ments of a circle, of which the radius is the length between the pencil and fulcrum of the beam. Further, while retained perpendicularly it will require to be moved horizontally, either towards the balance or from it, according as the hour lines are numbered from the right or left side of the plane. The clock-work that (as in the cylinder) gives motion to the plane, must be moved up towards the pencil end of the balance. The plane surface will best consist of glass, which when slightly ground (or frosted) with fine emery, will easily receive the pencil trace. It has the very great advantage of being little affected by heat or moisture, while the hour and horizontal lines may be drawn on the side opposite the one on which the pencil plays, so that this has a smooth surface on which to traverse, whereby the obstruction that might arise from those lines is avoided. From these planes it will be easy to read off to any degree of minuteness the pencil trace by means of a T ruler, the perpendicular limb of which will require to be graduated and furnished with a moveable vernier.
2d. With respect to the lower portion of the tube.—It was originally intended to make this sufficiently thick to displace the quantity of mercury necessary to balance that added to the length of the mercurial column by atmospheric pressure—instead of retaining so much solid iron, it is preferable to have a hollow cup of uniform area, and as the bulk of this that will enter the cistern will very nearly equal the bulk of mercury removed, very little alteration in the height of the latter in the cistern will occur. The cistern therefore need be no larger than will admit of the tube to move clear of it, and would best be of an oblong form as the horizontal motion of the tube, that, namely, occasioned by the beam describing an arc, is only in one direction. In the balance barometer the expanded bore at top is only of such length as to meet variations of atmospherical pressure accruing at any place, where the instrument is put in operation, and not for difference of relative height of situation—thus if the enlarged bore at top be only four inches long, it is evident that, at an elevation of about 4,000 feet, the mercury will have descended out of it—either the tube must be lowered or the cistern raised. This adjustment can easily be made by means of a screw—however, as a great portion of a tube that would suit at the level of the sea, must for an elevation of 6,000 or more feet be immersed in the cistern, this latter would require to be proportionally deep: by having tubes of different lengths, this increased depth might be avoided.
3d. With respect to the magnifying the indications—In addition to the mode mentioned in the description of the plan for this purpose, another suggests itself from what has been mentioned above respecting
