These means, combined with the 300 absolute
determinations, give 4' as the annual change of the declination.
M. Mascart's apparatus proved to be wonderfully useful in recording the
rapid and slight perturbations of the magnet. Comparisons between the
magnetic and atmospheric perturbations gave no result. There was,
however, little stormy weather and no auroral displays. This latter
phenomenon, according to the English missionaries, is rarely observed in
Tierra del Fuego.
The electrometer used at the Cape was founded upon the principle
developed by Sir William Thomson. The atmospheric electricity is
gathered up by means of a thin thread of water, which flows from a
large brass reservoir furnished with a metallic tube 6.5 feet long. The
reservoir is placed upon glass supports isolated by sulphuric acid, and
is connected to the electrometer by a thread of metal which enters a
glass vessel containing sulphuric acid; into the same vessel enters a
platinum wire coming from the aluminum needle. Only 3,000 observations
were given by the photographic register, due to the fact that the
instruments were not fully protected against constant wet and cold.
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