. 
42 Le Roy’s Memoir on the best Method 
Part f. 
Examination of different methods which ma y be tried to 
measure time at sea. 
Moving bodies being evidently the only measures of dura- 
tion or of time*, that nothing may be omitted in so im- 
portant a subject, let us first cast our eyes on those whose 
motions may appear capable of giving an exact measure’ of 
time. 
The first which present themselves are the stars. The 
perfection to which telescopes have been carried, the suc- 
cessful labours of many celebrated astronomers in the theory 
of Jupiter’s satellites, and the tables which they have given 
of their revolutions f, give us reason to believe that they will 
presently become of very great help for measuring time at 
sea: the same must be said of the theory of the moon. 
But when we shall haye given to these tables and these 
telescopes all the perfection that can be desired, we shall 
find that they are yet jnsufficient. We cannot always see 
the moon, still Iess the satellites of Jupiter: supposing, 
even, that we could obserye them as often as circumstances 
require, these observations are in some degree useless, with- 
out an instrument that would preserve the hour with exact+ 
ness after we have determined it by the sun f. 
The observations of the heavenly bodies cannot, therefore, 
entirely fulfil our wishes: Ict us therefore look among the 
bodies which are more at hand, if there be not some one 
which by motions arising from different causcs would be 
proper to give us the required measure of time. 
Those which offer themselves first to our examination are 
fluids, and solids reduced’ into insensibly small parts, form- 
ing clepsydras, or sand-glasses; bodies falling, or making 
oscillations by their gravity combined with their inertia; 
the vibrations of magnetic bodies; and those which solids 
make by the help of an elastic force, &c. By reflecting, 
* Le Monnier, Institutions Astronemiques, p.157. 
+ See the Essay on the Theory of Jupiter’s Satellites, by M. Bailly; and 
M. Jeaurat’s Tables. 
¢ This remark is M. Daniel Bernoulli’s, p. 21, of his Recherches Méchan. 
et Astron. sur la meilleure Maniere de trouver ? Heure en Mer, &c. é 
we 
