Report of Observations on the Transit of Mercury. 147 
In the transit of 1782, the French astronomers differed in respect 
to the ingress through the whole range from 19s. to 3m. and Qs. ; 
and at the third contact they differed from 21s. to Im. 44s. 
If we look back to former periods and see how far observation 
agreed with calculation, our present results, discordant and imper- 
fect as they are, indicate an encouraging advance in the knowl- 
edge of the motions of Mercury, and inspire the hope of soon be- 
ing able to tabulate them truly. In the transit of 1661, astrono- 
mers watched at their telescopes four days. Delambre in his 
System of Astronomy, (t. IT, p. 511,) has given us the full particu- 
lars of the observations of the French astronomers on the transit 
of 1786, showing that the best tables of that period are in error 
nearly three fourths of an hour. “ The ingress (says he) occurred 
at Paris during the night. At sunrise, it was rainy. All the astrono- 
mers of Paris were at their telescopes, but tired of waiting they 
had quit their posts half an hour after the predicted time of egress 
was past, abandoning all hope. Afterwards the sun came out. 
M. Messier, who had been making observations on the solar spots, 
the preceding days, wished to see them again, and thus gota 
sight of Mercury and observed his egress. I had remained at 
my telescope for another reason. Having made some researches 
respecting Mercury, I had seen that for the transit of 1786, the 
tables of Halley gave the egress an hour and a half later than 
those of Lalande. I had more confidence in the latter, but it 
was not demonstrated that Halley was decidedly wrong. I re- 
solved to wait, therefore, till the moment indicated by Halley’s 
tables, but I was not compelled to wait so long, since the phenom- 
enon arrived three fourths of an hour after the time of Lalande, 
but still three fourths of an hour earlier than that of Halley. Le 
Monnier, Pingre, Lalande, and his nephew, Mechain, Cassini, and 
his three adjuncts, deceived by the predicted time, had all missed 
the observation. I showed them mine that evening, and they 
would hardly believe it. This was the first observation which I 
had occasion to repeat to the Academy of Sciences, and it is from 
that epoch that I date my career as a practical astronomer.” 
Mercury appeared on the solar disk a round black spot, having 
an apparent diameter of 11.6, and of course occupying an ex- 
tent of only one hundred and sixty fourth part of the sun’s di- 
ameter. No traces of an atmosphere encircling the planet, could 
be discerned by an attentive examination for this special object, 
