ysikalischen Experimentierkunst_ (1899). Descartes suggested a method
which Huygens put into practice. The barometer tube was expanded into a
cylindrical vessel at the top, and into this chamber a fine tube partly
filled with water was inserted. A slight motion of the mercury occasioned a
larger displacement of the water, and hence the changes in the barometric
pressure were more readily detected and estimated. But the instrument
failed as all water-barometers do, for the gases dissolved in the water
coupled with its high vapour tension destroy its efficacy. The substitution
of methyl salicylate for the water has been attended with success. Its low
vapour tension (Sir William Ramsay and Sydney Young give no value below
70deg C.), its low specific gravity (1.18 at 10deg C.), its freedom from
viscosity, have contributed to its successful use. In the form patented by
C. O. Bartrum it is claimed that readings to .001 of an inch of mercury can
be taken without the use of a vernier.

The diagonal barometer, in which the upper part of the tube is inclined to
the lower part, was suggested by Bernardo Ramazzini (1633-1714), and also
by Sir Samuel Morland (or Moreland). This form has many defects, and even
when the [v.03 p.0419] tube is bent through 45deg the readings are only
increased in the ratio of 7 to 5. The wheel barometer of Dr R. Hooke, and
the steel-yard barometer, endeavour to magnify the oscillation of the
mercury column by means of a float resting on the surface of the mercury in
the cistern; the motion of the float due to any alteration in the level of
the mercury being rendered apparent by a change in the position of the
wheel or steel-yard. The pendant barometer of G. Amontons, invented in
1695, consists of a funnel-shaped tube, which is hung vertically with the
wide end downwards and closed in at the upper end. The tube contains
mercury which adjusts itself in the tube so that the length of the column
balances the atmospheric pressure. The instability of this instrument is
obvious, for any jar would cause the mercury to leave the tube.

[Illustration: FIG. 1. Siphon Barometer.]

The _Siphon Barometer_ (fig. 1) consists of a tube bent in the form of a
siphon, and is of the same diameter throughout. A graduated scale passes
along the whole length of the tube, and the height of the barometer is
ascertained by taking the difference of the readings of the upper and lower
limbs respectively. This instrument may also be