The Sidéromètre was designed in the late 1930's by Commandant Bastien to provide a means of rapid position "fixing" for aerial navigation. Special tables were required, called "Aeronautical Ephemerids" and these were produced to be used with the instrument. An almost identical instrument from the same series was described and illustrated by Anthony Randall in ?The Time Museum Catalogue of Chronometers?, Rockford, Illinois 1992, pp. 110-111, figs. 66 a-d. According to Anthony Randall, a Sidéromètre does not indicate sidereal time as such, although it is adjusted to run at sidereal, rather than mean time, and then converts this into degrees, minutes and seconds of Earth?s rotation. The center-seconds hand, therefore, rotates not in sixty seconds of sidereal time but in the time required by the Earth to rotate through 1°, in relation to the stars. As the Earth rotates through 1° in four minutes of time, the seconds hand rotates once every four miutes. The angular rotation of the Earth is recorded by the numbered discs appearing in the dial apertures: degrees are shown on the right-hand side, tens of degrees in the center, and hundreds of degrees on the left. There is an electrical input on the side of the box to connect to the power supply of the airplane - presumably about 24 volts. This provides the electrical energy for the bulbs and for a small thermostatically-controlled heating element inside the box. Since the temperature inside an airplane of the period could drop to a very low value at high altitude, precautions were necessary to prevent rapid cooling of the inside of the box, with a consequent risk of condensation in the mechanism and even the possibility of freezing of the oil. Although accurately compensated, the watch would otherwise also have been subject to a transient response due to rapid and extreme temperature change, and this had to be prevented.