68 Mr. Gilchrist on the Balance Self-Registering Barometer. [July 



instrument such as the present, where practical construction may not 

 have the correctness of its theoretical one, it would appear that the 

 more accurate way of dividing the scale will be to do so from the indi- 

 cations of a standard barometer of common construction. 



In the case of the cup float there will be a tendency to move the 

 tube from the perpendicular, inconsequence of the lower portion of the 

 tube being so much lighter than the upper. This tendency, however, 

 is easily prevented by lowering the centre of gravity of the tube, for 

 which purpose two rods attached to the tube may descend outside the 

 cistern to a convenient distance below it, to support a disc on which a 

 weight may be placed. This weight will if sufficiently heavy bring 

 the centre of gravity of the tube completely below the cup float. In- 

 deed, if this float be sufficiently large to buoy up the tube, the balance 

 part of the apparatus may be laid aside altogether, and the weight 

 below the cistern, acting on the principle of ballast, will preserve the 

 tube in the perpendicular. In order to keep the tube steady it 

 would be necessary to have guides on either side of the tube— the 

 pencil can be attached to any part of it, and made to bear direct- 

 ly on the register roller, which, as the tube moves perpendicular- 

 ly, may be made in the manner described in the original plan, and in 

 order to admit of the indications being magnified, the upper part of the 

 float can be contracted. 



Respecting form No. 2, (Fig. 1st, plate 15, Journal No. 14) the im- 

 perfection Mr. Taylor has pointed out as to the impossibility of know- 

 ing whether " air does not exist in the quicksilver between e and c," 

 is certainly a very serious one and indeed applies not only to the part 

 E C but to the entire tube, since the possessor of such an instrument 

 would not, as in the glass tube, have the advantage of ocular demonstra- 

 tion as to the presence of air ; but would be left in doubt on this essen- 

 tial point. Presuming that the objection to iron will be got over by 

 stating it as above mentioned, the following arrangement appears 

 to remove the imperfection under consideration. Let the end C of 

 the metallic tube rise into the middle of a cistern, after the manner 

 that the glass tube of the Englefield barometer descends in the iron 

 cistern — so that in what position soever the instrument be held the 

 termination of the tube will always be immersed in mercury. On the 

 top of this cistern let there be a stop-cock from which rises the 

 glass short limb— lastly, let a screw enter one side of the cistern, which, 

 when screwed home, will occupy a considerable portion of its area. 

 Suppose now the entire long limb filled with mercury, the stop-cock K 

 turned to prevent its return, also the cistern (the screw being out as 

 far as possible) filled with mercury. Let now the glass limb be at- 

 tached, and the cistern screw screwed home. Mereury will now 

 stand in the glass tube at a given height— which ought to be engraved 

 on the instrument. 



