of Nautical Astronomy. 323 
practically ; viz. the mutation of the earth’s axis,and the aber- 
ration of light. The former of these is a small periodical mo- 
tion of the earth’s axis produced by the action of the moon. ‘The 
latter is a small aynual change in the apparent places of fixed stars, 
resulting from the earth’s revolution in its orbit combined with the 
progressive motion of light. 
It was known from theoretical considerations, that if the di- 
stance of any one of the planets could be determined, the di- 
stances of all the others could be found from observations on 
their times of revolution. But the only method of finding the 
distance of the sun from the earth, that appeared likely to give 
results on which any confidence could be placed, was an indirect 
one depending on the duration of the transit of Venus as ob- 
served in different parts of the earth. And it was chiefly to ob- 
serve a transit of this planet that Captain Cook’s first voysge to 
the South Sea was undertaken. This leading object of the voy- 
age was accomplished as satisfactorily as all the subordinate ones 
were; and from the observations made on that occasion, and the 
only other oceasion of the kind that has happened during the 
last century, the distances of the planets from the sun and from 
each other have been determined with a precision which, from 
the nature of the problem, there is little reason to hope will ever 
be greatly exceeded. 
Not satisfied with simply comparing the masses of the planets 
with each other, philosophers have endeavoured to devise means 
of comparing their mean densities with that of some known ter- 
restrial substance. In this most ¢ ‘rious and elaborate research, 
the distinguished talents of Dr. Maskelyne and Dr. Hutton have 
been advantageously conjoined; and the result of their inquiries 
with respect to the density of the earth, appears entitled to all 
the confidence that can be placed in conclusions deduced from 
operations of so very delicate a nature. 
While the art of observation has been thus prolific in results, 
the intricate and difficult researches of those who have under- 
taken to trace the principle of gravitation into all its consequences, 
have produced discoveries that can scarcely be considered as less 
curious or important. They have shown that, notwithstanding 
the continual changes that take place in the planetary motions, 
notwithstanding the disturbing force which they are continually 
exerting on each other, their distances from the sun, the species 
of their orbits, and the mutual inclination of their planes, are sub- 
ject only to periodical changes, and return at regular though 
distant periods to the same state; that amidst the ever varying 
phenomena which their configurations present, the law that con- 
nects them with each other ensures the stability of the who'e 
system. Aud, what is of still greater importance, with the aid of 
data obtained from observation, they have furnished us with for- 
Ss2 niulas, 
