808 Description of a Ventilator 
above and below, as at a a ; and should be further secu- 
red by some hold-fast near to keep it steady in work- 
ing. 
Fig. 2, is a bottom of wood, four inches and a half 
thick, with a projecting rim at its base, for the metal cy- 
linder to rest on, when cemented and screwed to the 
wood. The centre of this bottom is excavated, for the 
reception of the crown of the nosle. In the same figure 
the nosle is represented with its crown like a bowl-dish, 
to condense the air gradually, without resistance, in its 
advance to the more contracted base of the inverted cone, 
i. e. the top or entrance of the nosle. About two-thirds 
down this nosle may be fixed a male screw, as c c, for 
the purpose hereafter mentioned. 
N. B. The forcing-pump should be cased in wood, to 
protect it from outward bruises, which would prevent the 
working of the piston, and ruin its effects. The leather 
round the embolus should be greased when used. 
Fig. 8, is a crutch-handle, fastened to the embolus A 
by its iron legs B, B. — A is a cylinder of wood, cased 
with leather, so as to fit well, but glide smoothly in the 
metal cylinder; having an opening as large as its strength 
will permit, for the free access of atmospheric air. C is 
a valve, well leathered on its top, and yielding down- 
wards to the pressure of the air when the piston is raised 
up. D is a cross bar of iron, to confine the valve, so that 
it may close instantly on the return of the piston down- 
wards. 
Fig. 4, is a tin pipe or tube, of less than four inches 
diameter, and of such length as, when fixed to the base 
of the cylinder, Fig. 1, shall admit the nosle d , Fig. &, 
to within half an inch of the valve E, at the bottom of the 
wooden cylinder F, in Fig. 4 ; which valve E will then 
yield to the pressure of air condensed in its passage 
through the nosle, and deliver it into the pipes below. 
