mercury, jar and rod rapidly
acquire the same temperature. This pendulum is less likely to break
than the form just described. The depth of mercury required in an iron
jar is stated by Lord Grimthorpe to be 8-1/2 to 9 in. The reason why it
is greater than it is when a glass jar is employed is that iron has a
larger coefficient of expansion than glass, and that it is also
heavier. In all cases, however, of mercury pendulums experiment seems
to be the only ultimate test of the quantity of mercury required, for
the results are so complicated by the behaviour of the oil and the
barometric errors that at its best the regulation of a clock can only
be ultimately a matter of scientifically guided compromise. A small
amount of compensation of a purely experimental character is also
allowed to compensate the changes which temperature effects on the
suspension spring. This is sometimes made as much as 1/6 of the length
correction.

As an alternative to the mercurial pendulum other systems have been
employed. The "gridiron" pendulum consists of a group of alternate
rods of steel and brass, so arranged that the expansion of the brass
acts upwards and counteracts that of the steel downwards. It was
invented in 1726 by John Harrison. Assuming that 9 rods are used--5 of
steel and 4 of brass--their lengths may be as follows from pin to
pin:--Centre steel rod 31.5 in.; 2 steel rods next the centre 24.5
in.; 2 steel rods farthest from centre 29.5 in.; from the lower end of
outside steel rods to centre of bob 3 in.; total 89.5 in. Of the 4
brass rods the 2 outside ones are 26.87 in.; and the two inside ones
22.25 in.; total 49.12 in. Thus the expansion of 88-1/2 in. of steel is
counteracted by the expansion of 49 1/8 in. of brass. Everything
depends, however, on the expansion coefficient of the steel and brass
employed, the requirement in every case being that of total lengths of
the brass and iron should be in proportion to the linear coefficients
of expansion of those metals. The above figures are for a very soft
brass and steel. Thos. Reid, with more ordinary steel and brass,
prescribed a ratio of 112 to 71, Lord Grimthorpe a ratio of 100 to 61.
It is absolutely necessary to put the actual rods to be used for
making the pendulum in a hot water bath, and measure their expansions
with a microscope.

John Smeaton, taking advantage of a far greater expansion coefficie