Page:Journal of the Asiatic Society of Bengal Vol 7, Part 2.djvu/86

656 are many strong points of internal evidence which would bear me out in asserting that the instrument now before me has been made a long time—has been patched up for experimental trials by its maker—has been thrown aside in favor of hi new invention, and has been now been finally brushed up for exportation to India I After bringing so serious a charge forward, it becomes my duty to support it with proof :—.and this I can do from Mr. BArK’S own written instructions, which bid me “where the coin is in high relief, to lessen the angle of axis B. to diminish the effects of distortion;” whereas in the following description of his patent, he prides himself on his sea’s having obviated all distortions. He begins with a description of the original or American instrument illustrated by a diagram, which I have introduced as fig. I. into the accompanying Plato XXIX. a, being the medal; 6, the copper plate covered with an etching-ground; c, the tracer; and d, the etching-point at right angles to it. “The arm c d having a ruling motion horizontally across the surfaces of a and 6, and likewise moving freely in the direction c d. Also vertical motion being given to a and horizontal to 6 by the same screw: a series of lines traced over the medal were described upon the plate in the following manner: so lore as the tracer moved over the pLane surface or ground of the medal, the point d de. acribed equidistant straight lines upon the plate; but so noon as the tracer touched a part of the raised surface or relief of the medal, It was rained shore it. plane a quantity equal to the height of auch relief, and the line described by the etching.point was no longer equidistant, but deviated an equal quantity upon the horizontal plate: in the succeeding line, the tracer being raised off still further by the increased height of the relief, the etching-point deviated still further from the former line described upon the plate: the continuation of this process produced a succession of deviating lines upon the plate, which opening as the tracer rose above the plane of the medal, and closing again as it approached that plane gave the effect of light and shade in the printed impression of the plate. But however pleasing the effect of these impressious, they were all distorted representations of the original, just so much as the lines producing the representation deviated from the straight line upon the medal—and I found that this distortion had suspended the use of the process which had been described 14 years before in the Manuel de Tourneur. The most valuable sabjects, those having the highest relief, being most distorted.” Here let me pause—the defects above condemned, are possessed in the fullest degree by the ruler sent to sue :—the tracer describes straight lines only across -the medal, while the diamond engraving point traces curves deviating in proportion to the relief of each part :—eo that if the relief of the central point of the medal be one-tenth of an inch raised, and the angle of axis b be fixed at the same point will be ranted not to break !—the bulbs were so thick that when heated even to 306’ Parb,, there was no chance of the mercury making Its appearance in the tube I It was doubtless ealculated by the makers that they would never even be tried, much less used! See Philosophical Magazine 1833, vol. 2, page 288. t Without a drawing of the instrument it is almost Impossible to explain what is meant by axis A and asia B, The first is the axis upon which the rod holding the tracing point turns in rising over the raised parts of tb.e medal, and