1837.] 



Plan for a Self-Registering Barometer. 



27 



point must be determined with the greatest nicety. C is the cistern, the 

 broad upper part of which is 12 inches in diameter, while the lower is of 

 such bore as to admit of the, tube moving up and down without touching 

 the sides of the former. The cistern consists of cast iron, and, when the 

 apparatus is being conveyed, the screw D is screwed into the top, there- 

 by preventing the escape of the mercury. The balance B is construct- 

 ed on the usual principles, the weight E on the top adjusts the centre 

 of gravity, while the screws F F, at the end of the beam, adjust the 

 length of each arm j other screws, which cannot be shown, adjust the 

 points of suspension to the plane of the point of support,* the screws 

 G G support the balance when not in use — H is the index of the 

 balance having a vernier at its lower end — I is a plummet, in the s^ape 

 of an arc, the two radii K K which support it, turn on a pivot which is 

 in the plane of the point of support of the balance — this plummet of 

 course will always preserve the perpendicular : it is graduated into 

 parts corresponding with the th of an inch of elevation or depression 

 of the mercury in the tube, so that, by means of this scale and the ver- 

 nier on the index, the exact height of mercury at any time may be 

 determined thereby— thus avoiding the necessity of disturbing the regis- 

 tering apparatus. 



The inclined arms L L and the pillar M support the balance and its 

 appendages. The stand N N is furnished with screws 0 0 by way of feet, 

 by which it is brought to the horizontal line, as indicated by spirit levels, 

 one of which is seen in front of the bottom of the pillar. The end of 

 another, at right angles to the former, is seen below the pillar is, 

 by means of the screw and nut R below the stand, easily attached to, or 

 removed from this. 



The registering apparatus is seen attached to the right arm of the 

 balance — S is a rod, on which at T is attached a disc of any metal, by 

 way of scale pan ; on this are placed weights U U U, which poise the 

 the barometer at different elevations of places of observation. In the 

 box W the clock work is contained, which turns the arbor on which the 

 register cylinder X is fixed, after the manner of the cap of a watch. 

 From the box W two rods Y Y rise up perpendicularly, which are guides 

 to the pencil frame Z — (a) is the pencil which is on the principle of 

 Mordan ; it is acted on by a delicate spring which causes the point to 

 press gently against the cylinder. This cylinder consists of ivory, the 

 surface of which is made smooth but not polished— longitudinally are 

 24 lines — which are hour lines — being numbered 1, 2, 3, &c. on to 24. 

 The horizontal lines correspond to fa of an inch of the barometrical , 

 tube at the other end of the balance. The middle horizontal line is call- 

 ed the zero line. By this I mean that when the instrument is put in 



* See Mr, Braddock's plans of the balance— Madras Journal of Science, No. 6. 



