50 Progress of European Science. [Jan. 
In extracting for this purpose the words of Mr. Sovru’s address on presenting 
the author with the gold medal, we are in fact giving a most luminous and inter- 
esting review of the history of this subject, so closely connected with the demon- 
stration of the Copernican system. 
“‘ Three hundred years have now elapsed since CopERNicUs proposed to the world 
that system which bears his name ; and if we except the labours of Tycuo BRAHE, 
who, besides a catalogue of 800 stars, made attempts to determine the altitude of 
the pole-star at different seasons of the year, little was done by practical observa- 
tion to support or refute the ideas of Copernicus till the time of GattLEO. Ob- 
servations of the eclipses of Jupiter’s satellites induced him to propose them as a 
means of determining differences of longitude, whilst his discovery of the phases of 
Venus removed a serious objection to the truth of the Copernican system, and 
which Copernicus himself predicted would be removed, though he had not the 
means of doing so himself. About the year 1665, HuycrEns, by his invention of 
the pendulum clock, gave to astronomical observations an accuracy hitherto un- 
known ; and Cassini, by means of the excellent glasses of CaMPANI, accumulated 
a vast mass of observations of the eclipses of Jupiter’s satellites, and deduced from 
them tables whereby astronomers could predict their occurrence. 
‘* Notwithstanding the powerful arguments advanced in its favour, the 
Copernican hypothesis was not generally entbraced ; for in the year 1669, nearly 
a century and a half subsequent to its promulgation by Copernicus, even the 
celebrated Hoox, to use his own words, ‘ would not absolutely declare for 
it*.’ To settle the matter, therefore, this extraordinary man, feeling that 
the instruments of Tycuo, although magnificent beyond all others, were, 
from the nature of their construction, and from their being unprovided 
with telescopic sights, incompetent to detect minute alterations of sidereal 
positions, and knowing that the laws which governed refraction were so little un- 
derstoodt as to render all observations in which that element was materially in- 
volved, liable to errors greater probably than the quantities he was in search of, 
invented the zenith sector. It was erected at Gresham College, and consisted of a 
telescope, 36 feet long, a divided arc, anda plumb-line. The star selected for observa- 
tion, and with reference to which, indeed, his instrument was entirely construct- 
ed, was one which passed within two or three minutes of the zenith of Gresham 
College ; it was visible in the day-time throughout the year, and was y Draconis : by 
observing its zenith distance when the earth was in opposite points of her orbit, he 
found (as he erroneously concluded) a sensible parallax, amounting to about 20 
seconds, and, consequently, determined that the Copernican system was the true 
onef. 
“ In the mean time, the eclipses of Jupiter’s satellites, thanks to the facilities of 
predicting them, afforded by Casstn1’s tables, had been assiduously observed ; and 
in the year 1675; the discordances found between the predicted and the observed 
eclipses enabled the celebrated Roemer to demonstrate that light was not instanta- 
neously propagated, and that the discordances between the tables and the observa~- 
tions might be considered as the measure of its velocity. 
“The year of RomMER’s discovery was further marked by another epoch in astro- 
nomical history, namely, the foundation of the Royal Observatory. FLAMSTEAD, 
* An attempt to prove the motion of the earth from observations made dy RoBERT 
Hooks, F. R. S. pp. 5 and7. 
“+ Tdem, pp. 10 and 11. 
+ Idem, p. 25, 
