A.—­The web, or base plate of the lower frame is 3/4 of an Inch thick, and a cooming is earned all round the cylinder, leaving an opening of sufficient size to permit the necessary oscillation.  The cross section of the upper frame is that of a hollow beam 6 inches deep, and about 3-1/2 inches wide, with holes at the sides to take out the core; and the thickness of the metal is 13/16ths of an inch.  Both the upper and the lower frame is cast in a single piece, with the exception of the continuations of the upper frame, which support the paddle wheels.  An oval ring 3 inches wide is formed in the upper frame, of sufficient size to permit the working of the air pump crank; and from this ring feathers run to the ends of the cross portions of the frame which supports the intermediate shaft journals.  The columns are 1-1/2 inches in diameter; they are provided with collars at the lower ends, which rest upon bosses in the lower frame, and with collars at the upper ends for supporting the upper frame; but the upper collars of two of the corner columns are screwed on, so as to enable the columns to be drawn up when it is required to get the cylinders out.  The cross section of the bottom frame is also of the form of a hollow beam, 7 inches deep, except in the region of the condenser, where it is, of course, of a different form.  The depth of the boss for the reception of the columns is a little more than 7 inches deep on the lower frame, and a little more than 6 inches deep on the upper frame; and the holes through them are so cored out, that the columns only bear at the upper and lower edges of the hole, instead of all through it—­a formation by which the fitting of the columns is facilitated.

620. Q.—­What are the dimensions of the condenser?

A.—­The condenser, which is cast upon the lower frame, consists of an oval vessel 22-1/2 inches wide, by 2 feet 4-1/4 inches long, and 1 foot 10-1/2 inches deep; it stands 9 inches above the upper face of the bottom frame, the rest projecting beneath it; and it is enlarged at the sides by being carried beneath the trunnions.

621. Q.—­What are the dimensions of the air pump?

A.—­The air pump, which is set in the centre of the condenser, is 15-1/4 inches in diameter, and has a stroke of 11 inches.  The foot valve is situated in the bottom of the air pump, and its seat consists of a disc of brass, in which there is a rectangular flap valve, opening upwards, but rounded on one side to the circle of the pump, and so balanced as to enable the valve to open with facility.  The balance weight, which is formed of brass cast in the same piece as the valve itself, operates as a stop, by coming into contact with the disc which constitutes the bottom of the pump; the disc being recessed opposite to the stop to enable the valve to open sufficiently.  This disc is bolted to the barrel of the pump by means of an internal flange, and before it can be removed the pump must be lifted