A S T R O N O M Y. 
tioiis will) the telefcope were made by Galileo, Harriot, 
Huygens, Hook, CalTini, &e. It was laid, that, from re- 
port only, and before he had feen one, Galileo made for 
himlelf telel'copes, by w hich he difeovered inequalities in 
the Moon’s furface, Jupiter’s fatellites, and the ring of Sa¬ 
turn ; al'fo fpots on the Sun, by which he found out the 
revolution of that luminary-on its axis ; and he difeover¬ 
ed that the nebula: and milky-way were full of fmall liars; 
Harriot alfo, who has hitherto been known only as an al- 
gebraid, made much the fame dilcoveries as Galileo, and 
as early, if not more fo, as appears by his papers before- 
ifientioned, in the poflefiion of the earl of Egremont. 
Mr. Horrox, a young altronomer of great talents, alfo 
made confiderable improvements. In 1633 he difeovered 
that the planet Venus would pafs over Sun’s dife on the 
24th of November, 1639; an event which he announced 
to his friend Mr. Crabtree ; and thefetwo were the only per- 
fons in the world that obferved this tranfif, which was alfo 
the fil'd time it had ever been feen by mortal eyes. Mr. 
Horrox made many other itfeful obfervations, and had 
even formed a new theory of the Moon, taken notice of 
by r Sir Ifaac Newton; but his early death, in the begin¬ 
ning of the year 1640, put a dop to his ufeful and valua¬ 
ble labours. About the fame time flouridied Hevelius, 
burgomader of Dantzic, who furnifhed an excellent obfer- 
vatory in his own houfe, where he obferved the fpots and 
phafes of the Moon, from which obfervations he compiled 
his Selenographia; and an account of his apparatus is con¬ 
tained in his work entitled Machina Coeledis, a book now 
very fcarce, as mod of the copies were accidentally burnt, 
•with tlie whole houfe and apparatus, in 1679. 
Dr. Hook, a cotemporary of Hevelius, invented indru- 
ments with telefcopic fights, and cenfured thofe of the lat¬ 
ter, which occafioned a fiiarp difpute between them; to 
fettle which, the celebrated Dr. Halley was lent over to 
Hevelius to examine his indruments. Thefe two adrono- 
mers made feveral obfervations together, very much to 
their fatisfaCtion, and amongd them was one of an occul- 
tation of Jupiter by the Moon, when they determined the 
diameter of the latter to be 30' 33". 
Before the middle of the feventeenth century the con- 
druclion of telefcopes had been greatly improved, parti¬ 
cularly by Huygens and Fontana. The former condruCt- 
ed one of 123 feet, with which he long obferved the Moon 
and planets, and difeovered that Saturn was enCompafled 
with a ring. With telefcopes of 200 and 300 feet focus, 
Cadini favv five fatellites of Saturn, with his zones or belts, 
and tlie diadows of Jupiter’s fatellites paffing over his bo¬ 
dy. In 1666 Azout applied a micrometer to telefcopes, 
to meafure the diameters of the planets, and other fniall 
didances in the heavens: but an indrument of this kind 
had been invented before, by Mr. Gafcoigne, though it 
was but little known. To obviate the difficulties of the 
great lengths of refracting telefcopes, and the abberration 
of the rays, it is faid that Merfennus fird darted the idea 
of making telefcopes of refledtors, indead of lenfes, in a 
letter to Defcartes ; and, in 1663, James Gregory of Aber¬ 
deen fiiewed how fuel) a telefcope might be condruCted. 
After fome time fpent by Sir Ifaac Newton, on the con- 
ftruCtion of both forts of telefcopes, he found out the great 
inconvenience which arifes to refractors from the different 
refrangibility of the rays of light, for which he could not 
then find a remedy; and therefore, purfuing the other 
kind, in the year 1672 he prefented to the Royal So¬ 
ciety two refleCtors, which were condruCted with fplie- 
rical fpeculums, as he could not procure other figures. 
The inconveniences, however, arifing from the different 
refrangibility of the rays of light, have fince been fully 
obviated by the ingenious Mr. Dollond. Towards the 
latter part of the feventeenth, and beginning of the eigh¬ 
teenth, century, practical adronomy it feems rather lan- 
guidied. But at the fame time the fpeculative part was 
carried to the highed perfection by the immortal New¬ 
ton in his Principia, and by the Adronomy of David Gre¬ 
gory. 
