er-in-law, Florin Perier (1605-1672), who measured the height
of the mercury column at various altitudes on the Puy de Dome. Pascal
himself tried the experiment at several towers in Paris,--Notre Dame, St
Jacques de la Boucherie, &c. The results of his researches were embodied in
his treatises _De l'equilibre des liqueurs_ and _De la pesanteur de la
masse d'air_, which were written before 1651, but were not published till
1663 after his death. Corroboration was also afforded by Marin Mersenne and
Christiaan Huygens. It was not long before it was discovered that the
height of the column varied at the same place, and that a rise or fall was
accompanied by meteorological changes. The instrument thus came to be used
as a means of predicting the weather, and it was frequently known as the
_weather-glass_. The relation of the barometric pressure to the weather is
mentioned by Robert Boyle, who expressed the opinion that it is exceedingly
difficult to draw any correct conclusions. Edmund Halley, Leibnitz, Jean
Andre Deluc (1727-1817) and many others investigated this subject, giving
rules for predicting the weather and attempting explanations for the
phenomena. Since the height of the barometric column varies with the
elevation of the station at which it is observed, it follows that
observations of the barometer afford a means for measuring altitudes. The
early experiments of Pascal were developed by Edmund Halley, Edme Mariotte,
J. Cassini, D. Bernoulli, and more especially by Deluc in his _Recherches
sur les modifications de l'atmosphere_ (1772), which contains a full
account of the early history of the barometer and its applications. More
highly mathematical investigations have been given by Laplace, and also by
Richard Ruhlmann (_Barometrischen Hohenmessung._, Leipzig, 1870). The
modern aspects of the relation between atmospheric pressure and the weather
and altitudes are treated in the article METEOROLOGY.

Many attempts have been made by which the variation in the height of the
mercury column could be magnified, and so more exact measurements taken. It
is not possible to enumerate in this article the many devices which have
been proposed; and the reader is referred to Charles Hutton's _Mathematical
and Philosophical Dictionary_ (1815), William Ellis's paper on the history
of the barometer in the _Quarterly Journal of the Royal Meteorological
Society_, vol. xii. (1886), and E. Gerland and F. Traumueller's _Geschichte
der ph