61 
between the time of their vibrations of different extents, and 
consequently, whether in these lengths there may not be one 
where the long and short are isochronous; so many reasons, 
drawn from the principles of philosophy and mechanics, 
appeat to lead us to this conclusion, that we shall find it 
difficult to conceive how we have hitherto heen ignorant of 
this important fact; much less can we Conceive that it was 
not till after twenty years researches that we arrived at this 
discovery. Happily, men of science are not ignorant that 
of measuring Time at Sea.  , 
‘the simplest things, almost always the most useful, are fre-~ 
quently so much the more difficult to discover, as, according 
to the remark of an illustrious Secretary to the Academy, 
we are less inclined to seek for them. 
. However it may be, it is constantly the case, as I have 
already''said, Article If. Part II., that in every spring of 
sufficient extent there is a certain length where all the vibra- 
tions, whether long or short, are tsochronous, 1 have expe+ 
rienced this in a great number of springs. 
To procure, therefore, in the vibrations the most perfect 
isochronism, I adjust the spiral springs to the balance, and 
I-set the marine watch to go (which, as we have seen, has 
no fusee,) twelve hours in the long ares and twelve hours in 
the short arcs; that is to say, twelve hours with the moving 
spring highly wound up, and twelve hours with it al- 
most unwound. If, in this last case, the going of the watch 
is more accelerated than in the first, it proves that these 
springs are too long, and I shorten them. On the con- 
trary, if it is slower, I lengthen them ; and thus I proceed 
until I have found the point where the watch goes very 
equably both in the high and low strain of the spring: I 
then diminish or increase the weight of the balance until 
the watch is regulated. This operation at first appears 
Jong; but practice renders it so easy, that at first sight L 
know actually, very nearly, the length of spring where all 
the vibrations are of equal duration. _ The two spiral springs 
are here of some help, because we can only act on the one, 
and the quantities which we lengthen or shorten it, produce 
less effect. For example, in my marine watch about one 
line of diminution in the lower spring makes it gain in the 
8 hizh 