Vol. II. No. 80. 
425 
Soon after this, great improvements of adronomical in¬ 
druments began to take place, particularly in Great Bri¬ 
tain. Mr. Graham, a celebrated mechanic and watch¬ 
maker, not only improved clocks and watch-work, but 
alfo carried the accuracy of adronomical indruments to a 
furprifing degree. He condruCted the old 8-feet mural 
arch at the royal obfervatory at Greenwich, and a final! 
equatorial feclor for making obfervations out of the me¬ 
ridian ; but he is chiefly remarkable for contriving the 
zenith feCtor of twenty-four feet radius, and afterwards 
one of feet, with which Dr. Bradley difeovered the 
abberration of the flxed flars. The reflecting telefcope of 
Gregory and Newton was greatly improved by Mr. Had¬ 
ley, who prefented a very powerful indrument of that 
kind to the Royal Society in 1719. The fame gentleman 
alfo immortalized his memory by the invention of the re¬ 
flecting quadrant, or feCtor, now called by his name, which 
he prefented to the fociety in 1731, and which is now fo 
univerfally ufeful at lea, efpecially where nice obfervations 
are required. It appears, however, that an indrument li- 
milar t@ this in its principles had been invented by Sir 
Ifaac Newton; and a defeription, with a drawing, of it 
given by him to Dr. Halley, when he was preparing for 
his voyage in 1701, to difeover the variations of the 
needle: it has alfo been allerted, that Mr. Godfrey, of 
Philadelphia in America, made the fame difeovery, and 
the fird indrument of this kind. About the middle of 
this century, the confiruCting and dividing of large adro¬ 
nomical indruments were carried to great perfection by 
Mr. John Bird ; and reflecting telefcopes were not lefs im¬ 
proved by Mr. Short, who alfo fird executed the divided 
objeCt-glafs micrometer, which had been propofed and de- 
feribed by M. Louville and others. Mr. Dollond alfo 
brought refraCting telefcopes to the greated perfection, by 
means of his achromatic glades; and the late difeoveries 
of Dr. Herlchel are owing to the amazing powers of re¬ 
flectors of his own condruCtion. 
Thus the adronomical improvements in the prefent cen. 
tury have been chiefly ow ing to the inventions and im¬ 
provements in the indruments, and to the edabhfhment of 
regular obfervatories in England, France, and other parts 
of Europe. Roemer, a celebrated Danifli adronomer, fird 
made life of a meridional telefcope ; ar.d, by obferving the 
eclipfes of Jupiter’s fatellites, he fird difeovered the pro- 
grellive motion of light, concerning which lie read a dif- 
lertation before the Academy of Sciences at Paris in 1675. 
Mr. Flamdead w r as appointed the fird aftronomer-royal at 
Greenwich in 1675. He obferved, for forty-four years, 
all the celedial phenomena, the Sun, Moon, planets, and 
fixed dars, of all which he gave an improved theory with 
new folar tables, and a theory of the Moon according to 
Horrox ; likewife, in Sir Jonas Moore’s Sydem of Mathe¬ 
matics, he gave a curious traCt on the doCtrine of the 
fphere, diewing how geometrically to condruCt eclipfes of 
the Sun and Moon, as well as occultatiuns of the fixed dars 
by the Moon. And it was upon his tables that were con¬ 
druCted, both Halley’s tables, and Newton’s theory of the 
Moon. Cadini alfo, the fird French adronomer-royal, 
very much didinguifhed himfelf, making many obferva¬ 
tions on the Sun, Moon, planets, and comets, and greatly 
improved the elements of their motions. He alfo erected 
the gnomon, and drew the celebrated meridian line in the 
church of Petronia at Bologna. 
In 1719 Mr. Flamdead was fucceeded by Dr. Halley, as 
afironomer-royal at Greenwich. The doCtor had been 
lent at the early age of twenty-one to the ifland of St. He¬ 
lena, to obferve the fouthern dars, and make a catalogue 
of them, w hich was publifhed in 1679. In 1705 lie pnb- 
liflied his Synopfis Adrononlias Cometicae, in which he 
ventured to predict the return of a comet in 1758 or 1759. 
He was the fird who difeovered the acceleration of the 
Moon, and gave a very ingenious method for finding her 
parallax by three obferved phafes of a folar eclipfe. He 
publidied, in tlie Phil. Tranf. many learned papers, and 
amongd them fome that were concerning the life that might 
5Q- be 
