294 French Royal Academy of Sciences. 
given time—a day for instance; or rather he does not count 
them, for it would be an endless task, but ascertains them by his 
clock ; and lest the time-piece should deceive him, he most mi- 
nutely compares it with the movements of the heavenly bodies— 
the great invariable clock, infallible at all times and in all places. 
Having determined how his pendulum goes, he measures its 
length with accuracy by his rule. He repeats these trials a great 
number of times, in order to be sure of their exactness.. That 
done, he carefully puts up his ball and his rule and betakes him- 
self to make the same proofs elsewhere. These being gone 
through, are sufficient to enable him to calculate with the utmost 
exactness, and perhaps more exactly than by actual measure- 
ment, the curvature of the terrestrial meridian, on which there 
have been so many observations made. In effect, the vibrations 
of the pendulum are caused by the gravity which tends to make 
bodies fall towards the earth. In the operation which has been 
just described, the metal ball, in returning to the vertical point 
ih each of its vibrations, does no more than fall towards the earth, 
as much as the length of the wire to which it is suspended will 
permit. It is then seen, that the rapidity of the vibrations of the 
pendulum, or of its fall in any place, according to the given length 
of the wire, must depend on the energy, more or less powerful, 
of its gravity in that particular place: so that the operator may 
compare, by this means, the intensities of gravity in different 
places. But, according to the theory of universal gravitation, 
this intensity is found connected with the form of the earth’s sur- 
face, and with the law of density in the inner beds of the earth, 
by mathematical analogies. Thus, then, we see that it is only 
necessary for the observer to have one of these elements, in order 
to be able to determine analogically with regard to the other. It 
is thus that, by means of impressions left upon trees and upon 
sand, the philosopher Zadig determined the form, height, and 
even the colour of the king of Babylon’s beautiful horse. In our 
case, the method is the same, though the results are only a little 
more serious. The sciences, indeed, afford innumerable exam- 
ples of those indirect methods, which conduct much further than 
any one could expect to go by means apparently more direct ; a 
sort of stratagem of inquiry which takes by surprise the secret of 
nature, just as the arts of a skilful general discover to him those 
points where the enemy is most assailable. 
‘© The two methods which we have just explained have been 
almost always used together, in order that their results might re- 
ciprocally confirm each other : and as endless perfectibility, how- 
ever doubtful in morals, is quite certain in the physical sciences, 
it has naturally happened that the most perfect operations belong 
tothe latter. Thus, at first, it was merely known that the earth 
was 
