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Mercer (Vaudrey), Edward John Dent, p. 341
Letter from E.J. Dent to the Astronomer Royal George B. Airy, 10/11/51; Will you sir, allow me to ask your opinion on a plan which I think might have the effect of correcting the want of agreement between the inertia of the balance and the varying elastic force of the balance spring in a chronometer, and so to get rid of the secondary compensation which at all times is objectionable. The plan I suggest for experiment is to alter the shape of the steel in the balance rim from that of the cylindrical form and having the steel and brass of the same figure, and is, to make the steel of an angular (or prism) shape. I presume then there will be a resistance from the steel ring of this shape to prevent the brass by its construction from carrying out the rim so far by a decrease of temperature, and the reverse on an increase. This form perhaps holds good mechanically, but having the expansion of the brass to deal with it alters the condition of things. Airy replied the following day in a letter saying that he had grave doubts as to whether the new balance would have any effect. But within a month Dent had made and tested the first prismatic form of balance and wrote to Airy full of enthusiam ...never before did I observe the rate so uniform in any other balance
Letter from E.J. Dent to the Astronomer Royal George B. Airy, 10/11/51; Will you sir, allow me to ask your opinion on a plan which I think might have the effect of correcting the want of agreement between the inertia of the balance and the varying elastic force of the balance spring in a chronometer, and so to get rid of the secondary compensation which at all times is objectionable. The plan I suggest for experiment is to alter the shape of the steel in the balance rim from that of the cylindrical form and having the steel and brass of the same figure, and is, to make the steel of an angular (or prism) shape. I presume then there will be a resistance from the steel ring of this shape to prevent the brass by its construction from carrying out the rim so far by a decrease of temperature, and the reverse on an increase. This form perhaps holds good mechanically, but having the expansion of the brass to deal with it alters the condition of things. Airy replied the following day in a letter saying that he had grave doubts as to whether the new balance would have any effect. But within a month Dent had made and tested the first prismatic form of balance and wrote to Airy full of enthusiam ...never before did I observe the rate so uniform in any other balance