4 DR. WOLLASTON ON A METHOD 



without cavities, and with uniformity, — a uniformity to be rendered perfect by 

 subsequent pressure. In order, however, to guard effectually against cavities, 

 the barrel may be weighed after filling it, and the actual weight of its contents 

 being thus ascertained, may be compared with that weight of platina and water 

 which it is known by estimate that the barrel ought to contain*. A circular 

 piece of soft paper first, and then of woollen cloth, being laid upon the surface, 

 allow the water to pass, during partial compression by the force of the hand 

 with a wooden plug. A circular plate of copper is then placed upon the top, 

 and thus sufiicient consistency is given to the contents to allow of the barrel 

 being laid horizontally in a forcible press. 



The press which I have generally used for this purpose, (Plate I.), consists of 

 a flat iron bar A B, set edgeways, and screwed down by a hook E, near its 

 middle, where it would otherwise be liable to bend, to a strong Avooden bench 

 C D. The bar is connected by a pivot at its extremity A, with the lever A F G. 

 An iron rod F II, which tunis at its two extremities upon the pivots F and H, 

 proceeds from the lever at F, and, as the lever descends, propells forward the 

 carriage I, which slides along the bar. A stopper or block being placed in the 

 vacant space I k, the carriage communicates motion to the cradle k 1 m, which 

 is also made to slide along the bar, and carries the barrel N, which lies 

 upon the cradle, straight against the piston O, which rests by its end against 

 P, a projection in the further extremity of the bar. 



The weight, which in this machine, when the angle of the lever's elevation 



is small, will keep the power, applied vertically at the extremity of the lever, 



•Ti • xu . AG X FH ^ ^ , , . , 



m equilibrio = that power X aF fAF + FHl ^ cotan. of the angle of the 



lever's elevation ; which expression, in the case of the press actually used, be- 

 comes, Power X 5 . cotan. of the angle of the lever's elevation. This expres- 



• From the mean weight of the ingots obtained in previous operations, it is known that the barrel 

 described in the text ought to contain 16 ounces troy of dry platina powder. The weight of the con- 

 tents of the barrel = 16 ounces X ^P- g^^v- ^f platina - J_ _^ ^^^ ^.^^ ^^ ^ ^^^.^ .^^^^ ^^ ^^^^^ 



sp. grav. of platina ° 



X capacity of the barrel in cubic inches = 16 ounces X ~-^-^— + .520 oimces X 7.05 = 18.9575 



21.25 



ounces troy. Should the contents of the barrel weigh materially kss than this estimated weight, there 



must be a want of uniformity in the disposition of the powder within the barrel. 



