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Art. !XVI. — Account of a new species of Chronometer adapted 
to the eye-pieces of Telescopes^ for reckoning fractional 
parts f a second in Astronomical Observations. Invented 
by M. Breguet, Member of the Institute, and of the Board 
of Longitude of France. 
BREGUET has been long known to the philosophers 
of our own country as well as to foreigners, as one of the most 
distinguished artists wliich France has for a considerable time 
produced. His various inventions connected with Horology 
and Chronometry, indicate an ingenuity and a fertility of in- 
vention which are not often united in the same person, and 
such is the degree of excellence with which his timekeepers are 
constructed, that we may justly rank him with the Harrisons, 
the Arnolds, and the Earnshaws of our own country. 
The ingenious instrument of which we propose at present to 
give a brief description, has been recently laid before the Board 
of Longitude of France; and though the commissioners have not 
yet given in their report, we have no doubt that it will be gene- 
rally considered as promising to supply an important desidera- 
tum in practical astronomy. In ascertaining the disappearance 
of a star behind the wires of a transit instrument, we think there 
are few astronomers who would venture to say, that in ordinary 
circumstances they could observe to the fifth part of a second 
of time ; but as this quantity corresponds to three seconds of 
right ascension, it becomes a matter of very high importance, in 
the present perfect state of astronomical instruments, to distin- 
guish more minute portions of time. 
The instrument by which M.M. Breguet propose to supply 
this defect, is shewn in Fig. 2. of Plate VII., where AB is a 
section of the eye-piece of a telescope through the diaphragm 
or field-bar placed in the anterior focus of the eye-glass next the 
eye; the black ring which it incloses representing the diaphragm 
itself : The box CD contains a chronometer, which, by means 
of the index G, points out every ten seconds upon the dial-plate 
EG, divided into ten minutes. Other two indices m, n, are made 
to revolve through the field of the telescope, and almost in the 
plane of the system of wires. The shortest of these w, marks 
