134 
THE ART OF REARING 
3. Of the manner of easily drying leaves in 
rainy weather. 
4. Of rearing silk-worms until maturity. 
1 . Of the necessity of the Barometer to measure 
the humidity of the Air in the Laboratory. 
We are surrounded by bodies which sometimes 
absorb the moisture of the atmosphere, and 
sometimes create the moisture ; this phenome- 
non is continually passing before our eyes. 
We see that salt, for instance, is more or less 
damp, according to the state of the atmosphere ; 
that is to say, in proportion as it has absorbed the 
moisture, or lost it by evaporation. 
Atmospherical air is generally dry when the 
wind sets from the north, and damp when from 
the south. 
Scientific men have thought it useful to invent 
instruments, fitted to measure the quantity of 
moisture which the air may contain in any cir- 
cumstances, using in their construction bodies 
which attract the dampness from the air easily, 
and by degrees, and which give it out again when 
the air is dry. 
These bodies, which stretch when they receive 
the moisture, and shrink on losing it, placed 
in certain instruments, shew, by degrees, the 
quantity of moisture which they lose, or im- 
bibe. These instruments are called barometers, 
