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    Re: Harrison chronometer no. 4 (1891 article)
    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.
    
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