a machine by which the noxious air of any close place, as an hospital, gaol, ship, chamber, &c. may be discharged and changed for fresh.

The noxious qualities of bad air have been long known; and no one has taken greater pains to set the mischief arising from foul air in a just light than Dr Hales; who has also proposed an easy and effectual remedy by the use of his ventilators; his account of which was read to the Royal Society in May 1741. In the November following Mr Tricawald, military architect to the king of Sweden, informed Dr Mortimer secretary to the Royal Society, that he had in the preceding spring invented a machine for the use of his majesty's men of war, in order to draw out the bad air from under their decks, the least of which exhausted 36,172 cubic feet of air in an hour, or at the rate of 21,732 tons in 24 hours. In 1742 he sent one of them, formed for a 60 gun ship to France; which was approved of by the Royal Academy of Sciences at Paris; and the king of France ordered all the men of war to be furnished with the like ventilators.

The ventilators invented by Dr Hales consist of a square box ABCD (fig. 1.) of any size; in the middle of one side of this box a broad partition or midriff is fixed by hinges X, and it moves up and down from A to C, by means of

an iron rod ZR, fixed at a proper distance from the other end of the midriff, and passing through a small hole in the cover of the box up to R. Two boxes of this kind may be employed at once, and the two iron rods may be fixed to a lever FG (fig. 2.) moving on a fixed centre O; so that by the alternate raising and pressing down of the lever FG, the midriffs are also alternately raised and depressed, whereby these double bellows are at the same time both drawing in air, and pouring it out, through apertures with valves made on the same side with, and placed both above and below, the hinges of the midriffs. In order to render the midriffs light, they are made of four bars lengthwise, and as many across them breadthwise, the vacant spaces being filled up with thin panels of fir-board; and that they may move to and fro with the greater ease, and without touching the sides of the boxes, there is an iron regulator fixed upright to the middle of the end of the box AC (fig. 1.) from N to L, with a notch cut into the middle of the end of the midriff at Z; so that the midriffs, in rising and falling, suffer no other friction than what is made between the regulator and the notch.

Moreover, as the midriff ZX moves with its edges only one twentieth of an inch from the sides of the box ABCD, very little air will escape by the edges; and, therefore, there will be no need of leathered fuses as in the common bellows. The end of the box at C is made a little circular, that it may be better adapted between A and C to the rising and falling midriff; and at the other end X of the midriff a lip of leather may be nailed over the joints if needful. The eight large valves through which the air is to pass, are placed at the hinge-end of the boxes BK (fig. 2.) as at 1, 2, 3, &c. The valve 1 opens inward to admit the air to enter, when the midriff is depressed; at the other end by means of the lever FG. And at the same time the valve 3 in the lower ventilator is shut by the compressed air which passes out at the valve 4. But when that midriff is raised, the valve 1 shuts, and the air passes out at the valve 2. And it is the same with the valves 5, 6, &c. of the other box; so that the midriffs are alternately rising and falling, and two of the ventilators drawing in air, and two blowing it out; the air entering at the valves 1, 3, 6, 8, and passing out at the valves 2, 4, 5, 7. Before these last valves there is fixed to the ventilators a box QQ NM (fig. 3.), as a common receptacle for all the air which comes out of these valves, which air passes off by the trunk P, through the wall of a building.

For a farther account of this machine we refer to the author himself, who gives a full detail of it and of its manner of working. See Description of Ventilators by Stephen Heles, D.D., Lond. 1743, 8vo.

The ventilators in large ships, since the order for ventilating the fleet issued by the lords of the admiralty in 1756, are fixed in the gunner's store fore-room, and generally ahead of the sail-room. The foul air is carried up through the decks and fore-castle near the topmast, sometimes afore it, and sometimes abaft it, but more frequently on its starboard side; the lever, by which the ventilators are worked, is under the forecastle in two deck ships, and between the upper and middle decks in three-deckers; sometimes the lever is hung athwart ships; in some ships afore and aft, and in others oblique. The iron rod, which communicates the motion from the lever, passes through the partners of the fore mast, and is connected with another lever, suspended at or near the middle; in some ships over the ventilators, in others under them. When it is found necessary to fix them up to the deck. The best method to save room is to place the ventilators over one another with their circular ends together; the air-trunk should be so high above deck, that the men on deck may not be inclosed by the foul air which blows out of it; and therefore the trunk comes through the upper deck, near and behind the forecastle. For the method of freeing mines, ships, prisons, &c. from noxious air by means of fire pipes, see Pneumatics, p. 371.