216 
Scientific Proceedings, Royal Dublin Society. 
90 cm. in length, as uniform in bore as possible, is fitted with a 
floating piston, as described by me upon a former occasion.1 Here 
I will describe a form specially suited to the present purposes (see 
accompanying figure). d, b, c, dis a cylindrical ivory box turned 
MMM 
TN 
tilt 
! | 
HTT 
2, Very flat and sharp-edged on the face a, 6. It fits the 
glass tube so truly that it just sinks slowly in it when 
the tube contains air. The end c, d screws off. /fisa 
solid cylinder of ivory turned with a neck g, by which 
it is attached to the box. The float is shown as it 
acts at the base of a column of mercury; it keeps the 
base from breaking, although the column be violently 
jogged up and down in the tube. Nor can air pass 
from below the mercury piston to above it except the 
tube be considerably inclined. It will be understood 
now that the box form is conferred upon 4a, 8, ¢c, d so 
that little weights (fine wire) may be put into it to 
counteract the flotation-pull of f if this is too strong. 
Of course this may also be accomplished by taking 
something off the bulk of / till the flotation force is 
adjusted. 
With this float we may suspend a column of mercury 
in the wide tube up to the barometric length, and a 
whole class may see it, the ivory piston being very 
conspicuous. 
I may add here that it seems very important not to 
speak of the vacuum as “sucking up” the mercury. 
It is my experience, in fact, that it is most important 
to impress upon the student the absolute inertness of 
the vacuum. In the vacuum there is nothing worth 
speaking of, and how can “nothing” pull up the 
mercury? The mercury is shoved up, not pulled up. 
Similarly this is of importance in explaining the oper- 
ation of lifting water by pulling up the piston of a 
pump. The water is being pushed up after the piston 
by a force equal to that which supported the hanging 
column of mercury; in fact, this push is helping us to lift the 
atmosphere pressing on the piston. When the shoving force is used 
1 Proceedings, ante vi1., p. 547. 
