‘800 TRANSHATIONS AND SELECTED ‘ARTICLES. 
«certainty of the principles of elementary geometry which have served 
as their base. 
The course which I have to pass over being sufficiently long, I ‘will 
not:enter upon any ‘detailed comparison between the results (which 
are really absurdly small) at which the ancients stopped in estimating 
the distance of the sun from the earth, and those which have been 
deduced from modern observations. °I shall limit myself to saying 
‘here, that it has been demonstrated—(and it is not without reason 
‘that I make use of so positive a word)—that it has been demon- 
strated, since the observation of the transit of Venus in 1769, that 
the mean distance of the sun from the earth is thirty-eight millions 
of leagues, and that, duting summer and winter, his distance from us 
varies by more than a million leagues. Such is the distance of this 
immense globe, whose physical constitution modern astronomers have 
made some progress towards determining. In the ancient philoso- 
phers we find nothing on this subject which is worthy of occupying 
us fora moment. Their disputes on the question as to whether the 
‘sun were a fire pure or gross, eternal or capable of extinction, not 
-being founded on observation, left in the deepest darkness the problem 
which the moderns have tried to solve. 
The progress which has been made on-this track dates from 1611. 
At that period, which is not far-from the invention of the telescope, 
a Dutch astronomer (Fabricius) observed distinctly the apparition of 
some dark spots on the eastern limit of the sun, which, after advan- 
cing gradually to the centre, crossed it, and moved to the western 
edge, disappearing finally after a certain number of days. From 
these observations, frequently repeated since then, we can infer that 
the'sun is a spherical globe, possessing a motion of rotation about an 
axis through its centre, the duration of which is twenty-five days and 
a half. 
These dark spots, variable and irregular, but well defined in outline, 
dave sometimes considerable dimensions; some of them have been 
observed of a magnitude more than five times that of the earth. 
They are generally surrounded by an aureola less luminous than the 
rest of the surface of the star, to which has been given the name of 
penumbra. This penumbra, first remarked by Galileo, and carefully 
observed, with reference to the changes it undergoes, by astronomers 
since his day, has led to a supposition with respect to the physical 
constitution of the sun, which at first sight looks singular enough- 
