George Graham, with his mercurial compensation pendulum, preceded John Harrison by a few years in solving the problem of temperature error caused by the expansion or contraction of the pendulum rod in heat or cold. Graham discovered that the upward expansion of mercury in a cylindrical jar, used as the pendulum bob, approximately 2 in. In diameter and 7 1/4 in. High, will compensate for the downward expansion of a one-second pendulum rod. Pendulums are sometimes made with two or more jars containing mercury. The increased surface area exposed to the air speeds the transfer of heat on change of temperature. Glass jars, usually adopted for appearance's sake, are not the best substance to contain the mercury. Steel or iron jars allow quicker heat transference because of their higher conductivity.