u6 



Mr. Gilchrist on the Balance 



[July 



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 t owards the balance or 

 from it, according as the hour lines are numbe red 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 gra- 

 duated 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 mer- 

 cury 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 ob- 

 long form as the horizontal motion of the tube, that, namely, occa- 

 sioned 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 propor- 

 tionally 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 



