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Re: Harrison chronometer no. 4 (1891 article)
From: George Huxtable
Date: 2008 May 12, 18:10 +0100
From: George Huxtable
Date: 2008 May 12, 18:10 +0100
I don't claim to know much about timekeepers, but can add a bit more information about Harrison's No. 4 from Rupert Gould's "The Marine Chronometer", from 1923. My reprint is 1978. He tells us that the watch beats 5 to the second, and says about the escapement- "It was impossible to use the "grasshopper" in a machine of such small size, and the escapement is a modification of the "verge" fitted to the "Nuremberg egg" and to the common watches of Harrison's day. But the modifications are extensive. The pallets are very small, and have their faces set parallel, instead of at the usual angle of 95 degrees or so. However, instead of being steel, they are of diamond, and their backs are shaped to cycloidal curves, as shown in fig.15. The action of this escapement is quite different from that of the verge, which it appears to resemble. In that escapement, the teeth of the crown wheel act only upon the faces of the pallets. But in this, as will be seen from fig. 15, the points of the teeth rest, for a considerable portion of the supplementary arc- from 90 to 145 degrres (limit of banking) past the dead point- against the backs of the pallets, and tend to assist the balance toward the extreme of its swing and to retard its return. This escapement is obviously a great improvement on the verge, as the train has far less power over the motions of the balance. The latter is no longer checked in its swing by a force equal to that which iriginally impelled it, but by the balance spring, assisted only by the friction between the tooth and the back of the pallet." =========== To help to give these words meaning, I attach (as Gould1), his diagram of the verge-and foliot escapement on the early "Nuremberg egg", from the days before the balance wheel had been invented, so it was controlled simply by the inertia of the dumbell arms. And also, as Gould2, his fig 15, referred to above. My guess, about the words that Frank quotes, is that its author was referring to the standard view of a chronometer, as being an instrument suspended in gimbals in a mahogany box, whereas Harrison's No. 4 looked exactly like a vastly oversized pocket-watch, which sat on a cushion, not in gimbals, and balanced accordingly. Gould tells us that the first account of it was by H M Frodsham, in Horological Journal for May 1878. George. contact George Huxtable at george@huxtable.u-net.com or at +44 1865 820222 (from UK, 01865 820222) or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---