144 Proceedings of Royal Society of Edinburgh. [sess. 
plug is in order that the passage through it may he short, so that 
as little dust as possible may be lost by settling in the narrow pas- 
sage. The second point to be attended to is to have the passages 
in the body of the stopcock joining A, B, and C as short as possible. 
The passage joining I is of no importance. The object of having 
these passages short is to ensure that the air in them shall be in the 
same condition as the air in the spaces into which they open. 
In order that the stopcock F may be worked with ease and pre- 
cision, three stops are provided for checking the movement of the 
handle when the ports are in the correct position. These stops are 
shown by the dotted lines at H x , H 2 , H 3 , fig. 4. The first and last 
of these stops are fixed ; the middle one is arranged so that it can 
be removed. When working with the cylinder measure, the handle 
G moves between the stops H x and H 2 . When at Hj the cylinder 
is in connection with A, and the air to be measured and tested is 
then taken in. When the handle is pushed up against the stop H 2 , 
the passage between the cylinder and the pipe C is opened, and 
the measured quantity of dusty air can be driven into D. When 
we wish to allow the air to pass direct from A to the receiver, the 
stop H 2 is removed and the handle brought up against H 3 , when the 
passage between A through I to C is opened. 
Turning now to the method of measuring by means of the cylinder. 
It will be seen that it is provided with a piston which is fitted with 
two cupped leathers. Two leathers have been used, because the 
piston must be quite tight, and as pressures act both ways, it was 
thought best to use a cup looking each way. The tube L is securely 
fixed to the end of the cylinder by means of a movable screw 
joint, in the manner shown. This tube is slightly larger in diameter 
than the cylinder, and acts as a guide to the piston rod; it also 
carries the scale and the stops used in measuring. To the piston is 
fixed the piston rod K, which is moved by means of the collar M, 
which slides on L. The piston and collar are connected by means 
of a pin N passing through them. The pin N slides in the slots 
00 cut in the pipe L. The collar M is grasped by the finger and 
thumb, and when moved back and forward carries the piston with 
it. In the pipe L, at right angles to the slots 00, are a series of 
holes P . . . P drilled at certain equal distances apart. In the 
drawing the cylinder is supposed to have an area of 1 square 
