322 Olservations on the present State 
of the science, and the rude kind of instruments employed in ob- 
serving, might lead us to expect. 
But from the moment that the principle of gravitation was dis- 
covered, and applied to account for the anomalies of the planetary 
motions, it is curious to observe how steadily and how rapidly 
the science has advanced towards perfection. It ‘resulted from 
this principle, That each planet in the system exerted a disturbing 
force on the motions of the others, and that the force of each 
depended both on its distance and its quantity of matter, or its 
weight. To determine the distances of the planets, might ap- 
pear a task sufficiently difficult ; but to estimate with any pre- 
cision the comparative weights of those bodies, would seem an 
undertaking which the limited faculties of man could scarcely 
be expected to accomplish. But, supposing the principle of gra- 
vitation univerally diffused, and the law of its operation invariable, 
it was easy to perceive, that on the correct determination of the 
distances and the masses of the planets, the ultimate perfection 
of the astronomy of the solar system entirely depended. It was 
also very obvious, that their comparative masses could only be 
discovered by observing the comparative magnitude of the effects 
which they produced on the motions of each other; and that 
though refined observations might show, at any time, the ulti- 
mate derangement in the motion of any planet produced by the 
action of a// the others; yet to disentangle the effect of one 
from this observed effect of the whole, and to assign to each that 
portion which was due to its weight in the system, was a task 
which required no ordinary attainments in science, and no small 
share of patience and sagacity. 
Before this, however, could be attempted with any prospect 
of success, the most accurate observations were necessary on the 
places of the fixed stars, and also on those of the planets in different 
parts of their orbits, and in a variety of situations with respect 
to each other. To make such observations with requisite cor- 
rectness, instruments of greater delicacy were required than any 
that had before been employed; and for fixed observatories such 
instruments were not long wanting. 
The positions of the principal fixed stars were then determined 
with such precision, and the phenomena of the planetary motions 
were so accurately ebserved, that the effect which the attraccion 
of the planets produced on the motions of each other was found 
in many cases to be distinetly perceptible. And repeated obser- 
vations have at length furnished data, from which the masses of 
the larger planets have been computed with an exactness that 
appears nearly sufficient for all practical purposes. 
From this refinement inthe art of observing, some curious, 
important, and previously unnoticed consequences, both of the 
laws of motion and the laws of gravitation, were first discovered . 
practi- 
