220 Blackman and Knight . — Controlling Air Movement . 
but from the records of a thermograph and hygrograph the variations 
were : 
Temperature, 65° F. to 65-5° F. 
Relative humidity, 70 % to 70*5 %. 
Table. 
30 -minute period 
Weight of water 
ending 
evaporated (in mg.). 
11.30 
a.m. 
198 
12 noon 
197 
12.30 
p.m. 
*95 
1.0 
198 
1.30 
» 
197 
2.0 
ft 
192 
2.30 
tt 
195 
3-o 
ft 
196 
3 - 3 ° 
tf 
195 
4.0 
>t 
193 
4-30 
tt 
196 
5.0 
tt 
193 
The maximum variation is about 3 per cent., and this includes all errors 
of weighing in addition to those due to slight variations in atmospheric 
conditions. 
The apparatus has been used and found satisfactory for air movement 
up to a speed of 25 metres per minute, although the higher speeds are 
generally not convenient for transpiration experiments. 
By means of experiments with atmometers it has been found desirable 
to use as high a speed as is conveniently possible, since greater regularity 
is obtained in this way. This is only to be expected, as chance air-currents, 
which are unavoidable in a laboratory, will have less influence on a rapid 
air-stream than on a slower one. 
