ARCHIMEDES’S SCREW.--There is no particular process for the making of this instrument. It is, however, necessary for one who would succeed in making it, to exercise himself in the art of well bending a tube. After a few attempts, you may finish by producing a pretty-regular spiral. The tube chosen for this instrument should be six or seven feet long, and about one-third of an inch in diameter. You commence by making a bend, nearly at a right angle, about four inches from one of its extremities. This bent portion serves afterwards as a handle, and very much facilitates the operation; it represents the prolongation of the rational axis which may be conceived to pass through the centre of the spiral. See pl. 4, fig. 10. * * * * * BARKER’S MILL.--_Apparatus for exhibiting the rotatory motion produced by the running of liquids._--Contract a tube at its two extremities, pierce it laterally about the middle of its length, and solder to the hole an additional tube, terminated by a funnel. Soften the principal tube at the side opposite to the part that was pierced, and form there a conical cavity by pressing the softened glass inward with the aid of a metallic rod. This cavity must be so carefully made that the whole apparatus can be supported on a pivot. Bend the contracted ends of the tube horizontally, and in different directions, cut off their extremities at a proper length, and slightly border the edges of the orifices. See pl. 3, fig. 33. You may produce this apparatus under a different form, as may be seen at pl. 3, fig. 5. * * * * * BAROMETERS.--Barometers serve to measure the pressure of the atmosphere. The following are the varieties most in use. * * * * * CISTERN BAROMETER.--Take a tube about thirty-two inches long, and at least one-third of an inch in diameter, internally; seal one of its extremities, free it with most particular care from moisture, fill it with mercury, and make the mercury boil in the tube, by heat, in order to drive out every particle of air which might be present. When the tube is full of mercury, and the boiling has taken place, turn it upside down, and plunge the open end into a cistern also filled with mercury which has been boiled. See pl. 2, fig. 4. * * * * * DIAL (or WHEEL) BAROMETER.--The tube intended for this barometer should be very regular in the bore. It should be thirty-nine inches long. Close it at one end, and bend it like the letter U at about thirty-two inches from the sealed extremity. See pl. 2, fig. 5, and _Graduation of the Dial Barometer_.