This confidence was warranted, for the decade of the 1850's had seen the appearance of major innovations in the basic instruments--thermometer, barometer, and wind velocity indicator--that made available instruments more adaptable to self-registration. It also saw the development of a new method of electrical registration derived from the telegraph. Sir Charles Wheatstone initiated this small revolution in 1843 when he reported to the British Association that he had constructed an electromagnetic meteorological register which "records the indications of the barometer, thermometer and the psychrometer [meaning wet-bulb thermometer] every half hour ... and prints the results on a sheet of paper in figures," running a week unattended. The working of this register involved the insertion of a conductor in the tubes to make a circuit, the thermometers having open tops.[26] This was ten years after the development of the electromagnetic relay and six years after Wheatstone's introduction of his own telegraph. Wheatstone's instrument left a very ephemeral record in the meteorological literature, and appears to have been defective or out of fashion with its time, which was concerned with the introduction of photographic instruments. Wheatstone's work was rediscovered, along with that of several other much earlier inventors, by the determined observatory directors of the 1860's. Of the five systems developed at that time, four used electromagnetic registration, only Draper adhering to a mechanical system (see fig. 11). For temperature measurement Secci and Hough used Wheatstone's electrical system with a mercurial thermometer (fig. 12), but the other four utilized a physical principle which had been proposed periodically for at least a century--the unequal thermal expansion of a bimetallic strip. This principle had been utilized by watchmakers for a quite different purpose--the temperature compensation of the watch pendulum--but its possibilities as a thermometer had been known long before the mid-19th century.[27] [Illustration: Figure 8.--Hipp's registering aneroid barometer, with a telegraphic printer. (_USNM 314544; Smithsonian photo 46740-D._)] For the measurement of pressure, Secci, Wild, and Draper adopted, or rediscovered, the balance barometer devised by Wren in the 17th century. In this type of instrument (see figs. 13, 15) either the tube or the reservoir of the barometer is attached to one arm of a balance, the equilibrium of which is disturbed by the movement of the mercury in the instrument.[28] [Illustration: Figure 9.--Front and rear views of Secci's meteorograph, 1867. (From Lacroix, _op. cit._ footnote 22.)]