. In such balances of precision great — 
