Robinson.

[Illustration: Figure 4.--Kreil's balance thermometer, 1843. (From Karl
Kreil, _Magnetische und meteorologische Beobachtungen zu Prag_, Prague,
1843, vol. 3, fig. 1.)]

[Illustration: Figure 5.--Osler's self-registering pressure plate
anemometer, 1837. The instrument is shown with a tipping-bucket rain
gauge. (From Abbe, _op. cit._ footnote 17.)]

Self-recording barometers and thermometers were more vulnerable to the
influence of friction than were wind instruments, but fortunately
pressure and temperature were also less subject to sudden fluctuation,
and so self-registration was less necessary. Nevertheless, two events
occurred in the 1840's which led to the development of self-registering
instruments. One event was the development of the geomagnetic
observatory, which used the magnetometer, an instrument as delicate as
the barometer and thermometer, and (as it then seemed), as subject to
fluctuation as the wind vane. The other event was the development of
photography, making possible a recording method free of friction. In
1845 Francis Ronalds at Kew Observatory and Charles Brooke at Greenwich
undertook to develop apparatus to register the magnetometer,
electrometer, thermometer, and barometer by photography.[18] This was
six years after Daguerre's discovery of the photographic process. The
magnetometers of both investigators were put into use in 1847, and the
barometers and thermometers shortly after. They were based on the
deflection--by a mirror in the case of the magnetometer and electrometer
and by the mercury in the barometer and thermometer--of a beam of light
directed against a photographic plate. Brooke exhibited his instruments
at the Great Exhibition of 1850, and they subsequently became items of
commerce and standard appurtenances of the major observatory until
nearly the end of the century (fig. 6). Their advantages in accuracy
were finally insufficient to offset the inconvenience to which a
photographic instrument was subject.

Before 1850 the British observatories at Kew and Greenwich (the latter
an astronomical observatory with auxiliary meteorological activity) had
self-registering apparatus in use for most of the elements observed.

Self-Registering Systems

In 1870 the Signal Corps, U.S. Army, took on the burden of official
meteorology in the United States as the result of a joint resolution of
the Congress and in accordance with Joseph Henry's dictum that the
Smithsonian