Meteorological Observations. 671 
Observations relating to Hygromeiry. 
One important attainment in meteorology ig 
the knowledge of the quantity of aqueous vapour 
existing in the atmosphere at any time, From 
this, and the observed temperature of the ais, we 
can easily ascertain its disposition for the eva- 
poration or precipitation of water; or in other 
words for fair or rainy weather. Various instru- 
ments have been used to determine the quantity 
of vapour in the air. The hygrometer of De 
Saussure seems to gainthe most credit ; but Ihave 
in a former essay assigned a reason for disallow- 
ing it. It is acknowledged that am hygrometer 
ought to indicate that degree of cold which is 
necessary to make the air begin to part with va- 
pour, or to form dew upon the surfaces of bodies; 
now as this is at all times capable of being effect- 
ed by a simple experiment, (see ‘page 581) it al- 
most totally supersedes the. necessity of a du- 
bious, very delicate, and consequently easily 
injured instrument. 
In my journal for a year and a half past, I 
have had acolumn entitled, vapour point, in lieu 
of thehygrometer column. By vapour point I 
mean that degree of the thermometer at which 
dew begins to be formed at the time, The 
* VOL. V. Y¥ x 4, 
