VOL. LV.] PHILOSOPHICAL TRANSACTIONS. 235 



where else ■^. The length of the arbor H; diameter of the pivots -j^. The 

 weight of the counterpoise one ounce avoirdupois. The hook, with the loop 

 and steel pin included, 0.68 avoirdupois. The division of 50 is at the top of 

 the ring. 



In making the beam, the point of the index e must be equally distant from 

 the centres a and b ; the whole beam and index made so as to poise itself, and 

 remain at rest in any position, before the counterpoise steel pin and loop are put 

 on. The counterpoise being at first made too heavy, to adjust it, take off the 

 beam, but let the two cocks and ring remain screwed on; hang then a fine wire 

 and plummet on the top division, and with a watch glass look through one pivot 

 hole till you see the wire against the other; turn the screw g till the wire bisects 

 the hole, and the top division is then perpendicularly over it. Take away the 

 plumb line, and put on the beam and counterpoise; on the loop hang a weight, 

 which together with the steel pin and loop makes one ounce avoirdupois; turn 

 now, by degrees, some inetal off the counterpoise, till the index points at the 

 top division, and the counterpoise will be truly adjusted, whether the arms of 

 the beam, ca, cb, are precisely equal or not : for though the arms should be 

 nearly equal, it is not necessary they should be exactly so, as in scales. 



The divisions are set on the ring by an instrument made on purpose, which 

 will very readily cut them, though unequal, with great exactness, on any circle, 

 large or small. To prove the beam, put a weight into the scale (that of 28 to 

 the pound is the best in this case) and see if it brings the index down to the 

 proper division on the ring: if it carries it too far, the angle of the beam acb 

 is too great; if the contrary, too little; and the arms ca, cb, must be set a 

 little in or out, till the angle is right: or the angle of the beam may be first 

 found experimentally, and the divisions calculated to it, which is not much 

 trouble; for having a table ready made for the intended angle, the alterations in 

 that table occasioned by a small variation from that angle, will be easily found. 

 When the balance is to be used, a weight of VW of a pound avoirdupois is to be 

 put on the hook: the screw g must then be turned, till the index e points at 

 the division of 50: the machine is then properly adjusted, and the weight may 

 be taken off, and a skain put on. 



XXFI. An Experimental Enquiry into the Mineral Elastic Spirit, or Air, con- 

 tained in Spa f Voter', as well as into the Mephitic Qualities of this Spirit. By 

 fVilliam Brownrigg, M.D., F.R.S. p. 218. 



Dr. B. here states that 24 years before, he had presented to the b. s. several 

 Essays on Damps, which were read at the meetings, and were preserved among 

 the paptrs of the k. s. Those essays contained a few of the observations and 



H H 2 



