38 QUANTITATIVE AGRICULTURAL AXALIMX

it to swing several times in both directions, noting when a pointer
attached to the beam swings equal distances on either side of a
"zero point3' on a fixed scale. The balance should be adjusted
so that without load it swings about the true zero of the scale
but thermal changes, settling of buildings, etc., will cause this to
change and the zero point must be determined occasionally and
the adjustment changed, if found necessary.

Differential Weighing. — Where the desired weights are found
by a differential process it is not necessary that the adjustment
of zero point should be made, or even that the zero point should
be known. It is sufficient to assume the zero point- to be t he same
as that of the scale. Although this may involve an error in
weighing, this error will be the same for both weights obtained
and the subtraction will eliminate it entirely. For example, a
crucible is being weighed empty and again containing a pre-
cipitate of barium sulphate, in order to find the actual weight of
the latter. A plus or minus error may have been made in the
recorded weight, due to an incorrect assumption of zero point,
but this will be the same for both weighings and when I he*
observed weight of the empty crucible is subtracted from the
observed weight of the crucible and barium sulphate, this error
disappears.

To Determine the Zero Point.— (''lose the hfilnntw ciuw nnd
lower the pan rests in such a manner a« to «top any hit oral winging f»f th«
pans, then lower the beam ro«t8 and net the hr»am in motion In* allowing
the rider to rest momentarily on the? beam, then mining it. Thin hhottid
cause the pointer fco swing five* to ten divinionM on either wiclc* of the j?w>
of the scale. Take at least throe roidingH on ono Hide and two OH the cither,
Subtract the less average from the greater and divide the* remainder by two.
This gives the zero point if the proper direction is noted.
The zero point may be determined with ttufficient accuracy for mmt
work by simple observations without compulation x, by noting that
the amplitude of vibration of the pointer diminixhvx regular ly with
each successive swing.
Weighing by the Single Deflection Method.— This rapid
method has been described by Brinton.1
The pan rests must first he adjusted HO that when rclfUHcd they
shall give no swinging impulse to the system. That int if the
1 /. Am. (them. /for., 41, 1151 (1919),