446 THE SCIENTIFIC MONTHLY
remembrance. He did not occnpy his time in calcnlations as to the number of cubic feet of air required per person per hour, but he designed a workable apparatus on the principle of the bellows for the purpose of abstracting air from places particularly badly situated as regards the changing of their air. The earlier forms were worked by hand, the later were driven by a windmill, their general design being much the same as that of bellows for church-organs. The velocity of outflow of air from the bellows. Hales expresses as 6^ miles an hour, a little over 9 feet per second.
His first paper on the importance of ventilators in mines, hospitals, prisons and ships was read to the Boyal Society in 1741, in which year an almost identical invention was announced by one Martin Friewald, '^ captain of mechanics to the King of Sweden. So useful was this latter apparatus that the French government ordered an installation of it on all the ships of their navy. Not once or twice in the story of invention have important discoveries been made simultaneously and often in countries widely distant, as in the present instance. The title- page of the treatise in which Hales describes his invention reads thus:
A description of Ventilatora whereby great quantities of freeh air maj with ease be conveyed into mines, gaols, hospitals, workhouses and ships in ex- change for their noxious air, and in preserving all sorts of grain dry, sweet and free from being destroyed by weevils both in granaries and ships ... as also in drying com, malt, hops, gunpowder &c and for many other useful purposes which was read before the Boyal Society in May 1741.
Truly it was not an age of succinct titles, but its length enables us to see that Hales had a clear idea of that one thing needful, namely, fresh in place of noxious air; he was under no doubt whatever that the air of mines, gaols, hospitals, workhouses and ships, left to itself, be- comes noxious and must be changed. This is corroborated by the quotation from Milton which follows:
and God made The firmament expanse of liquid, pure transparent, elemental air.
In this work he speaks of "the rancid vapors from human bodies"; from this it is not quite clear whether or not Hales was distinguishing respiratory carbon dioxide from the noxious vapors arising from the skin and limgs. We must at any rate remember, as has been already said, that he wrote without knowing of the discovery of respiratory carbon dioxide. It is certainly interesting to be told the very latest opinion as to the deleterious nature of breathed air is that it is not due to the carbon dioxide per se, but that the headache and distress are due to the moisture, the heat and the disagreeable, volatile, organic effluvia from the skin and lungs of the persons within the confined space. Without any technical chemical knowledge of the precise cause of impure air. Hales had grasped the far more important fact that
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