42 
Proceedings of Royal Society of Edinburgh. 
SESS. 
means of a screw to enable it to be taken to pieces for the con- 
venience of packing. The piston of the air-pump is packed with 
the usual cupped leather. To this I have added a small spring 
ring, as shown in drawing, at the lower part of the piston. This 
ring, so far as experience yet goes, has been found to be an 
advantage, and has kept the piston always tight with varying 
degrees of dryness of the leather. 
The second weak point in the portable apparatus, to which 
reference has been made, is the india-rubber tube making the air- 
tight joint between the rod of the stirrer and the receiver. Here, 
again, as will be seen from the drawing of the pocket instrument, 
the difficulty has been got over by simply removing it, and closing 
the end of the tube with a metal cap. This is possible in the 
pocket instrument, because we can move the stirrer without touching 
it. It is only necessary to invert the instrument, when the stirrer 
drops to the other end, and on again bringing the instrument to its 
original position, the stirrer again drops to the bottom. These 
movements are made two or three times to make the mixing 
thorough. Some little attention is necessary in the construction of 
the stirrer. It will not do to put a diaphragm into the receiver 
and allow it to fall anyhow. The effect of that would be to manu- 
facture a great quantity of nuclei, and copious showers would 
invariably follow its use. These showers are caused by nuclei 
formed by the wet surface of the stirrer splashing on the wet 
surface of the receiver. It will be seen from the drawing that the 
stirrer is caused to move truly by means of a small rod fixed in it, 
and projecting downwards. The lower end of this rod enters a 
tube which projects through the bottom of the receiver, the lower 
end of this tube being closed. Both ends of the guide-rod 
are pointed to reduce this splashing surface to a minimum. 
When the instrument is inverted, the falling stirrer keeps parallel 
to the top and bottom of the receiver, but touches neither, save at 
the points, and nuclei are rarely formed. As in the other instru- 
ments, both sides of the stirrer and the bottom of the instrument 
are covered with blotting-paper, cemented on with india-rubber 
solution. The blotting-paper is kept moist to saturate the air, and 
supply water for the rain drops, when the entering air is dry. 
On referring to the drawing it will be seen that there is no 
