M. Jurgensen’s Account of a Free Escapement y 149 
the moving-power, a rigorous exactness in the compensation for 
the variation of temperature, and perfect isochronism in the 
vibrations of the balance, as obtained by the spiral. The great 
regularity of its going, principally depends on the success in ob- 
taining these, abstraction being made of the quality of the oil 
employed. 
The principles on which compensation for temperature de- 
pend, are so well established, that great exactness is attain- 
able, and a want of success in obtaining a just compensation 
can only be imputed to a want of attention in the artist. 
Perfect isochronism in the vibrations of the balance is more 
difficult to attain than the compensation, and requires more 
trials and care ; but, although it is one of the parts most es- 
sential for continued regularity, machines have been known to 
go well without the isochronism being so rigorous as the com- 
pensation. 
Still, since a chronometer cannot be considered perfect with- 
out it, every care must be taken to succeed. The two modes 
of constructing the balance-spring, so that the great and small 
vibrations of the balance may be equal, are well known, and 
even the general theory of isochronism is thoroughly establish- 
ed, but still much tact and nicety are necessary for success, 
and on this point genius will do more than rule. 
The escapement, that delicate part of a chronometer, de- 
serves and requires particular attention. The escapement, with 
a constant force^ promises, by the theory, the greatest regula- 
rity, for in it the impulses which the regulator receives, are al- 
ways of equal force, and the resistance made to the regulator in 
disengaging the detent is also constantly the same ; hence there 
is nothing to disturb the regularity of the vibrations which, from 
the nature of this escapement, become always of equal extent. 
The late Thomas Mudge used this escapement in his chronome- 
ters, and their goodness in an equable temperature is no doubt in 
a great measure owing to it. Breguet also employed such an 
escapement, but more perfect, in many of his chronometers, and 
particularly in the celebrated one lately made for the Count Som- 
mariva; but this escapement, in spite of its advantages, can never 
become of general use, because it requires, in the execution and 
in the combination, precautions often beyond the abilities of even 
good workmen. However, were we still ignorant of the mode 
