1890 - 91 .] Mr J. Aitken on a Simple Pocket-Dust-Counter. 41 
Returning now to the first weak point, viz., the air-pump valves, 
reference to the Plate given with this paper will show how this 
difficulty has been overcome. The figures on the plate show the 
new pocket instrument drawn full size. In the figures, R is the 
receiver, and P the pump. It will be observed that the receiver R 
communicates with the pump P by means of the stopcock K, and 
it will be noticed that all difficulty with the valves is got over by 
simply removing them altogether, and making the stopcock K act 
the double part of air-pump valves, and valve for admitting the air 
to be tested. The passage through the plug of the stopcock K is 
not straight, but is bored at right angles as shown. It will also be 
noticed that there are three ports in the body of the stopcock — one 
communicating with the pump P, one with the receiver R, and one 
with the outer air. 
It will be observed that the lower part of the air-pump is similar 
in design to that of the portable apparatus. The continuation of 
the pump-barrel G forms a guide for the piston-rod of the pump, 
and has a scale graduated on it to enable the observer to introduce 
into the receiver different proportions of impure air for testing. At 
fig. 4 is shown a separate drawing of the guide-tube, and on it is 
shown the scale marked with the figiires Au, rh> t - This scale 
is so divided that when the guide-collar is drawn down to those 
respective marks it enables us to introduce into the receiver 
measured quantities of impure air, such that when mixed with the 
pure air in the receiver and expanded there will be these propor- 
tions of impure air in the receiver. So that on making a test and 
counting the number of drops per cubic centimetre in the mixed 
and expanded airs in the receiver, the number so obtained must 
be multiplied by the proportion of impure air used. Suppose, for 
instance, that we drew down the pump to the mark on the 
scale, and on testing the mixture of this amount of air with the 
pure air in the receiver, and observed, in say, ten tests, an average 
of two drops per square millimetre, then as there is one centimetre 
of air above the counting stage, two drops per square millimetre 
will be 200 per cubic centimetre in the air of the receiver, but 
this figure must in this case be multiplied by 20 to get the number 
of particles in the outer air, which in this case would be 4000. 
The guide G is fitted to the cylinder cover of the air-pump by 
