over the Disk of the Sun, &c. 217 
could have been distinguished from any other adjacent part of 
the solar disk. 
It will not be amiss to add, that very often, during the transit, 
I examined the appearance of Mercury with a view to its figure, 
but could not perceive the least deviation from a spherical form ; 
so that, unless its polar axis should have happened to be si- 
tuated, at the time of observation, in a line drawn from the 
eye to the sun, the planet cannot be materially flattened at its 
poles. 
OBSERVATIONS AND EXPERIMENTS RELATING TO THE CAUSES 
WHICH OFTEN AFFECT MIRRORS, SO AS TO PREVENT THEIR 
SHOWING OBJECTS DISTINCTLY. 
It is well known to astronomers, that telescopes will act very 
differently at different times. The cause of the many disappoint- 
ments they may have met with in their observations, is however 
not so well understood. 
Sometimes we have seen the failure ascribed to certain tremors, 
as belonging to specula ; and remedies have been pointed out for 
preventing them. Not unfrequently again, the telescope itself 
has been condemned; or, if its goodness could not admit of a 
doubt, the weather in general has been declared bad, though 
possibly it might be as proper for distinct vision as any we can 
expect in this changeable climate. 
The experience acquired by many years of observation, will 
however, I believe, enable me now to assign the principal 
cause of the disappointments to which we are so often exposed 
Unwilling to hazard any opinion that is not properly supported 
by facts, I shall have recourse to a collection of occasional ob- 
servations. They have been made with specula of undoubted 
mdccciii. F f 
