A clock without dial or hands regulates the other University time-pieces without deviating more than five seconds annually from the exact time. This enigma, one of the seven in the United States, stands inside a vault in the basement of the Geographical Institute, insulated from any vibration affecting the building by its foundation of sand, and kept at a constant temperature of 25 degrees. The master-clock itself, a three foot copper cylinder surmounted by a glass bell, encloses a tripod which supports a long steel pendulum; the air within the cylinder has been exhausted to 1-40th of an atmosphere. A slave clock (with a more prosaic appearance) electrically synchronized with the master every thirty seconds unlocks and resets the two-gram weight which provides all the energy needed by the big pendulum.

A year was required by the Short Company to build the apparatus, in Greenwich, England. From Rugby, England, come time signals every hour; they are recorded simultaneously with those of the master clock here on a film, which makes detection of an error of a two-thousandth of a second possible.