Dr. Herschel on the Nature and Construction, &c. 47 
has also pointed out the method whereby the quantity of 
matter it contains may be accurately determined. Dr. Brad- 
ley has assigned the velocity of the solar light with a degree 
of precision exceeding our utmost expectation. Galileo, 
Scheiner, Hevelius, Cassini, and others, have ascertained the 
rotation of the sun upon its axis, and determined the position 
of its equator. By means of the transit of Venus over the disc 
of the sun, our mathematicians have calculated its distance 
from the earth ; its real diameter and magnitude ; the density 
of the matter of which it is composed ; and the fall of heavy 
bodies on its surface. 
From the particulars here enumerated, it is sufficiently ob- 
vious, that we have already a very clear idea of the vast im- 
portance, and powerful influence of the sun on its planetary 
system. And if we add to this the beneficent effects we feel 
on this globe from the diffusion of the solar rays ; and consider 
that, by well traced analogies, the same effects have been 
proved to take place on other planets of this system ; I should 
not wonder if we were induced to think that nothing remained 
to be added in order to complete our knowledge : and yet it 
will not be difficult to shew that we are still very ignorant, at 
least with regard to the internal construction of the sun. The 
various conjectures, which have been formed on this subject, 
are evident marks of the uncertainty under which we have 
hitherto laboured. 
The dark spots in the sun, for instance, have been supposed 
to be solid bodies revolving very near its surface. They have 
been conjectured to be the smoke of volcanoes, or the scum 
floating upon an ocean of fluid matter. They have also been 
taken for clouds. They were explained to be opaque masses, 
