The 3 small brass clasps c, provided for attaching the tube to its support, may be readily cut from sheet-brass, polished, bent to shape, and drilled with a hole in each end to receive the appropriate small brass screws. The sliding-scale support d is a slip of cherry or mahogany, 1 in. wide, 3/16 in. thick, and 28 in. long, having two longitudinal cuts l made therein, through which pass the screws /, which fasten it to the walnut scale and allow of its motion upward and downward. These screws may be of brass with milled heads, or a cheap and excellent substitute may be found in 2 brass buttons with screw-stems sold for fastening carriage aprons. These are to have their stems passed through the longitudinal cuts e, and screwed into appropriate holes in the walnut support till their projecting shoulders bind on the scale support and prevent it from moving, except when required. The bottom k of the sliding scale support is a piece of sheet- brass cut to shape and attached by two small rivets or screws. Its angle or corner i is used as an index, as will afterward be explained. A scale k made of a piece of ivory veneer, 4 in. long and about 1 1/4 in. wide, is required for the upper end of the sliding support.
This must be carefully and accurately divided into inches and tenths, the lowest inch mark being numbered "29," the next "30," and the upper one " 31." It will be well to have the figures and lines done by an engraver; but, if economy be a consideration, the markings can be very well ruled with a fine pen, and after the ink has dried a coat of thin dammar varnish will protect the lines from injury by moisture. The ivory scale is now to be fixed to the sliding support, with the upper end of which its top exactly corresponds. If the measures have been correctly made, its 30-inch mark will now be situated exact 30 in. from the bottom of the brass index. An excellent cement for attaching the ivory to the wood is made of a little isinglass dissolved by heat in equal parts of alcohol and water. The walnut support b should receive two or three coats of copal varnish. The cherry wood slide d may either be finished with boiled linseed oil or varnish, according to taste.
All parts of the instrument being thus fitted, it only remains to introduce the mercury. For this purpose the tube a being detached from the support, is placed upon a level table and sustained by small pieces of wire, so that the short limb is uppermost, the long limb lying flat upon the table. The mercury used should be as pure as possible: though if freshly-distilled mercury cannot be had that of commerce may be used, provided it has not become contaminated by lead or kindred metals. A fair test of the goodness of mercury is made by dropping a little into a clean white plate and causing it to run about. If bright round globules are formed, which readily coalesce and leave no trails of discolouration on the china, the metal is sufficiently pure. If, however, the drops become pear-shaped and soil the plate with dull, metallic splotches, the metal must be rejected. Before being used for filling, the mercury should in any case be forced through small pinholes in a piece of thin chamois skin to remove mechanical impurities. The tube being filled, is next raised gently into a vertical position, with its closed end uppermost.
The mercury will descend a few inches, showing the Torricellian vacuum in the upper part of the longer limb, while at the same time it rises and overflows from the open orifice of the short limb. From the latter, enough of it should be displaced, by inserting a small round piece of wood into the bore, to leave a couple of inches empty. After this it only remains to finish the instrument by attaching the tube a to its support with the brass clasps c and screws. A narrow strip of green surface paper, 4-5 in. long, rlipped behind the upper part of the tube where the vacuum appears, is an improvement to the look of the instrument and an assistance when taking the readings. It will now be evident at a glance that by bringing the corner i of the brass index h level with the surface of the mercury in the short limb, as often as an observation is to be made, the height of the mercurial column in inches and decimals will at once be shown on the ivory scale.
A small thermometer l fixed beside the sliding scale is at once a useful and ornamental addition to the barometer. A small cap m of metal or wood must be loosely fitted over the open end to exclude dust. (A. F. Miller).
(8) Cistern barometer. The tube must be cleaned as already described, and closed at one end; but instead of being bent it is left straight, and cut off at a length of 32 in. Fig. 365 shows a section of the cistern, which is simply a small wooden cup turned neatly out of hard wood; its outside dimensions being 1 1/2 in. diameter and 2 1/4 in. high, and the inside cavity being 1 1/8 in. diameter and 2 in. deep. A cut made with a fine saw along the line a separates the underneath part of the cistern as a small wooden ring, to the bottom of which must be glued a piece of stout wash-leather b, made loosely convex so as to bulge readily inward and outward, forming the cistern-bottom and supplying a movable surface on which the atmospheric pressure is to act. A hole c in the closed top admits the pipe df, which passes down into the cistern till its end is level with the line of division a, and is secured in place by being cemented where it goes through the wood of the top. A small hole e for adjusting the height of the mercury is made £ in. below the closed top of the cistern, and stopped for the time with a little wooden plug.
The filling with pure mercury is to be done as already described in the case of the siphon, except that the tube may now be placed in a nearly vertical position with its closed end downward; a small straight funnel is to be used for pouring through. The cistern, which, owing to the position of the tube, is also to receive as much mercury as will fill it to the edge a, after which the ring-shaped piece, bearing the wash-leather bottom b is coated with glue on its sawn surface and pressed on in place, so closing the cistern. As soon as the glued joint is firm, the tube may be turned up into proper position by placing the finger on the washleather bottom, and pressing it inward till the orifice of the tube is felt, when the whole is quickly inverted. Thus no air enters the tube daring the moment of turning over; and as an instant later its opening is covered by the mercury of the cistern, the vacuum is now secured. Care should be taken, however, never again to turn the cistern bottom upward. The tube being now in a vertical position, the level of the mercury is adjusted by removing the plug from the hole e when the superfluous metal escapes and the column in the tube descends, leaving the vacuum above.