66 
HYGROMETER. 
rienced their violence, from which some or other of our ships, 
especially Indiamen, yearly suffer. It is also to be lamented that the 
hygrometer, especially Leslie’s, is not more used as a meteorological 
instrument. During my voyage I repeatedly experienced the pecu- 
liar delicacy of this instrument in indicating changes in the humidity 
of the atmosphere ; and as all winds are probably charged with their 
peculiar moisture, this instrument would express their slightest vari- 
ations. And I have no doubt that by repeated observations a 
hygrometrical scale might be formed which would be of great utility 
to the mariner.* Mr. Leslie’s instrument requires, however, a little 
modification before it can be applied with effect in those latitudes in 
which the air is saturated with moisture. At present its scale is 
graduated to about 120° marking the point of greatest dryness, and 
zero the point of greatest moisture : this last point is placed at the 
extremity of the scale near the bulb. In using this instrument in 
the China seas during the S. W. monsoon, I found that the fluid of 
the instrument remained in that part of the tube which is between 
the bulb and zero. Might not this part of the instrument be length- 
ened, and zero marked higher up ? 
Whenever this instrument indicates a saturated state of the atmo- 
sphere at periods when changes of wind are probable, such change 
may be expected to be sudden and violent. For if a large body of very 
humid air come in contact with a wind colder than itself, its moisture 
will suddenly be precipitated, which producing a comparative 
vacuum, either the wind which occasioned it will blow with great 
violence, or if the subversion of the equilibrium has been exten- 
sive, every point of the compass will contribute to its restoration. 
That something of this kind occurs in the Typhoons of the China 
seas, and in the hurricanes of the East and West Indies, appears 
very probable from the consideration of the circumstances attend- 
ing them. These storms are generally most severe near the land, 
and in narrow seas between the tropics, and during the hottest 
# 
See on this subject the Observations of Krusenstern, Peron, and Humboldt. 
