1890-91.] Mr J. Aitken on a Simple Pocket Bud-Counter. 45 
the receiver a measured quantity of air. The stopcock K is now 
turned one quarter turn to the left, so as to bring the inside of the 
receiver into communication with the outer air. When this is done 
the measured quantity of air rushes into the receiver. The quantity 
of air we have taken out of the receiver to make room for the air 
to be tested is, however, still in the pump, and must now be got 
quit of. To do this it is only necessary to return the piston to the 
top of its stroke before turning the stopcock back again to bring 
the receiver into connection with the pump. When the stopcock 
K is turned to the position to admit the outer air to the receiver, it 
will be seen from the drawing that the pump is then also in com- 
munication with the outer air by means of a small passage drilled 
longitudinally through the plug of the stopcock. By this arrange- 
ment only one movement of the stopcock is necessary for admitting 
the air to the receiver, and opening the discharge valve of the pump, 
and when the stopcock is again turned to bring the receiver into 
connection with the pump, the discharge valve is closed. From 
this it will be seen, that though we have dispensed with the air- 
pump valves in this new arrangement, we have not increased the 
number of stopcocks required, nor the number of movements 
necessary for making a test. 
The necessary quantity of air being admitted to the receiver, 
and the pump emptied, and the stopcock turned to its original 
position, so that the receiver is in communication with the pump, 
the instrument is then inverted so as to cause the stirrer to drop 
inside the receiver, and again brought to its vertical position when 
the stirrer again drops. This is done two or three times to mix the 
impure air with the pure air in the receiver. When this is done, 
the instrument is held, with the top of the receiver horizontal, 
and in a convenient position for the observer viewing the 
counting stage through the magnifying glass. Expansion is 
then made, and the number of rain drops counted in the usual 
way. If in this trial more than five drops fall per square milli- 
metre, then too much impure air has been taken in, and a smaller 
proportion of impure air must be used to get a correct test. From 
the number of drops observed it is easy to determine whether Jq 
or Jg. will be the best proportion to use for testing under the 
existing conditions. On the other hand, if, in this preliminary 
