0"T' LAERT> "! - USP

THE SEVENTEENTH CENTURY. 347
through tubes, invented the barometer (1643) and explained
that the rise and fall of the mercury are due to changes of
atmospheric pressure. PASCAL made some irrefutable
statements on this subject and pointed out that with the
help of the barometer the difference in height between
two places can be determined. MARIOTTE, J. PECQUET,
and SINCLAIR rendered these ideas more complete and
brought the practical application of them nearer. PASCAL
constructed a wine-barometer, while BERTI and O. V
GUERICKE enclosed water instead of mercury in the tube-
OTTO VON GUERICKE, burgomaster of Magdeburg and
formerly engineer of the fortress of Erfurt, devised the
air-pump and considerably astonished the princes assembled
in the imperial diet at Regensburg in 1654, by the experi­
ments he performed with it. He made excellent observa­
tions on the weight of air and constructed the first
manometer for measuring the density and weight of the
atmosphere. He also referred to the fact that in spaces
from which the air has been removed no sound can be pro­
duced and no combustion take place.s His observations
were completed by BOYLE, who studied more closely the
elasticity of air and discovered the law, erroneously named
after MARIOTTE, that the volumes of equal weights of air
stand in inverse ratio to the pressures to which they are
subjected. About the same time attempts were made to
determine the velocity of sound. GASSENDI asserted that
it travels 1,473 * eet i n a second. MERSENNE came somewhat
nearer the truth in estimating the number of feet at
1,380. Even if the results they arrived at were incorrect,
they nevertheless hit upon the right method of investiga­
tion and this was already an extraordinary advance. Even
a NEWTON was unable to avoid every source of error; he
calculated the velocity of sound at 906 Paris feet in a
second, not making a sufficient allowance, as LAPLACE has
pointed out, for the influence of temperature.
The most important advances were made in optics,
favoured, and, to some extent indeed, rendered for the first