Obtain a straight fine glass tube, about 33 inches long, and with as clean an interior as possible, sealed at one end, and having an even uniform bore of about 2-1/2 lines diameter. The mercury to be used should be perfectly pure and free from all air and moisture. This latter requisite may be assured by heating the mercury in a porcelain dish to nearly the boiling-point, previous to using it. The tube is then held securely, with the open end uppermost, and carefully filled with the liquid metal. The open end of the tube is then securely covered with the finger, the tube inverted, and the end covered by the finger plunged below the surface of a little mercury placed in a small vessel to receive it. The linger is then removed, when the mercury in the tube will immediately fall to a level of about

30 inches above the surface of that in the small reservoir below. In order to attach the scale correctly, it will be necessary to compare the indications with those of some good instrument.