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CLASS Xb.
REPORT ON IIOROLOGICAL INSTRUMENTS.
[The figures af.er the Names (between parentheses) refer to the Exhibitors’ Numbers and to the Pages in theOfficial Dfscuii’iive axd Illustrated Catalogue.]
Jury.
E. I>. Denisox, M.A. , Chairman and Reporter , 42 Queen Anne Street. _ (Juror Class X.)Baron Arm and Secslier, Deputy Chairman. France . (Juror Class X.)
The marine chronometer may be considered the most 'important of all machines for measuring time ; and it isalso the one in which an invariable rate of going is of !the most consequence, inasmuch as longitude at sea isdetermined by means of chronometers, and they havefrequently to go for a longer time without the means ofbeing corrected by astronomical observations, than astro-nomical clocks, which are generally accompanied byfixed transit instruments.
It will probably appear strange that we have notawarded any Council Medal for chronometers, of whichthere are a considerable number in the Exhibition, es-pecially in the English part, some of them by makers ofthe highest reputation. It is, however, this very cir-cumstance which has rendered it impossible to make anysuch award; not only because we are directed by theRoyal Commisioners to avoid the attempt to distinguishone article from others of the same kind, merely onuccount of superiority of execution, but because, amongseveral of the best makers, there really is no one whocould properly be so distinguished above all the rest.
The principles of the construction of chronometershave now been settled for some years without any mate-rial variation, except in one point, which will he men-tioned presently, and there is hardly any visible differencebetween one good chronometer and another; it was henceobviously out of our power to make such actual trialof them as can only be conducted in an observatory, andmust be continued for a considerable time in order toascertain their relative merits.
We therefore came to the conclusion that the onlysatisfactory plan on which we can distinguish any amongthe large number of chronometers in the Exhibition (atleast in the English part of it), is to adopt the results ofthe trials at the Royal Observatory, during the last fewyears, of chronometers by the same makers as lrave now’sent any to the Exhibition. But it must be understoodthat the same reports of the Greenwich trials whichjustify us in awarding prizes to several ehroncmeter-ruakers who are exhibitors, show that others who arenot exhibitors would have been equally entitled to Medalsif they had sent any chronometers here. The exhibitorswho have been awarded Prize Medals from having ob-tained high places in the trials at the Royal Observatoryin the last few years are—Messrs. Dent (Xo. 55, p. 413),C. Frodsham (No. 57, pp. 414, 415), Parkinson andFrodsham (No. 35, p. 411), Hutton (No. 7, p. 407), andLoskby (No. 12, p. 408); and we should have been gladto have the opportunity of similarly acknowledging themerits of Messrs. Poole, E. J. Massey, and Eiffe, asthe makers of chronometers, which, together with thoseof Lcseby and Dent, have exclusively occupied the firstthree places in all the trials of the last four years.
Among the foreign chronometer-makers we have hadthe satisfaction of being able to award Prize Medals tothree of the highest reputation in their respectivecountries, viz., M. Vissiere, of Paris (No. 733, p. 1215),M. Urban Jurgensen, of Altonat'No. 17, p. 1350), andM. Louis Richard, of Neufclnltel (Cl. I., Zollvereir)
'(No. 342, p. 1070); the last of whom has exhibited achronometer in which the necessity for a fusee is super-! seded by a remontoire escapement of a more simple con-struction than usual, the impulse being given to thebalance, not by any teetli of a scape-wheel, but by asmall spring which is wound up sufficiently for the pur-pose by the scape-wheel every time it escapes.
The single point in which there is any material differ-ence in the construction of the best chronometers, is themode of effecting what is called the secondary or aux-iliary compensation of the balance, or that which isrequired in addition to the ordinary compensation, toprevent the chronometer from gaining at mean tem-peratures if it is adjusted for two extreme ones, suchas 20° and 100°, or losing at the extremes of heatand cold if the compensation is adjusted only fortemperatures within moderate limits, as from 40° to 80°.
The earliest inventions for this purpose, as well as thediscovery of the necessity for a secondary compensation,appear to have been made by Mr. Eifi’e and Mr. Dentabout the same time, and independently of each other;and their methods are still in use, and evidently very suc-cessful. But we agree with the opinion expressed by theAstronomer Royal in a report made by him to the Admi-ralty, that the most ingenious contrivance for thispurpose, and probably the best adapted to the wants ofchronometers—at least of these which are likely to beexposed to extreme temperatures—is Mr. Lofeby’s(p. 408), in which small curved thermometer tubes are setupon the ends of the ordinary compensation bars, so thatthe mercury advances along the tubes towards the centreof the balance, and at an increasing rate the nearer itapproaches the centre. The effect of this, as of mostof the other contrivances for the same purpose, is evi-dently to diminish the moment of inertia of the balancestill more than it is diminished by the bending inwardsof the ordinary compensation bars, on the ends of whichthe mercurial tubes are fixed.
The three makers who have been mentioned in additionto these who have received Medals not being exhibitors,we can give no account of their methods of compensation.Mr. Dent’s (p. 413) consists in placing the balanceweights on small additional compensation bars set on theends of the common ones, in such a position that theweights advance and recede in a radial direction mererapidly the nearer they are to the centre, for any givennumber of degrees of heat. There is also a chronometerexhibited by Messrs. Barraud and Lund (No. 34,p. 411), with a contrivance which is substantially the samein principle, and perhaps equally likely to answer, thoughnot so simple in form; besides others which do not ap-pear to us to require particular notice.
We desire to draw attention to a chronometer with aglass balance and balance-spring, exhibited by Mr. Dent,on account of the remarkable fact that such a springonly requires about one-tenth of the compensation of asteel spring, and one-twelfth cf a gold one. We havebeen furnished witli a copy of the Observatory report ofthe rate of a chronometer with this balance ar.d spring,