50 Proceedings of Royal Society of Edinburgh. [sess. 
all that is desired, and there is little trouble from the needle 
blunting under the operation. In ruling these lines, it is, of course, 
necessary to keep the pressure on the needle constant, and to make 
the same number of strokes across the glass for each line, in order 
that the lines may be equally thick. 
The pocket instrument has been occasionally in use during the 
whole of this summer, first with a silver stage and then with a 
glass one, and has been working quite satisfactorily, and giving 
results agreeing with those of the larger instrument. The instru- 
ment appears to be now so simple, it can be easily worked by 
any one. So far as can be seen at present, the weakest point in 
the instrument now, and the only one likely to cause trouble, is the 
piston packing. If the piston is not tight, correct work cannot be 
done. Fortunately, the conditions of testing make it impossible, 
with ordinary care, to make a test with a badly-fitting piston ; 
because it would be impossible with it to thoroughly purify the 
air in the receiver. When air leaks in past the piston, nuclei are 
admitted, and these prevent the showers in the receiver ceasing 
completely. If the piston leaks a little, at each stroke of the pump, 
though no air has been admitted by the stopcock, a few drops will 
be seen falling, and call the attention of the observer to the 
imperfection. 
To reduce the trouble from this cause as much as possible, I have 
introduced the spring ring already referred to, under the leather 
cup packing, and so far this has worked well ; the piston has given 
no trouble since its introduction. One objection to the cup leather 
packing is, that if it gets out of order the repair of it might not be 
within the powers of the observer. To obviate this objection, the 
arrangement shown in fig. 5 has been designed, tried, and found 
satisfactory. It consists simply of the substitution of a plunger- 
pump for a piston one. The advantage of the plunger is, that any 
one can easily pack the stuffing-box, and some kind of material for 
doing it can always be obtained. In appearance, the plunger-pump 
is not so compact as the piston one, on account of the diameter of 
the stuffing-box requiring the guide-tube to be made of much 
greater diameter. Yet this is little disadvantage so far as compact- 
ness for packing is concerned, as the pump-barrel can be slipped 
inside the guide-tube, when unscrewed for packing in its case. 
