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 let the mischiefs arising from foul air in a jail light than Dr Hales; who has also proposed an easy and effectual remedy by the use of his ventilators; his account of which ventilator was read to the Royal Society in May 1741. In the November following M. Treswald, 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 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 midrift 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 midrift, and passing through a small hole in the cover of the box up to L. 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 midrifts 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 fame side with, and placed both above and below the hinges of the midrifts. In order to render the midrifts 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 midrift at Z; so that the midrifts, in rising and falling, suffer no other friction than what is made between the regulator and the notch. Moreover, as the midrift ZX moves with its edges only one-twentieth of an inch from the sides of the box ABCDFE, very little air will escape by the edges; and, therefore, there will be no need of leathern sides as in the common bellows. The end of the box at AC is made a little circular, that it may be better adapted between A and C to the rising and falling midrift; and at the other end X of the midrift a strip of leather may be nailed over the joints if needed. 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 midrift 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 midrift 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 midrifts 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 QQNM (fig. 3,) as a common receptacle for all the air which comes out of these valves; Ventilator which air passes off by the trunk P, through the wall of a building. See Description of Ventilators by Stephen Hales, D. D. Lond. 1743, 8vo.; and for the method of freeing mines, ships, prisons, &c. from noxious air by means of fire-pipes, see PNEUMATICS, No. 371.