1890 - 91 .] Mr J. Aitken on a Simple Pocket Dust-Counter. 51 
It may be asked — Does this simple instrument displace the more 
complicated earlier forms of the apparatus ? Have the earlier 
forms been unnecessarily complicated 1 The answer to this is — That 
the pocket instrument is designed for special work, and only for 
that work ; while the earlier forms are still necessary, and can do 
work in conditions in which the pocket instrument would be use- 
less. The large instrument fitted up in the Ben Nevis Observatory, 
with its arrangement of circulating pipes, aspirator, and artificial 
illumination, is still the best form for a first-class observatory, where 
observations have to be made in all ;weathers, and during night as 
well as day. The Portable instrument is still necessary when we 
wish to test locally poluted air, such as that near human inhabita- 
tions, that is for sanitary work ; while the use of the Pocket 
instrument is confined to meteorological work in the open air, 
and its advantages are simplicity and lightness. 
It may be remarked here that the Pocket instrument may be used 
to give a rough indication of the impurity of polluted air. The manner 
of using it for this purpose is as follows : — First, turn the stopcock K 
a quarter turn to the left, and draw down the piston. This takes 
the impure air into the cylinder. The whole of this air is then 
discharged by pushing the piston to the top of its stroke. By these 
movements nearly, but not quite, all the impure air is expelled from 
the cylinder. The small passage between the stopcock and the 
piston is still full of impure air. Immediately on pushing the 
piston to the top of its stroke, the stopcock is returned to its 
original position; the piston is then drawn down, and at once 
returned to its top position. By these movements we have taken 
some of the pure air out of the receiver and mixed it with the small 
amount of impure air in the pump passage, and the return stroke 
has sent the mixture into the receiver, where after being stirred, a 
shower is produced, and the drops counted. 
This cannot give a very accurate result, as some of the 
particles must be lost when the air is drawn in from the receiver to 
mix with the impure air in the pump passage. This loss, however, 
does not seem to be great, owing probably to the higher temperature 
of the pump-barrel, from contact with the hands, preventing con- 
densation. Owing to the possibility of some air being left between 
the top of the piston and the cylinder, it would be difficult to 
