VENTILATION. 
71 
capacity will no doubt be found very useful ou the 
island where water can only be had by such economy. 
As no boat would hold this large machine, the aper- 
tures were stopped, and it was floated on shore, and 
seen on its passage from the top of the mountain, it 
looked like a floating omnibus. 
In condemning so integral a portion of the costly 
ventilating apparatus, it may appear paradoxical to say 
that the theory was good, but that we found it in the 
‘ Wilberforce’ to be of no perceivable benefit in supply- 
ing improved air to the people. To carry out the 
principle to its full extent, all apertures except those of 
the ‘ purificator’ — about eighteen inches in diameter — 
ought to have been closed, so as to suffer no air to 
approach the many lungs which were gasping for it, 
except what had passed through the medicating 
substances. There can be no doubt that if a sufficient 
quantity could have been transmitted by these means 
it might have been deprived of much of its noxious 
nature; but the wind-sails which were intended to 
bring the air from the elevation of the mast-heads 
were never even inflated by the force of the fanners. 
This method, called the plenum impulse, was found 
in the ‘ Wilberforce’ to have merely the effect of com- 
pressing the stagnant and deteriorated air into all the 
remote extremities and corners of the ship ; but the re- 
verse process, — the vacuum which was always used in 
the Niger after the failure of the other was ascertained, 
— performed at least the good office of extracting it; 
