680. To ascertain, as near as possible, the degree of heat in an oven when it is very hot, Mr. Wedgwood, the ingenious manufacturer of earthen-ware from basaltic masses, has contrived a pyrometer which consists of small cubes of dry clay. The method of applying these cubes may be seen in Gregory's mechanics vol. 2, pa. 310.

The curious in these matters may find an account of Ellicot's pyrometer in number 443. Phil. Trans. of Smeaton's in vol. 48; of De Luc's in vol. 68; and of Ramsden's in vol. 75; but of all these Ramsden's is the most accurate.

Twelve lamps are placed under a trough of water, in which the rod of metal is put, and the expansion of the rod caused by the heat of the water, is measured by a microscope furnished with a micrometer.

On Pumps.

ON THE AIR PUMP.

631. Defs. The air pump is a machine by which the air is exhausted from a vessel, making in it what is commonly called a vacuum.

The vacuum produced by the air pump is, however, in fact only a very great degree of rarefaction; because, from the nature of the machine, it will 'appear, after the manner of operating has been explained, that to exhaust all the air is manifestly impossible.

So great a variety of air pumps has been constructed by different persons, that selection becomes a difficult task; we shall however describe one that is of a portable size, and convenient form, and tolerably well adapted to most of the purposes to which these machines can be applied.

DESCRIPTION. (see fig. 186).

632. Two cylindrical tubes or barrels of brass A, A, with valves at their bottoms opening upwards, are firmly retained in a situation perpendicular to the horizontal table EFGH, by the screws O, O, at the top of the brass pillars N, N, pressing upon the transverse beam T, T.

The cylinders A, A, communicate with a cavity in the table at D, and with a receiver LM, fixed upon a brass plate, smeared with wax or covered with wet sheepskin, by means of a canal or tube, one end of which opens into the cavity at D, and the other into the receiver at K.

Each barrel has a piston in it, exactly adapted to its cavity, so that the air cannot pass between it and the sides of the barrel; and each piston has a valve in it, that opens upwards. The pistons are connected to a rack CC, and are raised or depressed by a handle B, fixed to the axis of a cog-wheel, the teeth of which catch the teeth of the racks.

One piston is raised and the other depressed by turning the winch B, and the weight of the atmosphere pressing upon the rising piston, is counterbalanced by the same weight pressing upon the descending piston.

The screw at V, closes the end of the pipe terminating at K, and serves to admit the external air when wanted.

633.

METHOD OF WORKING. By turning the handle B, one of the pistons is raised, and a vacuum is left beneath it; thus the pressure being removed from the valve in the bottom of the barrel, it will rise, and the air contained in the receiver, communicating with the barrel by means of the orifice K, expands into the void space left by the piston; and thus a part of the air is drawn from the receiver.

If the handle be now turned the contrary way, the same circumstance takes place in the other barrel; but in the mean time, the first mentioned piston being depressed, it compresses the air beneath it, and shuts the lower valve, while the valve in the piston is forced open by the spring of the air in the barrel, and thus makes its escape into the atmosphere.

The motion of the handle being again reversed, the first piston exhausts more air from the receiver, while the second discharges a quantity in the same manner as the first piston did, when it was descending. Thus it is, that, during the time the pump is worked, one piston exhausts the air from the receiver, while the other discharges it through its valve into the atmosphere.