120 PHILOSOPHICAL TRANSACTIONS. [aNNO 1776. 



As it has been found difficult, after the final hardening of these bars, to pre- 

 serve among them a perfect equality in size; therefore, the contact of their sides 

 are perfected by thin iron plates, slipped in between the braces and the junction 

 of the ends of the bars: and these plates, being pressed by the screws passing 

 through the sides of the braces, keep the ends of the bars in as close contact 

 as their figures will permit; and, that the bars may be kept end to end in con- 

 tact, the iron plate at the north end and at the south end is perforated with 6o 

 holes, one against the end of each bar, with a screw fitted to each hole: every 

 screw having a square head may, by help of the key, be turned, and, by press- 

 ing against the end of the bar in the 4th length, force it against its abutting 

 bar in the 3d length, and so on till the bars, end to end, are brought into con- 

 tact and kept so. The braces are in 2 pieces; the sides and bottom in 1 ; and 

 the other piece forms the top, which is held close to the bars by the screws pass- 

 ing through it into the upright sides of the braces. 



As each of these magazines weighed about 500 lb. it became necessary to have 

 them so placed as to be conveniently used. Tiie Doctor, therefore, by screws 

 fixed the braces, containing the bars, to a strong mahogany plank, about l\ 

 inches thick; the screws passing through the plank entered the bottom parts of 

 the braces. Against the middle of the whole length, 2 strong brass plates are 

 well fixed to the sides of the plank ; to these brass plates are fixed 2 cylindrical 

 gudgeons, which projecting from the sides, like the trunnions of a cannon, lie 

 in the sockets of the standard, by which the magazine easily turns, as on an axis, 

 and is so well poised as to stand in any inclination of the line ns; and in this 

 the equilibrium is assisted by the strong mahogany semi-circular pieces, fixed in 

 a vertical position to the middle of the under part of the plank, on which the 

 magnetic apparatus rests. The standards are fixed to the squai-e frame, and the 

 whole supported on 4 trucks, by which the 2 magazines are easily brought end 

 to end, or set at a convenient distance, so as to admit a bar to be placed between 

 the ends, to be made magnetical. 



XLIII. A Demonslralion of tivo T/ieorems jiientioned in Art. XXV of ike 

 Philos. Trans., for the Year J 775. By Charles Hulton, Esq., F.R.S. p. dOO. 

 This is a geometrical demonstration of the 2 polygonal theorems, communi- 

 cated by Mr. Lexel of Petersburg, and printed in p. 647 of this volume of 

 Abridgments; but unnecessary to be here repeated. 



XLIV^. Experiments to Ascertain the Nature of some Mineral Substances; and, 

 in particular, to see how far the Acids of Sea Salt and of f'itriol contribute 

 to mineralize Metallic and other Substances. By P. Jf'oulje, F. R. S. p. 605. 

 The component parts of the several mineral substances on which Mr. W.'s 



