MOISTURE IN THE AIR. 
95 
four ” ; “ Clouds below moved toward the northwest, 
while a higher stratum moved toward the east.” 
After keeping a record for some time I began to 
study it. I noticed that usually when the thermome¬ 
ter rose the barometer fell. The conclusion was that 
the pressure of the atmosphere depends on the tem¬ 
perature, though not entirely, for several times the 
barometer fell very little though the thermometer 
rose considerably. There were also other deviations. 
Then I observed a connection between the barometer 
and the clouds; a cloudy sky with a rising thermome¬ 
ter, it seemed to me, always causes a corresponding 
fall in the barometer. Next I discovered that when 
there was much moisture in the air the barometer 
fell. 
MOISTURE IN THE AIR. 
To get some idea of the amount of vapor in the 
air, I fastened two good thermometers to a thin 
board, as shown in Fig. 52. This is a good home¬ 
made hygrometer. The bulb of one of the ther¬ 
mometers I wrapped with linen cloth, and tied with 
a thick cord, one end of which I inserted in a 
small vial fastened at the bottom of the board. 
When the vial was half-filled with water the string 
acted as a wick and kept the bulb moist. When the 
air contained as much water as it could hold, there 
was no evaporation from the cloth about the bulb, 
and the mercury stood at the same height in both 
thermometers. The drier the air the greater the 
evaporation. 
