INTRODUCTION.

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nomers of the age in which he lived. We owe to him a multitude of dis-coveries, which have secured for him an imperishable reputation. Heconstructed the first tables of Jupiter ’s satellites which could lay anyclaim to accuracy. He discovered four of Saturn’s satellites; determinedthe rotations of Jupiter and Mars , and arrived at a very approximate valueof the solar parallax. He also discovered the belts of Jupiter and thezodiacal light; established the singular coincidence of the nodes ofthe lunar equator and orbit; and, lastly, constructed an excellent tableof refractions.

While astronomical science was thus flourishing on the Continent, ithad already dawned upon England.

Harriot, the celebrated mathematician, was an assiduous observer ofcelestial phenomena. We owe to him some valuable observations of thecomet of 1607, which have since been found to refer to one of theperiodical apparitions of the famous comet of Halley. He was one of thefirst individuals who employed the telescope in exploring the heavens.His observations of Jupiter ’s satellites date from the 17th of October,1610. He also observed the solar spots very soon after their discoveryon the Continent.

Jeremiah Horrocks , a native of the north of England, displayed acapacity of the highest order for the cultivation of astronomy; but un-fortunately his career was soon brought to a close by a premature death.We owe to him the earliest observation of the transit of Venus. He andhis friend Crabtree were the only two individuals who witnessed this rarephenomenon on the 24th of November, 1639. He effected an importantimprovement in the lunar theory, and made many sagacious remarks onother subjects relating to astronomy. He died suddenly on the 3rd ofJanuary, 1641, at the age of about twenty-two years.

William Gascoigne, the contemporary of Horrocks , had the merit oforiginating some remarkable improvements in practical astronomy. Hewas one of the first who employed the Keplerian telescope in astronomicalobservations. He introduced the use of telescopic sights. He was theoriginal inventor of the micrometer, and was also the first who applied itto divided instruments. Like Horrocks , this highly-gifted individualperished in the flower of his age. He fell at the battle of Marston Moor,on the 2nd of July, 1644, when he had only attained the age of twenty-four years.

Hevelius was one of the most eminent observers of the seventeenthcentury. His labours extended over a period of about fifty years ; but ashe continued throughout his whole career to adhere to the ancient methodsof practical astronomy, the results achieved by him do not possess a valuecommensurate with his merits as an observer.

During the seventeenth century, a great revolution was effected in prac-tical astronomy. The application of the pendulum to clocks by Huyghens had the effect of introducing a method of observation which had beendevised in the preceding century, but which was found to be impracticablein consequence of the difficulty attending the measurement of time. Itconsisted in observing the altitude of a celestial body on the meridian,and noting the instant of its passage. By this means the declination andright ascension were obtained without any trigonometrical calculation. Inconsequence of this improvement, the observations of the celestial bodieswere henceforward made chiefly with instruments fixed in the meridian.The micrometer, the invention of which is due originally to Gascoigne,